Part 1. Energy and Mass

Chapter 4.

Atmospheric Pressure and Wind

IntroductionPressure = 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 1013.25 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 continuallyrecords 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

Air pressure measurements made at high elevations must be corrected to give the air pressure at sea level

High pressure gradient area (windy)

Low pressure gradient area (calm)

Pressure Gradient Force

Initiates air motion– High to lower pressure– Wind speed reflects gradient

Horizontal Pressure GradientsUsually small across large spatial scales

Vertical Pressure GradientsUsually 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

All columns have the same total mass

Warmer air has lower density and therefore greater column height

Both air columns are at the same temperature

The air in the right column is warmer than the air in the left column

500 mb height contours for May 3, 1995

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 5880 m

500 mb elevation 5280 m

Lines of equal elevation

Upper air heights decrease with latitude

Warmer air in south, 500 mb level at higher elevation

Colder air in south, 500 mb level at lower 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

(a) Air particle starts moving

(b) As air starts moving, it starts being affected by the Coriolis force

(c) Faster air movement results in a larger Coriolis force

(d) When the pgf and the Coriolis force become equal and opposite, the geostrophic wind results

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

Supergeostrophic flow

Subgeostrophic flow

High pressure “ridge”

Low pressure “trough”

Gradient Wind

Cyclones, Anticyclones, Troughs, and RidgesHigh 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 WindWind direction indicates direction from which wind blowsAzimuth = degree of angle from 0 to 360o Wind vanes indicate wind direction Anemometers record wind speedAerovanes indicate wind speed and direction

An aerovaneAn azimuth