Encyclopedic Entry

The skies over this barn are on the front line.

Photograph by Tina Magas, MyShot

Fronts on Weather Maps
On weather maps, cold fronts are illustrated by blue lines with sharp "teeth" pointing in the direction of the wind. Warm fronts are illustrated by red lines with rounded bumps pointing in the direction of the wind.

A front is a weather system that is the boundary separating two different types of air. One type of air is usually denser than the other, with different temperatures and different levels of humidity. This clashing of air types causes weather: rain, snow, cold days, hot days, and windy days.

Two major types of fronts are cold fronts and warm fronts.

Cold fronts often come with thunderstorms or other types of extreme weather. They usually move from west to east. Cold fronts move faster than warm fronts because cold air is denser, meaning there are more molecules of material in cold air than in warm air.

Strong, powerful cold fronts often take over warm air that might be nearly motionless in the atmosphere. Cold, dense air squeezes its way through the warmer, less-dense air, and lifts the warm air. Because air is lifted instead of being pressed down, the movement of a cold front through a warm front is usually called a low-pressure system. Low-pressure systems often cause severe rainfall or thunderstorms.

Warm fronts usually show up on the tail end of precipitation and fog. As they overtake cold air masses, warm fronts move slowly, usually from north to south. Because warm fronts aren't as dense or powerful as cold fronts, they bring more moderate and long-lasting weather patterns. Warm fronts are often associated with high-pressure systems, where warm air is pressed close to the ground. High-pressure systems usually indicate calm, clear weather.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

air mass

Noun

a large volume of air that is mostly consistent, horizontally, in temperature and humidity.

Encyclopedic Entry: air mass

atmosphere

Noun

layers of gases surrounding a planet or other celestial body.

Encyclopedic Entry: atmosphere

boundary

Noun

line separating geographical areas.

Encyclopedic Entry: boundary

cold front

Noun

interaction between two air masses, where the cool, dense mass is replacing the warm one.

dense

Adjective

having parts or molecules that are packed closely together.

fog

Noun

clouds at ground level.

Encyclopedic Entry: fog

front

Noun

boundary between air masses of different temperatures and humidities.

Encyclopedic Entry: front

high-pressure system

Noun

weather pattern characterized by high air pressure, usually as a result of cooling. High-pressure systems are usually associated with clear weather.

humidity

Noun

amount of water vapor in the air.

Encyclopedic Entry: humidity

indicate

Verb

to display or show.

low-pressure system

Noun

weather pattern characterized by low air pressure, usually as a result of warming. Low-pressure systems are often associated with storms.

molecule

Noun

smallest physical unit of a substance, consisting of two or more atoms linked together.

precipitation

Noun

all forms in which water falls to Earth from the atmosphere.

Encyclopedic Entry: precipitation

rain

Noun

liquid precipitation.

Encyclopedic Entry: rain

rainfall

Noun

amount of precipitation that falls in a specific area during a specific time.

severe

Adjective

harsh.

snow

Noun

precipitation made of ice crystals.

temperature

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

Encyclopedic Entry: temperature

thunderstorm

Noun

cloud that produces thunder and lightning, often accompanied by heavy rains.

warm front

Noun

mass of warm air that replaces a mass of cold air.

weather

Noun

state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

Encyclopedic Entry: weather

weather system

Noun

movement of warm or cold air.

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writer

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrator

Tim Gunther
Mary Crooks, National Geographic Society

Editor

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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