• 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.

    The skies over this barn are on the front line.

    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.

  • 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


    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.