5 Air Masses, Fronts, and Weather Systems

Air Masses: Temperature and Moisture

An air mass is a large body of air with broadly similar temperature and moisture. It takes on characteristics from the region where it forms, then may be modified by the surfaces it crosses.

Moisture and temperature labels

• Air over land is generally drier and is described as continental (c).

• Air over ocean is generally more humid and is described as maritime (m).

• Tropical (T) air forms at low latitudes and is warm.

• Polar (P) or Arctic (A) air is cold.

The labels can be combined to describe an air mass. For example, continental polar (cP) air is cold and dry, while maritime tropical (mT) air is warm and humid.

Fronts and Their Weather

A front is the boundary where air masses meet. Cold air is denser, so it commonly stays near the surface while warmer air is lifted over it. Rising air cools; if it contains enough moisture, clouds and precipitation can form. Weather depends partly on which air mass is advancing and how quickly the air is lifted.

Types of fronts

• Cold front: Advancing cold air pushes beneath warm air, lifting it relatively steeply. Showers or thunderstorms may form in a narrow band near the front. Conditions often turn cooler and drier afterward.

• Warm front: Advancing warm air glides gradually over retreating cold air. The gentle lift commonly produces broad areas of layered clouds and precipitation ahead of the surface front.

• Stationary front: Neither air mass advances much, so clouds or precipitation may persist near the boundary.

• Occluded front: A faster-moving cold front catches up with a warm front, lifting the warm air away from the ground. Occlusion often occurs as a midlatitude storm matures.

Front symbols on weather maps

• Triangles mark cold fronts and point in the direction of movement.

• Semicircles mark warm fronts and point in the direction of movement.

• Alternating symbols on opposite sides indicate a stationary front.

• Symbols on the same side indicate an occluded front.

How Midlatitude Cyclones Develop

A midlatitude cyclone, also called an extratropical cyclone, is a large low-pressure storm system that commonly develops where warm and cold air meet in the middle latitudes.

A simplified development sequence

1. A boundary forms between air masses.

2. A disturbance aloft can help the boundary develop a wave.

3. A low-pressure center forms, and warm and cold fronts take shape.

4. As the system strengthens, rising air and fronts bring clouds and precipitation.

5. The cold front often moves faster than the warm front and may catch it, forming an occluded front.

6. As the warm air is lifted away and the storm loses its source of energy, the system weakens and dissipates.

This is a useful model, not a guarantee that every storm follows the same sequence.

Cyclone Movement and Local Weather

Winds circulate around a low-pressure center in opposite directions in the two hemispheres: counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Many midlatitude systems generally travel from west to east. Upper-level winds, including the jet stream, help steer them. The exact track and speed vary as surrounding winds and pressure patterns change.

Common front-related weather patterns

When a cold front approaches with a warm, humid air mass ahead of it, denser cold air can force the warm air upward. A brief line of showers or thunderstorms may accompany the front, followed by a wind shift and cooler air.

A warm front may bring gradually thickening clouds and more widespread, steady precipitation before warmer air arrives. These are common patterns, not certain outcomes: local geography and moisture also affect the weather.