geography worksheet 2
Cooper2021
geographyweek8material2.pdf
The Nature of Atmospheric Pressure
Now let’s talk about the atmospheric pressure and winds. As you know, if there is a pressure gradient, the air starts to move from high pressure to low pressure – just like energy moves from high to low!
• Mapping pressure with isobars – Pressure measured with
a barometer
– Typical units are millibars or inches of mercury
– Contour pressure values reduced to sea level
– Shows highs and lows, ridges and troughs
The Nature of Atmospheric Pressure
To clarify, atmospheric pressure is the pressure of the atmosphere on top and around us! Therefore, if you fly in an airplane, at high altitude, your bag of chips/tooth paste/hand cream inflate, and may even explode! This happens because you are no longer experiencing the same pressure as you were on ground. Can you imagine what might happen if you transport a precious collection of wine by airplane?
In the following pressure distribution draw the path of an air parcel. Lines are isobars – contours of equal pressure.
H L
The Nature of Wind • Origination of wind
– Uneven heating of Earth’s surface creates temperature and pressure gradients
– Direction of wind results from a pressure gradient
– Winds blow from high pressure to low pressure
Of course, wind blows from high pressure to low pressure. The driving force for all air motion is variations in atmospheric pressure.
Then, what determines the speed of wind moving down a pressure gradient?
Pressure gradient can be steep, when large changes in pressure occur over small distances. The rate at which air
moves depends on the steepness of the gradient.
As we all know, the direction of the wind is determined by both the Coriolis effect and pressure gradient force. On the ground, however, there is an additional force that we need to consider – friction. Friction will slow the wind and reduce the Coriolis effect, so eventually, wind will either diverge from the center of a high pressure system or converge into the center of low pressure system.
This is perhaps a familiar phenomenon for all of us, since we have seen so many satellite images showing hurricane wind is converging towards the center of the super low pressure system (see following slides).
http://en.wikipedia.org/wiki/Hurricane_Gloria
Hurricane Gloria - 1985
http://en.wikipedia.org/wiki/Hurricane_Bob
Hurricane Bob - 1991
http://en.wikipedia.org/wiki/Image:Hurricane_Katrina_August_28_2005_NASA.jpg
Hurricane Katrina was the costliest and one of the deadliest hurricanes in the history of the United States .
It was the sixth-strongest Atlantic hurricane ever recorded and the
third-strongest landfalling U.S. hurricane ever recorded.
Category 5 hurricane ( SSHS )Hurricane Katrina near peak strength on August 28 ,2005 Highest winds 175 mph (280 km/h)(1-
minute sustained) Lowest pressure 902 mbar (hPa) Damages $81.2 billion (2005 USD) (costliest Atlantic hurricane in history)
Fatalities ≥1,836 total
And then there was Irene….
Hurricane Irene was a big rain maker
http://en.wikipedia.org/wiki/File:Hurricane_Irene_Aug_24_2011_1810Z.jpg
Hurricane Irene was a large and very destructive tropical cyclone, which affected much of the Caribbean and East Coast of the US in 2011. It became the fifth costliest US hurricane on record.
Hurricane Harvey
Hurricane Harvey as seen from the International Space Station on Friday. NASA European Pressphoto Agency
The Hurricane Harvey hit Texas on Friday, August 25, 2017 between Port Aransas and Port O'Connor. Although the Category 4 storm quickly became a Category 1 and downgraded to a tropical storm, Harvey dumped a large amount of rain. Huston, the 4th largest US city remained underwater for days and weeks in some places.
So, now we know why the wind converges into the center of a low pressure system.
What happens in the upper atmosphere where the wind does not experience friction?
Without friction, wind will continue to turn until the pressure gradient force and Coriolis effect reaches equilibrium. And it continues to circle around the pressure system. The motion is clockwise around a high pressure system and counterclockwise around a low pressure system.
This flow, in which pressure gradient force and Coriolis effect is balanced out, is called GEOSTROPHIC FLOW.
To review, wind direction around a high and low pressure system is different on the ground versus the upper atmosphere. Also, the wind direction is clockwise around a high pressure system and counterclockwise around a low pressure system. In addition, the direction of wind is opposite in the southern hemisphere versus the northern hemisphere since the Coriolis effect moves in an opposite direction in the southern versus northern hemisphere.
Trade Winds NE Trade Winds
Westerlies
Polar Easterlies
Trade Winds SE Trade Winds
Westerlies
Polar Easterlies
90�N (North Pole)
90�S (South Pole)
60�N
30�N
0�(Equator)
30�S
60�S
L
H
L
H
L
ITCZ
H
H
Ekman Transport ~90o to the
prevailing winds
to the right of the prevailing winds in the N. Hemisphere, to the left of the prevailing winds in the S. Hemisphere
This is a slide from an earlier lecture. You learned about prevailing winds and the wind driven ocean current, which are all affected by the pressure gradient force and Coriolis effect. This is why the ocean currents move in a giant circulation, called ocean gyre (see next slide).
Geostrophic currents flow around subtle �hills� and �valleys� on the ocean surface
This is also a slide from a previous lecture that shows the wind-driven ocean gyres. Ocean gyres are another example of geostrophic flow. Please note that in this figure that the colors are based on the topography of the ocean surface. The western side of each ocean is slightly higher than eastern side. This is because strong easterly winds (trade winds), constantly push the body of water toward the western side of the ocean. So, the prevailing winds provide the energy to drive the surface currents of the world’s oceans. Ekman transport and the Coriolis effect cause surface water to converge (“pile-up”) in the subtropics and diverge (move apart) at the equator and in the subpolar waters. This creates subtle “hills” and “valleys” on the ocean surface of <2m (<6.6ft.).
(continue)
http://www.seos-project.eu/modules/oceancurrents/oceancurrents-c06-s02-p01.html
(continued)
Gravity acts on the water to pull it back from these hills or into these valleys. This continuous tug-of-war between opposing forces results in a partial balance that eventually reaches an equilibrium and that keeps water moving around these subtle domes and valleys (= geostrophic flow/current). Therefore, the ocean gyre permanently goes around clockwise in the northern hemisphere, and counterclockwise in the southern hemisphere.
Ocean CirculationOcean Circulation
http://oceanmotion.org/html/background/western-boundary-currents.htm
Wind Driven Surface Currents: Western Boundary Currents
Surface currents located on the western side of the subtropical gyres are called western boundary currents, and are faster than their eastern counterparts. In fact, they are among the fastest surface currents in the ocean. One reason for the westward intensification of boundary currents has to do with the strengthening of the Coriolis effect with latitude. The Coriolis effect is stronger in the latitudes of the westerlies than in the latitudes of the trade winds. Transport of surface waters toward the western boundary of the ocean basins causes the ocean-surface slope to be steeper on the western side (versus eastern side) of a gyre (in either hemisphere). A steeper ocean-surface slope translates into a faster geostrophic flow on that side of the gyre.
Please view the animation video below. This video demonstrates how strong western boundary currents transfer energy (heat) from low the latitude to high latitude. http://media.pearsoncmg.com/bc/bc_0media_geo/geo_animations/gulf-stream- meanders/meanders.html
