In this topic you will learn about how weather is affected by air masses and fronts.

An air mass is a large region of the atmosphere in which the air has similar properties. Air masses get their properties from the regions where they form. These air masses, pushed along by global winds, carry their properties with them as they move.

When air masses meet, they do not mix. The boundary between air masses of different temperatures is called a front. Fronts are the leading edge of an air mass. Fronts often cause rainy, unsettled weather.

Cold air that moves under a warm air mass is called a cold front. Cold fronts usually bring high winds and heavy storms like thunderstorms. After the storms, the weather is usually cooler and drier and the skies are clearer.

Warm air that moves over a cold air mass is called a warm front. Warm fronts bring light, steady precipitation with low winds. Warm fronts may also bring fog. After the passage of the front, the weather is usually warmer and more humid.

A front formed when a cold front and a warm front meet is an occluded front. A cold-front occlusion takes place when a cold front meets cool air and warm air is pushed up between them. This produces cold front weather. A warm-front occlusion is the result of cool, but not cold, air meeting a warm front. This produces warm front weather.

A stationary front is the result of a cold air mass and a warm air mass meeting and not moving for several days. Calm weather is usually the result.

Fronts help tell how the weather will change. The region just ahead of a front can expect to have a weather change. When fronts meet, weather may change quickly and dangerously. Sudden storms may develop.

Weather maps are used to record, monitor, and predict weather. Weather patterns in the United States tend to move from west to east. A weather system can be followed as it crosses the country. The information can be used to predict weather changes in the path of an air mass.

Pictures taken from satellites in space also help scientists follow weather patterns and make predictions. Satellite photographs show large portions of Earth and show fronts and storms.

Many people and industries rely on accurate weather forecasts, but no one can be exactly sure how the weather will change.