# Chapter 4. Atmospheric Pressure and Wind

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Chapter 4. Atmospheric Pressure and Wind
Part 1. Energy and Mass Chapter 4. Atmospheric Pressure and Wind

Introduction Pressure = Force per unit area
Gases exert equal pressure in all directions Average atmospheric pressure is controlled by the “weight” of overlying air--it decreases with height Average sea level air pressure is mb Air pressure changes depending on the air density and temperature Dalton’s Law: Where several different gases are mixed, the total gas pressure is equal to the sum of the partial pressures of the individual gases

Air pressure is less at a higher elevation (p2) than at a lower elevation (p1)
Gravity is always trying to pull the air downward toward the Earth’s surface

Air pressure decreases with elevation according to this curve
Meteorologists use air pressure as a measure of elevation in the atmosphere (i.e., the 500 mb level or the 200 mb level)

The Equation of State (Ideal Gas Law)
For a gas, the following measurable parameters are inter-related: Pressure Temperature Density Changes in air pressure occur with changes in air temperature or density (or both)

Molecular movement in a sealed container
Pressure increases by increasing density (b) or temperature (c)

Aneroid barometer (left)
and its workings (right) A barograph continually records air pressure through time

The distribution of air pressure is important for determining weather patterns
Air always tries to move from higher pressure to lower pressure The greater the pressure difference between high and low pressure, the greater the force trying to move the air Isobars = lines of equal air pressure Pressure gradient = change in pressure with distance Steep pressure gradients are represented by closely spaced isobars

Sea level air pressure depicted on a weather map
Low pressure gradient area (calm) High pressure gradient area (windy) Air pressure measurements made at high elevations must be corrected to give the air pressure at sea level

Pressure Gradient Force Initiates air motion
High to lower pressure Wind speed reflects gradient Horizontal Pressure Gradients Usually small across large spatial scales Vertical Pressure Gradients Usually greater than horizontal gradients Pressure always decreases with altitude

Hydrostatic Equilibrium = Force of gravity balances vertical air pressure gradient
Local imbalances in hydrostatic equilibrium cause updrafts and downdrafts

Heating causes a density decrease in a column of air
Both air columns are at the same temperature Heating causes a density decrease in a column of air All columns have the same total mass Warmer air has lower density and therefore greater column height The air in the right column is warmer than the air in the left column

500 mb height contours for May 3, 1995
Lines of equal elevation Upper air pressure maps depict the height to the specific air pressure level (such as the height to the 500 mb air pressure level) 500 mb elevation 5280 m 500 mb elevation 5880 m

Upper air heights decrease with latitude
Colder air in south, 500 mb level at lower elevation Warmer air in south, 500 mb level at higher elevation

Forces that Affect the Speed and Direction of Winds
1) The Pressure Gradient Force (pgf): Air tries to move from areas of high pressure to areas of low pressure; a larger pressure gradient gives a larger pgf and faster winds

2) The Coriolis Force Free-moving objects are affected by the Earth’s rotation; the coriolis force causes an apparent deflection to the right in the northern hemisphere and to the left in the southern hemisphere The coriolis force is greater at high latitudes than at low latitudes The faster the air is moving, the greater the coriolis force on the air

Coriolis Deflection

3) The Friction Force The friction force acts in the opposite direction from the direction of movement of the air; it acts to slow the air movement Air friction if greatest near the Earth’s surface Above an elevation of 1.5 km (1500 m or about 4500 ft), air friction is negligible

Winds in the Upper Atmosphere are affected by only the pressure gradient force and the Coriolis force When the pressure gradient force balances with the Coriolis force, the result is the geostrophic wind (parallel to the isobars)

Free Atmosphere (no friction) Pressure Gradient
This plot shows the direction of the pressure gradient force at the 500 mb level. The pgf is always perpendicular to the isobars.

Geostrophic Flow Development
(b) As air starts moving, it starts being affected by the Coriolis force (a) Air particle starts moving (d) When the pgf and the Coriolis force become equal and opposite, the geostrophic wind results (c) Faster air movement results in a larger Coriolis force

If the pgf and Coriolis forces never balance, Supergeostrophic and Subgeostrophic Flow results
Supergeostrophic and subgeostrophic flows follow curved air pressure contours Supergeostrophic flow occurs in ridges Subgeostrophic flow occurs in troughs These flows are called Gradient flows

High pressure “ridge” Supergeostrophic flow Low pressure “trough” Subgeostrophic flow

Cyclones, Anticyclones, Troughs, and Ridges
High pressure areas (anticyclones) Clockwise motion in northern hemisphere Descending air Clear skies Low pressure areas (cyclones) Counterclockwise motion in northern hemisphere Ascending air Clouds Upper atmosphere Ridges = surface anticyclones Troughs = surface cyclones

Due to friction, near surface air crosses isobars at an angle

Northern and Southern Hemisphere anticyclonic air patterns

Northern and Southern Hemisphere cyclonic air patterns

Ridges and troughs in the northern hemisphere

Maps depicting troughs, ridges, cyclones, and anticyclones

Measuring Wind Wind direction indicates direction from which wind blows Azimuth = degree of angle from 0 to 360o Wind vanes indicate wind direction Anemometers record wind speed Aerovanes indicate wind speed and direction

An aerovane An azimuth