Module 2
Atmospheric Motion
A. Gases Respond to Changes in Temperature and Pressure?
The atmosphere consists largely of gases, with lesser amounts of liquids, such as
drops of water, and solids, such as dust and ice. By their nature, gases expand easily or
contract in volume in response to changes in temperature and pressure. Variations in
temperature and resulting changes in pressure are the main drivers of motion in the
atmosphere. The amount of insolation at the top of the atmosphere varies considerably
from place to place and through time. These variations in insolation in turn lead to
differences in temperatures, to which gases in the atmosphere respond.
Consider what happens when we want to make a hot air balloon rise. Typically, a
propane-powered burner heats ambient air, causing the air to expand in volume. This
increase in volume inflates the balloon. Since the same amount of gas now occupies a
much larger volume, the density of the heated air is less than the density of the
surrounding air, so the balloon rises. So, as air increases in temperature, it tends to
increase in volume and become less dense. The figure below shows how a quantity of gas
responds to either an increase in temperature (heating) or a decrease in temperature
(cooling). The starting condition is represented by the cube of gas on the left. An increase
in the temperature of a gas means more energetic molecules, so a larger volume is needed
to accommodate the same amount of gas. If a gas cools, the molecules within it have less
kinetic energy (motions) and can therefore be packed into a smaller volume. The gas has
a higher density and will tend to sink. This example shows that temperature and volume
of a gas are directly related — in fact they are proportional if pressure is held constant.
Such a proportional relationship means that if temperature is doubled, volume doubles
too. If temperature decreases by half, volume does too. This specific relationship is called
Charles’s Law, which is one of the fundamental laws governing the behavior of gases,
and it explains why a hot air balloon rises.
If a gas is held at a constant temperature but forced to occupy a smaller volume,
the pressure of the gas increases. Pressure is proportional to the number of collisions of
the molecules. If the same gas fills a larger volume, the collisions and amount of pressure
both decrease. In both cases, if we instead change the pressure, the volume of the gas will
adjust accordingly. A material, like a gas, that can be compressed, is said to be
compressible.
Molecules of gas in the sealed container in the left canister below are under
pressure, represented by the two weights resting on top. At some temperature, the
molecules have a corresponding amount of energy, and some of the moving molecules
are hitting the movable lid, resisting the downward force of the attached weight.
Removing a weight reduces the downward pressure on the gas. However, the gas retains
its same average energy level (temperature) and therefore exerts the same upward force
on the movable lid as before. The upward force from the gas molecules exceeds the
downward force of the weight and so raises the lid, increasing the volume occupied by
the gas. In this way, a decrease in pressure results in an increase in volume, if the gas
does not change temperature. Increasing the downward pressure by adding a third weight
on the original canister causes the lid to slide down. This increase in pressure causes a
decrease in volume. As the gas is compressed into a smaller volume, the number of the
molecules impacting the lid increases. When this upward force from the gas molecules
equals the downward force from the weight, the lid stops moving, and the volume and
pressure of the gas stop changing. The relationship between pressure and volume of a
gas, under conditions of constant temperature, is inversely proportional — if pressure
increases, volume decreases. If pressure decreases, volume increases. Either pressure or
volume can change, and the other factor responds accordingly, changing in the opposite
direction by a proportional amount. That is, if the volume is cut in half, the pressure
doubles. If the volume doubles, the pressure is cut in half. This inversely proportional
relationship between pressure and volume, under constant temperature, is called Boyle’s
Law.
Since Charles’s Law relates volume to temperature, and Boyle’s Law relates
volume to pressure, we might suspect that we can relate temperature and pressure.
Combining Charles’s Law and Boyle’s Law leads to the Ideal Gas Law, which relates
temperature, pressure, and density (mass divided by volume). Basic aspects of the Ideal
Gas Law help explain the processes that drive the motion of matter and associated energy
in the atmosphere.
We can represent the Ideal Gas Law with a figure, with words, or with an
equation. We begin with this figure, which expresses the two sides of the equation. On
one side of the equation (the left in this figure) is pressure. On the right side of the
equation are density and temperature. The Ideal Gas Law states that if we increase a
variable on one side of the equation (like increasing pressure), then one or both of the
variables on the other side of the equation have to change in the same direction — density
or temperature have to also change, or perhaps both do. Examine this figure and envision
changing any one of the three variables (pressure, density, or temperature), and consider
how the other two variables would respond to satisfy the visual equation. What happens
if pressure increases? If temperature does not change, then density must increase. If
pressure increases but density does not change, then temperature has to increase.
Alternatively, temperature and density can both change. This three-way relationship
partly explains why temperatures are generally warmer and the air is more dense at low
elevations, where the air is compressed by the entire weight of the atmosphere, than at
higher elevations, where there is less air. Higher pressure often results in higher
temperatures. What does the relationship predict will happen if a gas is heated to a higher
temperature? If the density does not change, the pressure exerted by the gas on the
plunger must increase. If the pressure does not change, the density must decrease. This is
because density and temperature are on the same side of the equation, so an increase in
one must be matched by a decrease in the other — if the other side of the equation
(pressure) does not change. The relationship indicates that heated air can become less
dense, which allows it to rise, like in a hot air balloon.
The way gas responds to changes in temperature and pressure is the fundamental
driver of motion in the atmosphere. Since temperature changes are largely due to
insolation, we can examine how insolation affects the physical properties of gas and how
this drives atmospheric motion. The Sun is the major energy source for Earth’s weather,
climate, and movements of energy and matter in the atmosphere and oceans. In the figure
above, insolation strikes Earth’s surface (land or water), which in turn heats a volume of
gas in the overlying atmosphere. The increase in temperature results in expansion of the
gas because of the increased kinetic energy of the molecules in the gas; expansion is an
increase in volume. If the same number of gas molecules occupy more volume, the
density of the air decreases (the air becomes less dense). The increase in volume can
result in a decrease in pressure (less frequent molecular collisions). As a result, the air
mass is now less dense than adjacent air that was heated less. The more strongly heated
and expanded air rises because it is less dense relative to surrounding air (which was not
heated as much and so is more dense).
As the heated, less dense air rises, adjacent air flows into the area to replace the
rising air. The end result is vertical and lateral movement of air — vertical motion within
the rising air, and lateral motion of surrounding air toward the area vacated by the rising
air. In this way, the response of gas to changes in temperature, pressure, and density (or
volume), as expressed by the gas laws, is the primary cause of motion in the atmosphere.
Variations in insolation cause changes in temperature, pressure, and density, which in
turn cause air to move within the atmosphere.
B. Air Pressure
Pressure of gases within the atmosphere is highly variable, both vertically and
laterally. These variations in pressure determine the nature and direction of atmospheric
motions. If one place in the atmosphere has higher pressure than another place, this
imbalance of pressure (and therefore also atmospheric mass) tends to be evened out by
the flow of air. How do we describe and measure pressure, and how do we use these
measurements to understand or even predict the flow of air?
Pressure is an expression of the force exerted on an area, usually from all
directions. In the case of a gas, pressure is related to the frequency of molecular
collisions, as freely moving gas molecules collide with other objects, such as the walls of
a container holding the gas. It is such collisions that keep a balloon, soccer ball, or
bicycle tire inflated. Molecules of gas in a sealed glass container move rapidly in random
directions, and some strike the walls of the container. The force imparted by these
collisions is pressure. The more collisions there are, the more pressure is exerted on the
walls of the container. If we push down on the lid of the container, the same number of
molecules are confined into a smaller space. Lower parts of the container walls are now
struck by a greater number of the more closely packed gas molecules, so the pressure is
greater. Decreasing the volume of a gas increases its pressure, consistent with Boyle’s
Law. What happens if we put a weight on top of the lid (center container) and then either
cool or heat the gas in the container?
If we cool the container by placing it in ice, the molecules become less energetic
and so strike the walls and lid of the container less often — the gas pressure decreases
and the lid moves down. If we instead heat the container, the gas molecules become more
energetic and strike the walls and lid of the container more often — the gas pressure
increases and lifts the lid. The equation to the right illustrates what pressure actually
measures and the units we use to describe it. The units of pressure are used throughout
this book in describing weather, climate, and the flow of water. Pressure is a force
exerted on a given surface area. According to Newton’s second law, force is the product
of mass and acceleration. The unit of mass is the kilogram (kg), acceleration is in
meters/second per second (m/s2 ), and area is in square meters (m2 ) A force of one
kg·m/s2 is called a Newton. Pressure, measured in Pascals, is an expression of the
number of Newtons of force exerted on a square meter of surface. The air pressure at
Earth’s surface is many Pascals, so we express pressures in a larger, related unit called a
bar, or in millibars (1/1000 of a bar).
We can measure air pressure with an instrument called a barometer. The
barometer shown to the left is a sealed glass tube fixed in liquid mercury. Changes in air
pressure cause the liquid level in the tube to rise or fall, allowing the measurement of
relative pressure. Such barometers have units of inches (or centimeters) of mercury.
Pressure is also reported in units of a bar, with one bar being approximately equal to the
average air pressure at sea level. Modern digital instruments record pressure in millibars.
Meteorologists measure air pressure at vertical heights in the atmosphere using hydrogen-
or helium-filled balloons, like this one. An instrument package called a radiosonde is
suspended from the balloon. Sensors measure pressure, temperature, humidity, and
position (using a GPS) as the balloon ascends, and these measurements are transmitted
via radio waves to a central computer. Wind speed and direction are inferred from
successive positions of the radiosonde. The balloon eventually pops and the radiosonde
parachutes to the ground.
Air pressure in the atmosphere is not constant. The largest variation is vertically,
with an abrupt decrease in pressure upward from near the surface. The red curve on this
figure shows how the air pressure, measured in millibars (mb), decreases from Earth’s
surface to the top of the atmosphere. This diagram depicts the main layers of the
atmosphere (troposphere, stratosphere, etc.) and highlights some of the features observed
in each part, such as auroras in the thermosphere, shooting stars that mostly burn up in
the mesosphere, and the restriction of most clouds and weather to the troposphere. The
top of the troposphere is the tropopause. Colors along the left edge of the diagram convey
temperature variations within and between the atmospheric layers. These vertical
temperature variations affect the density of the air, impacting air motions caused by the
Sun heating the Earth’s surface. In the thermosphere and mesosphere, gas molecules are
relatively sparse and temperatures are low (−90°C at the thermospheremesosphere
boundary). As a result of the sparseness of molecules, air pressures are very low (less
than one millibar).
The abundance of gas molecules increases down into the stratosphere, and this is
accompanied by an increase in air pressure (the bending of the red curve to the right as it
goes downward). Air pressures at the base of the stratosphere (the tropopause) have
increased to about one-fifth of pressures measured at sea level. The pull of Earth’s
gravity holds most gas molecules close to Earth’s surface, in the troposphere. Air
pressure increases downward in the troposphere because of a greater abundance of
molecules downward and the larger total number of molecules pressing down from the
layers above. The highest air pressures are close to the surface, and at the lowest
elevations. Sea level is the reference level for air pressure, with an average pressure of
1,013 mb (a little over 1 bar).
Air pressure also varies laterally, from area to area, and from hour to hour, and
these variations are typically represented on maps, like the one shown here. Such maps
either show the pressure conditions at a specific date and time or show pressure values
averaged over some time period, like a month or a year. To allow us to compare different
regions and to see the larger patterns, the map uses pressure values that are corrected to
sea level, or their sea-level equivalent. In this way, we eliminate the effects of differences
in elevation from place to place. Such maps of air pressure contain numbered lines, called
isobars, that connect locations with equal pressure. If you could follow an isobar across
the countryside, you would follow a path along which the pressure values, once corrected
to their sea-level equivalents, would be equal. Successive isobars are numbered to
represent different values of air pressure, usually in millibars (e.g., 1,024), and there is
generally a constant difference in pressure between two adjacent isobars (a 4 mb
difference on this map). Note that isobars do not cross, but can completely encircle an
area. Most maps of air pressure feature the large capital letters H and L. An H represents
an area of relatively higher pressure called a high-pressure area or simply a high. An L
represents a low-pressure area, commonly called a low. An elongated area of high
pressure can be called a ridge of high pressure and an elongated area of low pressure is a
trough. The map patterns change with time, corresponding to changes in air pressure.
Patterns typical for a region also change from season to season.
C. Pressure Variations and Winds
The movement of air in the atmosphere produces wind, or movement of air
relative to Earth’s surface. Circulation in the atmosphere is caused by pressure
differences generated primarily by uneven insolation. Air flows from areas of higher
pressure, where air sinks, to areas of lower pressure, where air rises. Wind speed and
direction are among the most important measurements in the study of weather and
climate. On short timescales, wind can indicate which way a weather system is moving
and the strength of a storm. When considered over longer timescales, winds indicate
general atmospheric circulation patterns, a key aspect of climate.
Wind directions can be assessed as easily as throwing something light into the air
and tracking which way it goes, but it is best done with a specially designed device,
called an anemometer, that can measure the wind speed and direction. Wind speed is
expressed in units of distance per time (km/hr) or as knots, which is a unit expressing
nautical miles per hour. One knot is equal to 1.15 miles/hr or 1.85 km/hr. Wind direction
is conveyed as the direction from which the wind is blowing. Wind direction is
commonly expressed with words, such as a northerly wind (blowing from the north). It
can instead be described as an azimuth in degrees clockwise from north. In this scheme,
north is 0°, east is 090°, south is 180°, and west is 270°. Go ahead and write these
numbers on the appropriate place on this figure. The atmosphere also has vertical motion,
such as convection due to heating of the surface by insolation. A local, upward flow is an
updraft and a local, downward one is a downdraft.
Air moves because there are variations in air pressures, in density of the air, or in
both (recall that pressure and density are related via the Ideal Gas Law). Such pressure
and density variations are mostly caused by differential heating of the air (due to
differences in insolation) or by air currents that converge or diverge. The atmosphere is
not a closed container, so changes in volume (i.e., air being compressed or expanded)
come into play. These volume changes can make air pile up or spread out, resulting in
variations in air pressure. Movement of air occurs to equalize a difference in air pressure
between two adjacent areas, that is, a pressure gradient. Air molecules in highpressure
zones are packed more closely together than in lowpressure zones, so gas molecules in
high-pressure zones tend to spread out toward low-pressure zones. As a result, air moves
from higher to lower pressure, in the simplest case (as shown here) perpendicular to
isobars. High-pressure zones and low-pressure zones can be formed by atmospheric
currents high in the atmosphere that converge or diverge. Converging air currents
compress more air into a smaller space, increasing the air pressure. Diverging air currents
move air away from an area, decreasing pressure. Forces associated with converging and
diverging air are called dynamic forcing.
Most variations in air pressure and most winds, however, are caused by thermal
effects, specifically differences in insolation from place to place. This cross section ( ⊲)
shows a high-pressure zone (on the left) caused by the sinking of cold, high-altitude air
toward the surface. In the adjacent lowpressure zone (on the right), warmer near-surface
temperatures have caused air to expand, become less dense, and rise, causing low
pressure. Near the surface, air would flow away from the high pressure and toward the
low pressure. Different air currents would form higher, in the upper troposphere, to
accommodate the sinking and rising of the air.
Differences in air pressure, whether caused by thermal effects or dynamic forcing,
produce a pressure gradient between adjacent areas of high and low pressure. Associated
with this pressure gradient are forces that cause air to flow. Pressure gradients can exist
vertically in the atmosphere or laterally from one region to another. Elevation differences
cause the largest differences in air pressure. At high elevations, there is less atmospheric
mass overhead to exert a downward force on the atmosphere. As a result, density
decreases with elevation, and air pressure does too. These vertical variations in air
pressure cause a pressure gradient in the atmosphere, with higher pressures at low
elevations and lower pressures in the upper atmosphere.
This pressure gradient can be thought of as a force directed from high pressures to
lower ones. This pressure-gradient force is opposed by the downwarddirected force of
gravity, which is strongest closer to Earth’s surface. Lateral variations in air pressure also
set up horizontal pressure gradients, and a pressure-gradient force directed from zones of
higher pressure to zones of lower pressure. On the map below, the pressure-gradient force
acts to cause air to flow from high pressure toward lower pressures, as illustrated by the
blue arrows on the map. Places where isobars are close have a steep pressure gradient,
and a strong pressure-gradient force, so movement of the atmosphere (i.e., winds) will
generally be strong in these areas. Places where isobars are farther apart have a more
gentle pressure gradient, thus a weak pressure-gradient force, and generally lighter winds.
Although winds tend to blow from high to low pressure, other factors, such as Earth’s
rotation, complicate this otherwise simple picture, causing winds patterns to be more
complex and interesting.
As is typical for nature, some forces act to cause movement and other forces act to
resist movement. The pressure-gradient force acts to cause air movement, where friction
acts to resist movement. Friction occurs when flowing air interacts with Earth’s surface.
Wind is slowed near the surface because of friction along the air-Earth interface, as
represented in this figure by the shorter blue arrows low in the atmosphere. As the air
slows, it loses momentum (which is mass times velocity). Some momentum from the
moving air can be transferred to the land, such as when strong winds pick up and move
dust or cause trees to sway in the wind. It is also transferred to surface waters, causing
some currents in oceans and lakes and forming surface waves.
Friction with Earth’s surface, whether land or water, also causes the wind patterns
near the surface to become more complicated. On land, air is forced to move over hills
and mountains, through valleys, around trees and other plants, and over and around
buildings and other constructed features. As a result, the flow patterns become more
curved and complex, or turbulent, near the surface, with local flow paths that may double
back against the regional flow, like an eddy in a flowing river. Friction from the surface
is mostly restricted to the lower 1Ekm of the atmosphere, which is called the friction layer.
Stronger winds occur aloft, in part because these areas are farther from the frictional
effects of Earth’s surface. Some friction occurs internally to the air, even at these heights,
because adjacent masses of air can move at different rates or in different directions.
Friction can also accompany vertical movements in updrafts and downdrafts.
D. Significant Regional Winds
Differences in air pressure cause a variety of regional to local wind conditions,
such as those associated with storms, which are discussed in the chapter on weather.
Some local winds are not so much related to weather systems as they are to differences in
pressure that tend to occur at certain times of the year or after the establishment of an
area of high pressure. These local to regional winds have interesting names, like Chinook
winds or Santa Ana winds, and can have profound impacts on people.
The term Chinook originated in the Pacific Northwest and can refer to several
types of winds. The most common usage is for a warm, dry wind that blows down the
flanks of a mountain range. Chinooks are so warm and dry they are called “snow eaters,”
for the way in which they can cause a sudden melting of snow and ice on the ground. The
onset of a Chinook wind can cause a sudden rise in temperatures, especially during the
winter. A Chinook in Loma, Montana, caused temperatures to rise from −48°C (−59°F)
to 9°C (49°F) within a 24-hour period, the most change recorded for a single day in the
U.S. In Spearfish, South Dakota, a Chinook off the adjacent Black Hills caused
temperatures to rise 27 C° (49 F°) in two minutes, the world’s fastest rise in temperature
ever recorded!
This figure depicts the formation of a Chinook wind. The process begins when
winds push moist air against the windward side of a mountain, where windward refers to
the side from which wind is blowing. As the moist air rises up the mountain, it cools,
causing the formation of clouds, a process that also releases latent heat. The heat warms
the air, which continues rising toward the mountain peaks. Once the air reaches the peak,
it begins flowing down the other side — the leeward side, of the mountain (the side
opposite the windward side). As the air descends, it continues drying out and is
compressed and heated. It was also warmed from the release of latent heat on the
windward side. The warm, dry air descends from the mountain and spreads across the
adjacent lowland, forming a Chinook wind. This map shows the locations where Chinook
winds are relatively common. As expected, Chinooks occur on the leeward side of
mountain ranges (prevailing winds are from the west to east in this region), in Alaska,
and elsewhere.
On the previous two pages, we introduced the term katabatic wind for a wind that
blows downslope, forming a cool mountain breeze. More regional and pronounced
katabatic winds affect Antarctica and Greenland, both of which have a high central
landmass surrounded by ocean. Air over the middle of the landmasses is very cold and so
also very dense, flowing off the central topographic highs and down the icy slopes. These
two perspective views show that Antarctica (on the left) and Greenland (on the right)
both have a broad, high area centered in the middle of the ice. Katabatic winds blow
down off these high areas in all directions. These winds are especially strong where they
are channeled down valleys, such as the famous Dry Valleys of Antarctica, so named
because strong katabatic winds have stripped most ice and snow off the land surface.
Katabatic winds in Antarctica generally involve cold but dry air, but they can interact
with clouds along the coast, sometimes creating stunning effects as the cold air and
clouds spill off the highlands, similar to the scene in the photograph above.
Winds in Southern California typically blow from west to east (that is, they are
westerlies), bringing relatively cool and moist air from the Pacific Ocean eastward onto
land, especially in areas right along the coast, like Los Angeles and San Diego. These
coastal cities also often have onshore sea breezes during the day and offshore land
breezes at night. At other times, however, regional winds, called Santa Ana winds, blow
from the northeast and bring dry, hot air toward the coast, causing hot, uncomfortable
weather and setting the stage for horrendous wildfires. Santa Ana winds are regional
winds that blow from the northeast (⊲), typically developing during spring and fall, when
high pressure forms over the deserts of eastern California and Nevada. Circulation of air
associated with the area of high pressure pushes winds south and westward, toward the
coast, in marked contrast to the normal onshore flow. This air is coming from the Mojave
Desert and other desert areas to the north and east, so it is very dry.
Santa Ana winds from the Mojave Desert are partially blocked by the mountains
on the northern and eastern sides of Los Angeles. The winds spill through mountain
passes, such as Cajon Pass northeast of Los Angeles, and are funneled down the canyons
and into the Los Angeles basin. The funneling effect causes winds to be especially strong
within the canyons. As the air moves from higher deserts (to the northeast) down toward
Los Angeles, the air compresses and heats up. As a result, during an episode of Santa
Ana winds, the coastal areas of Southern California experience much hotter and drier
weather conditions than are normal. Due to this behavior of air flowing from high to low
areas, Santa Ana winds are considered to be a type of katabatic wind.
The hills and mountains of coastal Southern California receive enough
precipitation to be covered with thick brush, such as oak, or by forests at higher
elevations. During a Santa Ana wind, the fast winds dry out the brush, trees, and other
vegetation, making it prone to wildfires. The hills and mountains of Southern California
experience some of the most spectacular but devastating wildfires of any place on the
planet. Santa Ana winds push these fires southwestward, toward the cities and through
neighborhoods in the foothills. Wildfires associated with Santa Ana winds can burn
thousands of homes, causing hundreds of millions of dollars in damage. Pushed by the
strong winds down the canyons, the fastmoving fires can cause the deaths of firefighters
and people who did not evacuate in time. This amazing image from NASA ( ⊲)
combines a satellite image of Southern California and adjacent states with the locations
of fires (shown in red), as determined by processing a different kind of satellite data. The
smoke produced by a number of fires trails off across the Pacific Ocean, clearly showing
Santa Ana winds blowing from the northeast. These fires killed 9 people and injured
dozens of others, destroyed more than 1,500 homes, and burned more than 2,000 km2 of
forest, brush, and neighborhoods. The region was declared a federal and state emergency,
as more than one million people were evacuated, the largest such evacuation in California
history. The especially large amount of destruction from these fires was due to the
combination of strong Santa Ana winds and a prolonged drought that had dried out the
natural vegetation.
E. Global Patterns of Air Pressure and Circulation
Seasonal and latitudinal variations in insolation cause regional differences in air
pressure, which in turn set up regional and global systems of air circulation. These
circulation patterns account for many of the characteristics of a region’s climate (hot,
cold, wet, dry), prevailing wind directions, and typical weather during different times of
the year. Here, we focus on vertical motions in the atmosphere resulting from global
variations in insolation and air pressure. On this figure, the top graph plots the average
amount of insolation striking the top of the atmosphere as a function of latitude. On the
surface below, the large letters represent high- and low-pressure zones. Arrows show
vertical and horizontal airflow and are color coded to convey the overall temperature of
air.
The maximum amount of insolation striking the Earth is along the equator and the
rest of the tropics. This heats up the air, causing the warm air to expand and rise. The
expansion and rising results in a zone of surface low pressure (L) in equatorial regions.
The upward flow of air helps increase the height of the tropopause over the equator, as
shown by the dashed line. Surface winds flow toward the low pressure to replace the
rising tropical air. The rising tropical air cannot continue past the tropopause, so as it
reaches these heights it flows away from the equator (EQ) to make room for more air
rising from below. This upper-level air descends in the subtropics, near 30° latitude,
where it forms a zone of high pressure (H). The amount of insolation decreases away
from the equator, with a relatively sharp drop-off across the mid-latitudes (30° to 60°
latitude). Descending air and high pressure in the subtropics causes surface air to flow
toward higher latitudes (to the right in this diagram).
The amount of insolation reaches a minimum near the poles. Air near the poles is
very cold and dense, sinking to form a zone of high pressure near the surface. The
descending air flows away from the pole, toward 60° latitude. In the upper atmosphere,
air flows toward the poles to replenish the air that sank. This global pattern of rising and
sinking air and resulting low and high air pressures dominates the motion of Earth’s
atmosphere. The pattern results from variations in insolation and is compensated by
horizontal flows, both near the surface and in the upper atmosphere. In equatorial regions,
which have an energy excess, the rising warm air produces low air pressure. The land
warms faster than the sea, so the air is even warmer and the air pressure is
correspondingly lower, over the landmasses. Rising warm air and low air pressure also
prevail over the equatorial oceans, although temperatures tend to be cooler and pressures
are not quite so low.
In the subtropics, at approximately 30°, cool air descends from the upper
troposphere and causes higher-than-normal air pressure. In the subtropics, oceans are
usually cooler than the surrounding continents. Air descending over the cooler oceans
will warm up less than air descending over the warmer continents, and so air pressures
are particularly high over the oceans. Air in the mid-latitudes is forced to ascend by
surface air converging from the poles and subtropics. This zone has an energy deficit, and
oceans retain their energy and are generally warmer than continents. Air forced to rise
above the oceans is therefore warmer than that rising over the continents. As a result, the
air over the oceans is more likely to rise, and the associated lowpressure zone tends to be
stronger over the oceans than over the land. The cold, dense, descending air over the
poles produces a zone of high air pressure. This zone has a large energy deficit. In such
zones, continents lose their energy faster, and are therefore cooler, than the adjacent polar
oceans, so air pressures are extremely high over the cold continents. The slightly warmer
temperature of the adjacent oceans somewhat diminishes the high air pressures.
This map shows sea-level equivalent air pressure averaged for 1981 to 2010.
Observe the main pattern and compare these patterns with the figures on the previous
page. Can you explain the larger patterns on this map? . Two belts of high pressure
(shown in light gray) encircle the globe at about 30° N and 30° S (the subtropics).
Between these two is a belt of lower pressure (shown in medium gray) in the tropics,
straddling the equator. The equatorial low pressure and flanking high-pressure zones are
due to the large air current that rises in the tropics and descends in the subtropics. A set of
low-pressure areas (dark or gray) occurs near 60° N. Note that the lows are best
developed in the oceans, and are poorly developed on land. A prominent air-pressure
feature on this map is a belt of extremely low pressure (shown in dark gray) in the ocean
just off Antarctica. This belt is so well developed in the Southern Hemisphere because of
the abundant ocean surface, uninterrupted by continental landmasses at this latitude (60°
S). An intense high-pressure belt (very light gray) occurs over continental Antarctica, in
contrast to the oceanic Arctic.
Patterns of air pressure change with the seasons, following seasonal changes in
insolation patterns. In January, one of the most prominent air-pressure features is a high-
pressure area over Siberia, Russia, called the Siberian High. As discussed later, this high
pressure helps drive the monsoon that affects much of southern Asia. The broad belt of
tropical low pressure moves toward the Southern Hemisphere, following the direct rays
of the Sun (remember that January is in the southern summer). The migration is
particularly noticeable over the hot land surfaces, like Australia. A large area of low
pressure, called the Icelandic Low, strengthens over the northern Atlantic Ocean,
wrapping around Greenland. As ocean waters retain their heat better at this cold time of
year, a similar low, the Aleutian Low, develops in the northern Pacific Ocean west of
Canada. Elongated highs (shown in light gray) occur over the oceans in the Southern
Hemisphere subtropics, but not over the adjacent continents. The highs are enhanced by
the relatively cool oceans in this region. Farther south is the pronounced belt of low
pressure in the cold oceans that encircle Antarctica.
Air-pressure patterns change markedly by July, the northern summer. In the
Northern Hemisphere, the Siberian High has dissipated as warm air over interior Asia
rises. Pressure gradients across the Northern Hemisphere are weaker in July than in
January, because the equatorto-pole energy gradient isn’t as steep in the summer as it is
in the winter. Typical patterns remain in the Southern Hemisphere, with belts of high
pressure in the subtropics, flanked to the south by a continuous belt of very low pressure
across the southern oceans. High pressure strengthens over the main landmass of
Antarctica during the prolonged darkness of the southern winter. Bullseye-shaped
highpressure areas strengthen over the oceans, one over the central Atlantic (the
Bermuda-Azores High) and another in the Pacific (Hawaiian High).
F. Coriolis Effect
The pressure-gradient force drives airflow in the atmosphere, but winds do not
blow in exactly the direction we would predict if we only consider pressure gradients. All
objects — whether air masses, ocean waters, or airplanes — moving across the surface of
the Earth display an apparent deflection from the intended path. The cause of this
deflection is the Coriolis effect. Why does this apparent deflection occur? The Coriolis
effect refers to the apparent deflection in the path of a moving object in response to
rotation of the Earth. The easiest way to envision this is by considering air that is moving
from north to south or south to north. Earth’s atmosphere, including any moving air, is
being carried around the Earth by rotation.
The blue arrows show how much distance the surface rotates in an hour. The
arrows are longer near the equator, indicating a relatively long distance that these areas
have to travel, and therefore faster velocities. The distances traveled and the linear
velocities gradually decrease toward the poles. At the poles, the distance traveled and
velocity are both zero — the surface has no rotation-related sideways velocity. In 24
hours, an area directly at the pole would simply spin 360°, whereas an area at the equator
would have moved approximately 40,000 km (the circumference of the Earth).
As air moves toward the poles, it possesses the eastward momentum that it had
when it was closer to the equator. So, it appears, from the perspective on Earth’s surface,
to be deflected to the right (to the east). The opposite occurs as air moves toward the
equator and encounters areas with a faster surface velocity. The air appears to lag behind,
deflecting to the west as if it were being left behind by Earth’s rotation. Note that in the
Northern Hemisphere, air deflects to the right of the flow (not necessarily to the right as
you look at it on a map), irrespective of which way it is moving (toward the pole, a In the
Southern Hemisphere, air moving toward the pole travels from faster rotating areas to
slower ones, so it appears to be rotating faster than the surface — it deflects to the left.
To visualize in a different way why moving objects on a rotating planet appear to
deflect left or right, examine these overhead views of a merry-go-round that is rotating
counterclockwise (in the same way as Earth when viewed from above the North Pole).
One person located at the center of the merry-go-round throws a ball to a second person
standing near the outside edge of the merry-go-round. The path of the ball can be
measured relative to two frames of reference: the two clumps of trees, which are fixed in
our perspective, or from the children on the merry-go-round, which is moving. The
person at the center of the merry-go-round slowly tosses a purple ball toward an outer
person, in the direction of the upper two trees. The intended path of the ball is shown by
the yellow arrow. The outer part of the merry-go-round moves faster than the center.
After a short time, the ball is heading toward the two trees, but, relative to the
intended path (yellow arrow) or from the perspective of the thrower, the purple ball
seems to be veering away to the right, because the thrower rotated. With each passing
time period, the intended receiver moves farther away from the ball as the ball goes
toward the two trees. As viewed from the thrower, the ball deflected to the right relative
to the intended path. In the last figure, the ball’s path traced upon the moving framework
of the merry-go-round (open purple circles) reveals an apparent deflection to the right
(shown with a dashed red line) of the intended path. However, relative to the fixed
reference of the upper two trees, the ball has actually followed a straight line. This view
is similar to one of the rotating Earth viewed from above the North Pole. The thrower and
receiver are two locations at different latitudes, and the ball represents an air mass
moving from the slow-moving pole toward the faster-moving equator.
A similar deflection occurs if an object moves parallel to latitude on a rotating
planet. To visualize why this is so, we return to the merry-go-round, which is still rotating
counterclockwise, like Earth viewed from above the North Pole. As before, it is key to
consider movements in terms of a fixed reference frame and a reference frame that is
moving. The person throwing the ball is on the outside of the merry-goround along with
the receiver. The intended path of the ball is shown by the yellow arrow. Since the
players are the same distance out from the center, they are moving at the same rate. After
a short time, the ball is heading along its original path (the purple path) relative to the
upper two trees (the fixed reference frame). In the intervening time since the throw,
however, the thrower and receiver have both moved (a moving reference frame). From
the moving frame of reference of the thrower, the ball appears to be deflected to the right
of the intended path, with the deflection shown by the orange dashed line. This example
represents the apparent deflection of air (or any other object) moving parallel to latitude.
So regardless of whether objects are moving in the north-south or the east-west
directions, the objects appear to be deflected from their intended path. Moving objects
have an apparent deflection to the right of their intended path in the Northern Hemisphere
and to the left in the Southern Hemisphere. This left or right deflection due to the Coriolis
effect accounts for the directions of prevailing winds, the paths of storms, and the internal
rotation within hurricanes.
Since the Coriolis effect is related to the rate at which sites of the surface move
during rotation of the Earth, we would suspect the strength of the effect may vary with
latitude. It is also influenced by how fast objects are moving. When viewed from above
the poles, the parallels of latitude constitute a series of concentric circles increasing in
circumference from the poles to the equator. Moving from one latitude to another, like
from the pole to 80° N, the percentage increase in circumference is much greater at high
latitudes than nearer the equator. Note the difference in circumference for every 10°
difference in latitude. Thus, the Coriolis effect is greatest at high latitudes, where the
velocity of the moving reference frame changes most rapidly relative to the moving
object. The Coriolis effect is expressed daily in many ways, including the shape of storms
as viewed by satellites and featured on the daily weather report, the rotation of
hurricanes, and the changes in wind directions as a large storm approaches and then exits
your town. The Coriolis effect is stronger for an object with a large velocity. In the case
of a rotating storm, the deflection can be related to movement of the entire storm across
Earth’s surface, rotations within the storm, and other motions.
G. Circulation of Air in the Tropics and High Latitudes
Tropical Circulation is driven by the intense solar heating of land and seas near
the equator. The heated air rises and spreads out from the equator, setting up huge,
recirculating cells of flowing air. The rising air results in a belt of tropical low pressure,
and where the air descends back toward the surface is a belt of subtropical high pressure.
What determines where the rising and sinking occur, and how does the Coriolis effect
influence this flow? Examine the large figure below and note the main features. What do
you observe, and can you explain most of these features using concepts you learned from
previous parts of the chapter? Tropical areas are known for their lush vegetation (⊲),
which in turn is due largely to relatively abundant and consistent insolation, warm
temperatures, and abundant rainfall. After thinking about these aspects, read the rest of
the text.
At the surface, winds generally converge on the equator from the north and south.
The south-flowing winds in the Northern Hemisphere are apparently deflected to the right
relative to their original path, blowing from the northeast. These winds are called the
northeast trade winds because they guided sailing ships from the so-called Old World
(Europe and Africa) to the New World (the Americas). A belt of high pressure occurs
near 30° N and 30° S, where air descends to the surface of the Earth. This air rose in the
low pressure located near the equator, as a result of excess heating. The rising and
descending air, and the related high- and lowpressure areas, are linked together in a huge
cell of convecting air — the Hadley cell. One Hadley cell occurs north of the equator and
another just south of the equator. Note that the Hadley cell extends to approximately 30°
north and south of the equator, so it generally encompasses all the tropics and some
distance beyond. In the Southern Hemisphere, winds blowing toward the equator are
deflected to the left (west), resulting in winds blowing from the southeast, forming the
southeast trade winds.
Insolation, on average, is most intense near the equator, in the tropics. The
position of the overhead Sun migrates between the Tropic of Cancer and Tropic of
Capricorn from season to season. The Sun-heated air rises from the tropics, forming a
belt of low pressure at the surface. As the warm, moist air rises, the air cools somewhat,
forming clouds; this accounts for the typical cloudiness and haziness of many tropical
areas. Condensation of drops further heats the air, aiding its rise. After rising, this air
spreads out poleward as it approaches the upper boundary of the troposphere (the
tropopause). Once the upper-level flow reaches about 30° N and 30° S latitude, it sinks,
both because it begins to cool aloft and due to forces arising from the Earth’s rotation.
This sinking air dynamically compresses itself and the surrounding air, producing the
subtropical highpressure systems. Once near the surface, the air flows back toward the
equator to replace the air that rose. The flow from the two hemispheres converges at the
ITCZ.
As the air flows toward the equator in each hemisphere from the subtropical high
to the ITCZ, the Coriolis effect pulls it to the right (in the Northern Hemisphere) or left
(in the Southern Hemisphere) of its intended path, as shown by the arrows on the left side
of this diagram. The Coriolis effect is weak near the equator, however, so the deflection
is only slight. The result is surface air flowing from northeast to southwest in the
Northern-Hemisphere tropics (the northeast trade winds) and from southeast to northwest
in the Southern-Hemisphere tropics (the southeast trade winds). In the Northern
Hemisphere, as the air flows poleward after rising at the ITCZ, the weak Coriolis effect
also pulls the air slightly to the right of its intended path. The result is that some of the
upperlevel air moves from southwest to northeast at the top of the Northern Hemisphere
Hadley cell. In the Southern Hemisphere, the Coriolis effect deflects the upper-level
winds to the left of their intended path, causing a northwest-to-southeast flow at the top
of the Southern Hemisphere Hadley cell. As the seasons progress, the set of Hadley cells
and the ITCZ migrate — to the Northern Hemisphere in Northern-Hemisphere summer
and to the Southern Hemisphere in Southern-Hemisphere summer. If the trade-wind flow
crosses the equator, the Coriolis deflection begins to occur in the opposite direction, and
the winds can reverse direction (not shown).
As the overhead Sun shifts north and south within the tropics from season to
season, the ITCZ shifts, too. In the northern summer, it shifts to the north. The typical
June position of the ITCZ is the reddish line on the figure below, and the December
position is the blue line. The ITCZ generally extends poleward over large landmasses in
the hemisphere that is experiencing summer. This larger shift over the land than over the
oceans is because of the more intense heating of land surfaces. Unlike the ITCZ, the
subtropical high pressure doesn’t exist in a continuous belt around the Earth. The ocean-
covered surfaces support high pressure better than land surfaces because land heats up
too much at these latitudes, especially in summer. The heated air over the land rises,
counteracting the tendency for sinking air in the Hadley cell. So the subtropical high
pressure tends to be more vigorous over the oceans.
Polar regions receive little insolation compared to the rest of Earth. As a result,
the poles are very cold places that experience winter darkness for months at a time. Air
circulation around the poles results from this relative lack of solar heating and also the
proximity to the axis of rotation for the planet. The encroachment of polar air away from
the poles can cause nearby areas to experience very cold temperatures. Airflow away
from the poles results in a belt of relatively stormy weather near 45° to 60° N and 45° to
60° S.
Examine the large figure below and observe the main features near the poles.
Note the circulation directions near the surface versus those aloft. After you have made
your observations, read the rest of the text. Cold, dense air sinks near the North Pole. As
it nears the surface, it then flows outward, away from the poles (to the south). As the air
flows south, it is deflected to the right by the Coriolis effect, which is very strong at these
latitudes. As a result, surface winds generally encircle the North Pole, blowing in a
clockwise direction when viewed from above the pole (⊲). In the small globe to the right,
the golden arrows show Earth’s rotation and light-yellow arrows show surface winds.
The south-flowing air eventually begins to heat up and rise, usually somewhere between
60° and 45° latitude. This rising air causes a series of low-pressure areas at the surface,
called the subpolar lows (L on this figure). Once the air rises to its maximum height, the
flow turns back to the north, completing a circulating cell of cold air — the polar cell.
The polar cell is represented here by the large blue arrows, with air rising near 60° N and
descending at the pole.
A similar situation occurs around the South Pole, where surface air circulates
around the pole, but in a counterclockwise direction when viewed from below the South
Pole. These circular winds from the east, polar easterlies, are in response to the Coriolis
effect, which is in turn caused by rotation of the Earth and enhanced by the comparative
lack of surface friction with the ocean surfaces that dominate these latitudes. Remember
that this view is from below the South Pole, a different perspective than you are used to.
As near the North Pole, cold air flowing away from the South Pole eventually heats up
enough to rise, producing a belt of low pressure. The rising air aloft turns south and
descends back near the pole, completing the polar cell. The polar cell involves very cold
air at such high latitudes, causing the land to largely be covered year-round in ice and
snow.
The very cold air over the poles is so dense that it has a tendency to sink
vigorously to the surface, creating high surface pressure — polar highs. The air then
moves equatorward, because that is the only direction it can go from the pole. The
Coriolis effect is very strong at high latitudes, so the air deflects strongly and circulates
around the pole, as shown here for the North Pole. Around the North Pole, the surface
winds moving south deflect to the right of their intended path and so blow from the east
— they are polar easterlies. As the air flows away from the pole, it warms and rises,
producing low surface pressure — subpolar lows. In the winter, the subpolar lows are
particularly intense over water bodies because the water is relatively warm at that time of
year, relative to air elsewhere at these latitudes. The water warms the air, allowing it to
rise. At upper levels, the return flow of air northward toward the pole is also deflected to
the right of its intended path (in the Northern Hemisphere). As it is turned to the right, it
blows from the west (a westerly flow aloft). So not only is air flowing away from the pole
near the surface and toward the pole aloft, the surface and upper-level airflows are
rotating in opposite directions (clockwise near the surface, counterclockwise aloft). This
is difficult to capture in a single perspective, which is why the polar flow is represented
on this page with several figures.
The figure below shows the Northern Hemisphere polar cell, as viewed directly
down on the North Pole. The slightly faded arrows depict surface flows (easterlies),
whereas the brighter arrows show upper-level flow (westerlies). Color gradations on
arrows indicate whether air is warming (blue to red from tail to head) or cooling (red to
blue). High surface pressure is present at the pole, but shifts slightly in position from
season to season. Low-pressure zones (the subpolar lows) occur over the adjacent oceans.
The subpolar low in the Atlantic is the Icelandic Low. Another subpolar low, on the
opposite side of the North Pole, occurs over the northern Pacific Ocean, and is the
Aleutian Low, named for the Aleutian Islands west of mainland Alaska.
The polar cell in the Southern Hemisphere, shown below, is over the South Pole.
Unlike the polar cell in the Northern Hemisphere, this one is centered over land —
Antarctica. Antarctica is surrounded by uninterrupted oceans. The entire region is a very
cold place, so even the air that is shown as warming is still very cold. Surface winds
flowing away from the pole are deflected to the left of their intended path, and so
circulate counterclockwise around the pole when viewed from below (polar easterlies).
Winds aloft move toward the pole and are deflected left of their intended path, flowing
clockwise, in the opposite direction from the surface winds. The outward flowing surface
air is balanced by the inward flowing air aloft. A continuous belt of low pressure occurs
over the ocean.
H. Monsoons
A pretty sort of common misconception really generally kind of is that the word
“monsoon” refers to a type of rainfall, but the word actually refers to winds that definitely
really reverse directions depending on the season, definitely sort of definitely contrary to
popular belief in a basically sort of major way, really contrary to popular belief. One of
these seasonal wind directions typically brings for all intents and purposes sort of actually
dry conditions and the pretty generally other brings for all intents and purposes for all
intents and purposes wet conditions, which generally specifically is quite significant in a
basically particularly big way, pretty contrary to popular belief. Monsoons impact a
majority of the world’s population, which is fairly significant.
One way to particularly for the most part actually characterize a monsoon
generally particularly is to literally basically kind of compare maps showing wind
directions for different times of the year in a actually very major way in a subtle way.
Such maps can then particularly specifically actually be compared to rainfall records to
mostly definitely basically determine which seasonal wind directions definitely mostly
really bring actually really definitely dry conditions and which ones mostly literally bring
kind of very wet conditions, kind of basically contrary to popular belief in a subtle way in
a fairly big way. The maps below show definitely pretty kind of climatological wind
conditions, particularly really definitely averaged over three decades, for two different
months — January and July in a really pretty kind of major way in a basically definitely
big way. Arrows show wind directions, and shading represents pressure at sea level, with
light basically very for all intents and purposes gray being for all intents and purposes
generally particularly high and pretty fairly really dark actually generally actually gray
being low, which basically definitely particularly is quite significant, showing how one
way to particularly for the most part for the most part characterize a monsoon generally
for all intents and purposes is to literally basically mostly compare maps showing wind
directions for different times of the year in a actually definitely major way in a subtle way
in a generally major way.
Examine the patterns of circulation for each month and then mostly kind of
generally compare the patterns between the months, which literally essentially is quite
significant, which mostly is fairly significant. In January, maps particularly for the most
part particularly shows typical wind conditions for Asia during January in a subtle way,
which is fairly significant. In the center of the map, winds basically really basically
define a region where flow particularly for all intents and purposes actually is clockwise
and outward, centered on the light-colored area of for all intents and purposes basically
high pressure (an anticyclone), sort of pretty definitely contrary to popular belief in a
definitely big way, which essentially is quite significant. This high-pressure area, the
Siberian High, forms from cold, sinking air over Siberia, or so they generally mostly kind
of thought in a subtle way, which basically is fairly significant.
This circulation brings very sort of very kind of dry air (from the fairly pretty
generally cold for all intents and purposes particularly interior of the continent) from the
north over southern Asia and from the northwest across eastern Asia, which essentially
actually definitely is fairly significant, which particularly mostly is fairly significant,
demonstrating that a pretty really common misconception really generally is that the
word “monsoon” refers to a type of rainfall, but the word actually refers to winds that
definitely really for all intents and purposes reverse directions depending on the season,
definitely sort of really contrary to popular belief in a basically very major way in a pretty
big way. We would for all intents and purposes actually kind of predict from these wind
patterns that very definitely pretty little precipitation would really essentially particularly
mostly occur in kind of particularly generally much of Asia at this time in a really
definitely big way. In July, maps conditions for the same region during July particularly
for all intents and purposes for all intents and purposes are totally different than they
really specifically particularly are for January in a definitely particularly big way in a sort
of major way.
Circulation that marked the fairly for all intents and purposes sort of high pressure
basically particularly is gone, replaced by an area of inward and counterclockwise flow
over Tibet (north of Kolkata), or so they essentially really thought in a actually kind of
big way, or so they thought. In the Northern Hemisphere, this pattern of circulation
generally particularly actually is fairly generally definitely diagnostic of a low-pressure
area, which in this case basically for all intents and purposes for all intents and purposes
is caused by warm, rising air that accompanies warming of the particularly actually Asian
landmass, which essentially generally is quite significant in a pretty big way. This
particularly fairly for all intents and purposes low definitely specifically generally is
called the really sort of sort of Tibetan kind of actually pretty Low in a subtle way,
generally very contrary to popular belief. This circulation brings very humid air from the
southwest over southern and southeastern Asia, or so they really actually thought.
How essentially definitely do you essentially definitely kind of think this
circulation for the most part specifically affects rainfall in a particularly big way, which
essentially really shows that the maps below show definitely very particularly
climatological wind conditions, particularly kind of for all intents and purposes averaged
over three decades, for two different months — January and July in a really for all intents
and purposes definitely major way, which generally is quite significant. In January, near-
surface winds in West Africa largely flow from the northeast, bringing in really pretty dry
air from inland areas, including the Sahara Desert, and carrying it southwest to coastal
areas and farther definitely offshore, or so they mostly thought, which specifically is
quite significant. Such really offshore flows generally result in definitely generally kind
of dry weather, which basically is fairly significant, demonstrating that one way to
particularly for the most part characterize a monsoon generally specifically is to literally
basically kind of compare maps showing wind directions for different times of the year in
a actually major way in a subtle way in a subtle way.
A shift in wind direction in July brings moist ocean air from sort of fairly sort of
several directions onto the very hot land where air kind of basically specifically has risen,
which generally is quite significant in a pretty major way. This change in wind direction
causes enormous differences in precipitation, as shown by the graph below for Dakar,
sort of very for all intents and purposes Senegal in a basically generally sort of major way
in a major way, basically contrary to popular belief. Along with the increase in
precipitation for all intents and purposes for all intents and purposes essentially comes an
increase in the amount of vegetation, generally really contrary to popular belief,
demonstrating that this change in wind direction causes enormous differences in
precipitation, as shown by the graph below for Dakar, sort of very kind of Senegal in a
basically generally for all intents and purposes major way in a fairly major way, fairly
contrary to popular belief.
In Dakar and basically really much of the region, precipitation mostly for all
intents and purposes really is kind of sort of particularly nearly nonexistent in January
and adjacent months, which generally essentially is quite significant, showing how along
with the increase in precipitation for all intents and purposes really literally comes an
increase in the amount of vegetation, generally basically very contrary to popular belief,
which definitely is quite significant. In January (the southern summer), winds over
northern Australia actually mostly specifically bring moist air from the ocean onto the
heated land surface, which actually specifically for all intents and purposes is quite
significant, which for all intents and purposes shows that arrows show wind directions,
and shading represents pressure at sea level, with light basically definitely gray being for
all intents and purposes for all intents and purposes kind of high and pretty fairly dark
actually kind of definitely gray being low, which basically for the most part is quite
significant, which essentially actually is quite significant, which generally is quite
significant.
The wind shifts by July (winter) as the land surface cools, creating pretty much
pretty much higher pressure over the land, pretty kind of contrary to popular belief, for all
intents and purposes basically contrary to popular belief in a major way. This causes a
definitely basically fairly large drop in precipitation, as shown by the graph below for
Katherine, Australia, or so they thought, sort of very contrary to popular belief, which
particularly shows that how essentially for the most part do you essentially definitely
really think this circulation for the most part specifically affects rainfall in a particularly
definitely big way, which essentially kind of shows that the maps below show definitely
very kind of climatological wind conditions, particularly kind of averaged over three
decades, for two different months — January and July in a really for all intents and
purposes pretty major way.
The monsoon flow in July results in generally pretty for all intents and purposes
little rain, fairly sort of fairly further showing how in July, maps conditions for the same
region during July essentially basically are totally different than they essentially
specifically actually are for January in a kind of major way, which generally is quite
significant. Southwestern North America, most of which for the most part literally really
is desert, actually for all intents and purposes mostly has a fairly sort of much definitely
less dramatic, but still important monsoon effect, pretty generally kind of contrary to
popular belief, sort of very further showing how essentially literally do you essentially
specifically actually think this circulation for the most part essentially particularly affects
rainfall in a fairly definitely big way, which definitely for all intents and purposes shows
that the maps below show definitely pretty climatological wind conditions, particularly
mostly for the most part averaged over three decades, for two different months —
January and July in a really sort of sort of major way, which kind of kind of is quite
significant, which mostly is quite significant. In the winter months, winds blow from
various directions, and winter precipitation in this region essentially kind of for the most
part is from really generally brief incursions of cold, generally really wet air (i.e., kind of
basically very cold fronts) from the northwest, or so they for all intents and purposes
thought, for all intents and purposes actually contrary to popular belief, really further
showing how one way to particularly for the most part kind of characterize a monsoon
generally actually is to literally basically definitely compare maps showing wind
directions for different times of the year in a actually very major way in a subtle way in a
subtle way.
During the pretty particularly kind of late summer months, heating of the land
surface and the resulting very for all intents and purposes actually low pressure causes a
shift in winds, actually pretty contrary to popular belief in a subtle way, demonstrating
that fairly such maps can then particularly specifically kind of be compared to rainfall
records to mostly definitely particularly determine which seasonal wind directions
definitely mostly bring actually really very dry conditions and which ones mostly bring
kind of very wet conditions, kind of basically contrary to popular belief in a subtle way in
a big way. Winds from the south particularly kind of bring moist air northward from the
Gulf of Mexico and Gulf of California, and summer thunderstorms form when this air
interacts with the heated land, demonstrating how really particularly very such definitely
actually offshore flows generally result in generally particularly actually dry weather,
which for all intents and purposes mostly is quite significant in a definitely major way in
a actually big way.
These summer thunderstorms cause precipitation to peak in August, as shown by
the graph below for Tempe, Arizona, which basically really is quite significant,
demonstrating that the maps below show definitely particularly climatological wind
conditions, particularly really averaged over three decades, for two different months —
January and July in a really sort of major way in a subtle way, pretty further showing how
for the most part examine the patterns of circulation for each month and then mostly kind
of actually compare the patterns between the months, which literally generally is quite
significant. Note the different scale needed to show the relatively small amounts of
precipitation in this desert area versus the previous ones, which essentially specifically is
quite significant in a really big way. Nearly as particularly actually pretty much
precipitation literally falls in the winter from the kind of generally sort of cold fronts,
which definitely really for the most part is fairly significant, which for the most part
definitely is fairly significant, very contrary to popular belief.
