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Air pressure and atmospheric motion

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Presentation on theme: "Air pressure and atmospheric motion"— Presentation transcript:

1 Air pressure and atmospheric motion
Explain the forces that generate winds Explain why there is well-developed westerly flow in the upper troposphere Differentiate between high and low pressure systems Compare and contrast surface and geostrophic winds Describe/read maps of constant pressure surfaces Identify and discuss wind types and the forces that generate them Recognize surface and upper-level atmospheric maps and identify general patterns of windflow Explain monsoonal flow and land-sea breezes

2 Air pressure and atmospheric motion
Q: What makes the wind blow? A: Air pressure differences.

3 Air pressure Force exerted by molecules in atm due to gravity and temperature July 11, 2006 NY Times As the World Wobbles Late last November, as a big low-pressure system built over Europe and Asia and high pressure settled in over the Pacific and Atlantic Oceans, the shifts in the atmosphere caused the earth to jiggle ever so slightly, like a hiker adjusting to a shifting load in a backpack. As a result, the North Pole and its southern counterpart moved about four inches by one measure. (There are several ways to define the poles.) Despite its diaphanous appearance, the atmosphere weighs about 5,000 trillion metric tons, and its mass is unevenly distributed. All those ridges and troughs on a weather map reflect differences of billions of tons of gases. Scientists have long known that as the atmosphere shifts, it influences the earth’s rotation. The recent advent of satellites’ global positioning systems made it possible to confirm even the tiniest movements. There were well-known, regularly occurring wobbles in the earth’s rotation that could shift the poles 30 feet over a year or more. These shifts blocked the detection of subtler, quicker movements caused by day-to-day changes in the atmosphere and the oceans. Now, these small shifts are being measured by institutions devoted to tracking the planet’s behavior, including the earth orientation department of the United States Naval Observatory and the “time, earth rotation and space geodesy section” of the Royal Observatory of Belgium. Experts at the Belgian observatory and the Paris Observatory found the November polar shift and a series of other little loops by looking particularly closely at a period from last November through February, when two of the larger regular wobbles in the axis canceled each other out. They reported their analysis in the July 1 issue of Geophysical Research Letters.

4 Air pressure


6 Surface air pressure variability
Average sea level pressure 1013 mb

7 Mercury barometer 1013 mb = inches Aneroid barometer


9 Pressure systems Two types: high and low
Low: associated with clouds and instability. High: associated with clear conditions and stability

10 Low pressure systems Cyclone Converging rising air at surface
Diverging air aloft Winds rotate counterclockwise in NH Areas of “light” atmosphere; air is forced into these locations Unstable surface conditions

11 High pressure system Anticyclone Converging air aloft
Diverging sinking air at surface Winds rotate clockwise in NH Areas of “heavy” atmosphere; air is forced out of these locations Stable surface conditions


13 Is the location for these pressure systems the northern or the southern hemisphere?

14 High and low pressure systems
Occur on a variety of spatial and temporal scales Some pressure systems may be stationary for a long period of time, others may migrate rapidly around the planet Some pressure systems are closed, others are more belt-like and open

15 Low pressure systems Types of low pressure systems: tornados, thunderstorms, hurricanes….

16 Low pressure systems ….., midlatitude cyclones, the ITCZ, thermal lows

17 Dust devil versus ITCZ

18 Another secret of weather forecasting
The atmosphere is a heterogeneous collection of pressure systems in three dimensions. Weather forecasting involves looking at surface conditions as well as upper level conditions (aloft, in the upper troposphere)

19 “Deep” tropospheric phenomena
A midlatitude cyclone is a low pressure at the surface coupled to a low pressure aloft in the upper troposphere A hurricane is a high pressure aloft and a low pressure at the surface

20 Midlatitude cyclones Strong, “deep” interaction between surface and upper levels May travel large distances around the globe Midlatitude cyclone

21 “Shallow” tropospheric phenomena
Thermal low (warm) Thermal high (cold) Weak interaction between surface and upper levels May occur on a daily basis or persist over many months L H

22 Pressure differences initiate…..

23 …….winds Named according to the direction they blow from
Winds can blow at different directions at different altitudes in the atm Forces that act on winds: PGF CF Surface friction



26 1. PGF: Pressure gradient force – winds blow from high to low
Where are winds the fastest?

27 2. Coriolis Force (CF) Apparent deflection of the winds due to rotation of the Earth NH winds deflected right SH winds deflected left

28 Coriolis Force (CF) CF is not a true force; it is an apparent force arising from the effect of the Earth’s rotation Deflection is strongest at poles and zero at the Equator CF acts perpendicular to the direction of motion CF deflects to the right in the Northern Hemisphere

29 Bart versus Australia

30 Misconceptions about the Coriolis Force
The CF does not determine the rotation in a drain.

31 3. Surface friction (SF) Topography (mnts, elevated plateaus) deflect winds

32 Two major types of winds
Geostrophic (upper troposphere winds) Influenced by PGF and CF only Wind flow is parallel to isobars/geopotential heights Geostrophic flow is westerly (west to east) in NH Shown on geopotential height maps

33 Geostrophic winds on geopotential height map

34 Two major types of winds
2. Surface winds Influenced by PGF, CF, and SF Winds cross isobars

35 Types of surface winds Monsoonal flow
Creates extreme wet and dry seasons

36 Location of thermal highs and lows associated with monsoonal flow Monsoon in India


38 Monsoon precipitation in the Sonoran desert, AZ

39 Winter Summer Weak monsoonal flow in southwestern US

40 Types of surface winds Land-sea breeze circulation
Many other different types of surface winds based on local physiography and arrangement of pressure cells. Many have unique names.

41 Sea breeze blowing from ocean to land

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