Winds and Air MassesUnit 4 – Atmospheric Processes

Winds…Earth’s atmospheric circulation is an

important transfer mechanism for both energy and mass

The imbalance between equatorial energy surpluses and polar energy deficits is partly resolved

Wind – the horizontal motion of air across Earth’s surfaceDifferences in air pressure (density) between

one location and another produce windNames for the direction from which they

originate

Driving Forces within the Atmosphere

Gravitational forcePressure gradient forceCoriolis forceFrictional force

Earth’s Gravitational ForceEarth’s gravitational force on the atmosphere

is practically uniform, equally compressing the atmosphere near the ground worldwide

Density decreases as altitude increases

Pressure Gradient ForceHigh- and low-pressure areas exist in the

atmosphere because of uneven heating of the Earth’s surface

A pressure gradient is the difference in atmospheric pressure between areas of higher pressure and lower pressureA steep gradient causes faster air movement from a

high-pressure area to a low-pressure area.High and low pressure areas are caused by

differences in surface heating and cooling.Land versus water.Different albedos and energy absorbing abilities of

land, vegetation and human built surfaces

Isobars Isobars are lines of equal atmospheric pressure drawn on

a map.Widely spaced isobars display a gradual pressure gradient.

Gradual pressure gradients imply light winds.Closely spaced isobars display a steep pressure gradient.

Steep pressure gradients imply strong winds.

Coriolis Force

Deflects from a straight path any object that flies or flows across Earth’s surfaceWind, airplane, ocean currents

Deflection to the right in the Northern Hemisphere; to the left in Southern Hemisphere

The strength of the Coriolis force strengthens with an increase in latitude.

Friction ForceDrags on the wind as it moves across surfaces; up

to 500mVaries with surface texture, wind speed, time or day

and year, and atmospheric conditionsSince surface friction decreases wind speed, it

reduces the effect of the Coriolis force Anticyclone

Winds in the Northern Hemisphere spiral out from a high-pressure area clockwise

Cyclone Spiral into a low-pressure area counterclockwise

In the Southern Hemisphere these circulation patterns are reversed

When the “forces” combine…When the gravitational force, pressure gradient force,

Coriolis force and frictional force combine we see the development of ‘predictable’ global wind patterns.

Air MassesAir masses are bodies of air that develop

over large areas of the Earth’s surfaceTwo conditions needed:

1. Air must stay over the source region for a long period of time so that it can take on the characteristics of that region

2. Air must be stableUsually form where air is descending back to

Earth These high-pressure cells line up roughly over

the polar circles and the Tropics

Your taskLook at the defining characteristics of the 6 major air masses.

This will be done by completing the “Characteristics of Air Masses” chart

You will then look at cloud formation along fronts Complete the “cloud development in fronts” diagram and associated

questions on handout

Additional questions:

1. Explain a mid-latitude storm.2. Why are storms more common in the mid-latitudes than

elsewhere?3. Explain the differences between continental and maritime

air masses.4. Explain the role of convections currents and the rotation of

the Earth in the development of a storm.5. Explain how studying jet streams could help long-range

weather forecasting.All info is found in Chapter 16, pages 262-268