Air Masses, Global Winds,

and Fronts

Meteorology is the study of atmospheric phenomena. Meteorology includes weather and climate.

• Weather is the short-term variations in atmospheric phenomena that interact and interfere with the environment and life.

• Climate is the long-term variations of weather in a certain area.

Meteorology, Weather and Climate

An air mass is a large volume of air that has the same characteristics, such as humidity and temperature, as its source region.

A source region is the area over which an air mass forms.

Air Masses

Source Regions

Tropical

Polar

Arctic

Continental

Maritime

Global Wind Systems

The directions of Earth’s winds are

influenced by Earth’s rotation.

This Coriolis effect results in fluids and

objects moving in an apparent curved path

rather than a straight line.

Global Wind Systems

The directions of Earth’s wind systems, such as the polar easterlies and the trade winds, vary with the latitudes in which they occur.

Global Wind Systems

Polar easterlies

The polar easterlies are the wind zones between 60 N latitude and the north pole, and 60 S latitude and the south pole.

Prevailing westerlies

The prevailing westerlies are the wind systems on Earth located between latitudes 30 N and 60 N, and 30 S and 60 S.

Global Wind Systems Trade winds

Between latitudes 30 N and 30 S are two circulation belts of wind known as the trade winds.

Near latitudes 30 N and 30 S, the sinking air associated with the trade winds creates an area of high pressure. This results in a belt of weak surface winds called the horse latitudes.

Global Wind Systems Trade winds

Trade winds from the North and the South meet and join near the equator. The air is forced upward, which creates an area of low pressure.

This process, called convergence, can occur on a small or large scale. Near the equator, it occurs over a large area called the intertropical convergence zone (ITCZ).

Global Wind Systems

Global Wind Systems

Jet Streams

A jet stream is a narrow band of fast, high-altitude, westerly wind.

Weather in the middle latitudes is strongly influenced by fast-moving, high-altitude jet streams.

Jet Streams

Types of jet streams

The major jet streams, called the polar jet streams, separate the polar easterlies from the prevailing westerlies.

The minor jet streams are the subtropical jet streams. They occur where the trade winds meet the prevailing westerlies.

There are four types of fronts:

Cold Front

Warm Front

Stationary Front

Occluded Front

Fronts

Cold Fronts occur when cold, dense air

displaces warm, less dense air forcing it up along

a steep slope. This collision results in intense

precipitation and sometimes thunderstorms.

Cold Fronts

Warm fronts occur when advancing warm air

displaces cold air developing a gradual boundary

slope. Often causes widespread light

precipitation.

Warm Fronts

Stationary Fronts occur when two air masses

meet but neither advances resulting in light

winds and precipitation.

Stationary Fronts

Occluded fronts occur when a cold air mass

moves so rapidly that it overtakes a warm front

forcing the warm air upward. Strong winds and

heavy precipitation are common along an

occluded front.

Occluded Fronts

Cold Front

Occluded Front Stationary Front

Warm Front

Pressure Systems

In the northern hemisphere, winds move counterclockwise around a low-pressure center, and clockwise around a high-pressure center.

Low-pressure center High-pressure center