Map assignment

PHYSICAL GEOGRAPHY; EXAM TWO

LECTURE 6: WIND AND ATMOSPHERIC MOVEMENT

As you will note from the first section, the movement of the atmosphere is important to the

Earth's energy equilibrium. Movement of the atmosphere is caused by the uneven distribution of

solar radiation, causing differential heating patterns. Wind is caused by temperature and pressure

differences. The processes which dominate wind are significant in understanding how ocean

currents, storms and droughts occur.

The second section of physical geography focuses upon weather patterns and the earth's ability to

redistribute energy on a global basis. Please keep in mind the pattern of solar radiation receipt

(temperature) as this pattern is key to understanding and predicting patterns of weather, climate

and vegetation patterns.

WIND:

1. CAUSED BY TEMPERATURE AND PRESSURE DIFFERENCES

2. THE GREATER THE DIFFERENCE IN TEMPERATURE AND PRESSURE (the steeper the gradient) THE GREATER WIND SPEED.

3. WIND IS NAMED FOR ITS SOURCE REGION (ie; north wind is from the north)

WIND OCCURS AT THREE SCALES:

1. LOCAL- land and sea breezes, upslope , downslope 2. REGIONAL- such as monsoons, santa ana winds 3. GLOBAL- such as the jet stream/geostrophic wind

The processes which create wind reflect the scale of wind which occurs. Also this reflects the

elevation at which each wind occurs (near the surface or high in the atmosphere). Local and

regional winds can be explained by examining pressure and temperature differences. I like to

begin explaining wind according to temperature differences which are easier to comprehend.

The processes which dominate local and regional winds are:

1) CONDUCTION: the heating of a surface (by the sun on Earth)

2) CONVECTION: the vertical movement of air caused by heat rising

3) ADVECTION: the horizontal movement of air, as known as wind. This occurs as warm air rises, cool air moves vertically to replace the air that rose.

In areas of differential heating (coastlines where water and land heat at different rates) we find a

cyclical pattern of air movement during the 24 hour period. Pages 85 and 137 illustrate the land-

sea breeze. During the day the land heats more rapidly than water (a lower specific heat),

causing

conduction on the land. We measure conduction through surface temperatures. This heating

causes the upward movement of air over the land (convection), causing a vacuum effect which

pulls in surrounding cool air, in this case from the ocean. As this cool air moves along the

surface, it is heated due to friction with the surface, and this perpetuates the upward movement of

the convection cell. As the as rises in the atmosphere, it cools by the environmental lapse rate

(recall it is 3.5 degrees Fahrenheit for every 1000 feet). In the Bay Area we consider this daily

wind from the ocean, or bay as our "air conditioning" due to the cool air that is drawn inland as

land areas heat up rapidly (such as San Jose). At night, the air moves in the opposite direction.

Because water retains heat longer than land (higher heat capacity) the air over water is warmer.

This causes conduction and convection over water, pulling in cooler air from the land, creating a

land breeze.

We experience land-sea breezes most intensely during the summer due to the variation in

temperature which occurs. During the summer land may heat up to 105 degree while the ocean

remains at 55 degrees. This wind is not as strong in the winter since the inland temperature

around most of the Bay Area rarely drops below 50 degrees, and the ocean remains near 55

degrees.

Thus, the greater the temperature difference, the greater the wind speed and land-sea breeze

effect.

Page 138 illustrates another example of local wind caused by differential heating; mountain and

valley breezes. The basic premise behind mountain and valley breezes is the idea that warm air

rises and cool air sinks. Due to the angle of mountain slopes, the mountain regions do not receive

as much direct SR as the valley. During the day, heat rises upslope causing a valley breeze, and

at night as high elevations cool more rapidly (due to the lack of atmosphere in high elevations)

than valleys, cold air sinks into the valley causing a mountain breeze.

Differential heating can also create winds on a REGIONAL and GLOBAL scale. Regional

winds occur on large land areas, such as Europe, Asia, and North America which sit in high

latitudes and, therefore, experience a greater annual range in temperature (see page 106).

Monsoon winds characterize Asian climates, due to the tremendous rain and drought that they

can bring (page 139).

On page 129, examine the top map labeled a) Northern Hemisphere winter conditions. This

map depicts pressure in January. The dotted line represents the ITCZ (intertropical convergence

zone). This should connect in your mind to the concept of CONVERGENCE above, or the

location of warm air rising. If you correlate this map to the January Temperature Map, p. 104,

you will see the thermal equator corresponds to the ITCZ. This line represents the location of

warm air rising which stimulates the horizontal movement of air. In Dec-Jan, you will see the

wind movement is coming off of the continent, producing cold, dry winds blowing across South

and Southeast Asia. In summer, June-July, the thermal equator and the ITCZ move north to the

tropic of cancer, stimulating upward movement of air over inland Asia. Cool, wet air is pulled

into the ITCZ causing tremendous rainfall (some areas in India have measured 50 inches in a

day!).

Thus, regional wind correspond to the land-sea breeze on a much larger scale.

Globally, we can see this same pattern which stimulates large scale wind movements. If you

examine page 106 or page 17 in the atlas, you will see land areas in the Northern Hemisphere

experience an extreme range in temperature. During the winter, the difference in temperature

between Siberia and the Pacific Ocean, as well as the temperature difference between Northern

Canada and the Atlantic Ocean, cause large-scale winds. Winds appear more clearly in the

atlas on the pressure map, pages 18-19, see if you can explain where the winds are located and

why they move in the direction illustrated on the map.

You will see that the jet stream, or geostrophic wind (p. 135), creates a west wind (from the

west) moving across the USA. As the land temperatures on land increase in the summer, the

strength of the jet stream dissipates. Below, I will add pressure to this discussion of wind

processes. For greater clarity, try the USA Today website at: www.usatoday.com or the Weather

channel website at: www.weather.com, these sites have excellent visuals to aid your

comprehension of these concepts.

LECTURE 5: pressure and atmospheric movement

In order to stimulate your thought concerning pressure and wind, please fill in the following

descriptions:

LOW PRESSURE:

temperature:

movement:

weather:

HIGH PRESSURE

temperature:

movement:

weather:

Hopefully, you listed that low pressure is warm air, rising or ascending, and is related to rainy

weather. High pressure is the exact opposite. Cold in temperature, it sinks or descends, and it is

associated with clear skies. These conditions are important to remember because they allow us to

predict movement of air between the two pressures and their differences stimulate storm tracts

and violent weather phenomena, such as tornadoes and hurricanes.

There are three processes which explain atmospheric movement according to pressure

differences. These are:

1) Pressure gradient force - this phrase describes the movement of high pressure into low pressure, moving perpendicular to the isobars (or lines of pressure). This movement is caused by

low pressure rising (where land/water is heated) causing a vacuum of air which pulls in cooler

surrounding air. The key here is to recognize that the warm, rising air causes this movement.

Please refer to the ITCZ (intertropical convergence zone) discussed last lecture. This movement

is greatest close to the earth's surface due to differential heating. When the text discusses a

"steep" pressure gradient, this means a wide difference in pressure, causing a fast wind (see

lecture 4). Conversely, a "gentle" gradient implies that there is little difference in pressure (much

like a gentle hill for you bicyclists), and there is little wind.

2) Coriolis Force - this term refers to the friction force caused by the earth's rotation. Since the earth is spherical, the atmosphere moves at different speeds as the earth rotates. The atmosphere

has little distance to travel to circumvent the poles, but a much longer distance around the

equator. This creates a deflection of air to the poles. If you were to drop a bomb in the northern

hemisphere, it would be deflected as it descends to the right. And, if you also dropped a bomb in

the southern hemisphere it would be deflected to the left. This force mostly affects high pressure,

since it also descends in the atmosphere. The Coriolis force causes the pressure patterns to spin

in a characteristic manner; high pressure spins clockwise in the Northern Hemisphere and low

pressure spins counterclockwise. This is important to visualize since our storms (low pressure

cells) in the northern hemisphere spin in a counterclockwise direction. This is the exact opposite

in the southern hemisphere, allowing us to predict the path of hurricanes by satellite image.

Where we see this the most is through the movement of ocean currents. Please see page 126,

and 133, which illustrate these patterns well. Then correlate the ocean current map, page 142, to

the map of pressure, page 133 and 143. Hopefully you will see the high pressures over the

oceans, and see the characteristic spin of the Coriolis force upon them.

3.) the Geostrophic Wind - when we discussed this last week we described this as the jet stream.

Another way to describe the cause of the jet stream is through pressure differences. The jet

stream is an upper air movement, also called the secondary circulation. It occurs high in the

atmosphere when Coriolis force is equal to the PGF (pressure gradient force). This causes the

pattern of air movement to flow parallel to the isobars, or between high and low pressure cells.

This wobbly pattern of air flow, over high pressures (ridge) and below low pressures (trough) is

known as Rossby waves. There are very good visualizations in the text on page 134, 135 and

130.

PRESSURE PATTERNS: the General circulation refers to the predictable pattern of wind

movement on a global basis due to predictable pressure cells. Visualize high and low pressure

movements, sinking and falling. Now go to page 130 - 131 in the text and you will see the

patterns I describe below. Take a few minutes and write down locations affected by these air

pressures.

EQUATORIAL LOW: centered upon the ITCZ, shifting as the ITCZ shifts

SUBTROPICAL HIGHS: between 20 -40 degrees north and south.

SUBPOLAR LOWS: between 40-60 degrees north and south

POLAR HIGHS: above 60 degrees north and south.

These patterns cause rising air along the equator and subpolar regions and sinking air in the

subtropical and polar regions. Now recall from the text that precipitation occurs with low

pressures and rising air. If you look in the atlas on page 16-17 at the inset map on the lower right

hand corner, you will see an illustration which shows this pattern. This is also depicted on page

131, in the text.

In the tropics, precipitation occurs due to low pressure (referred to as the doldrums) and at the

subpolar regions, precipitation also occurs where ever low pressure is found near the coastline.

For California, and any subpolar location, this changing pressure pattern determines our

seasonality of precipitation. During January, we have cold air over land, causing a high pressure-

yet, off the Pacific coast, the water is relatively warm, causing a low pressure off shore. This

situation sets up a cold front if the low pressure is from the Aleutian region, and a warm front if

it is from Hawaii or Baja (see page 129). During the summer, the air over land is hot (sometimes

104 degrees in San Jose) causing low pressure over California, and relatively cool air over the

Pacific, or a high pressure over water and this limits any precipitation for California during our

summer months (once this pattern of heating gets established!).

The last item that I want to think about this week is to pull these concepts forward to discussing

air masses. Air masses are similar to wind, in that we are talking about pockets of air moving

out of their source regions. Source regions (such as the Arctic) create temperature and humidity

characteristics for such masses. This will determine their interaction as you assess the

temperature of each air mass (which one sinks and which one rises).

Please characterize each type of air mass below. List its source region, temperature and humidity

level. These are identified on page 185. This will get you prepared for next week's lecture on air

mass interaction and the hydrologic cycle.

cP-

mP -

cT -

mT -