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17.3: Air Masses and Weather Fronts

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    Air Masses

    An air mass is a large mass of air that has similar characteristics of temperature and humidity within it. An air mass acquires these characteristics above an area of land or water known as its source region. When the air mass sits over a region for several days or longer, it picks up the distinct temperature and humidity characteristics of that region.

    Air Mass Formation

    Where an air mass receives its characteristics of temperature and humidity is called the source region. Air masses are slowly pushed along by high-level winds. When an air mass moves over a new region, it shares its temperature and humidity with that region. So the temperature and humidity of a particular location depends partly on the characteristics of the air mass that sits over it. Storms arise if the air mass and the region it moves over have different characteristics. For example, when a colder air mass moves over warmer ground, the bottom layer of air is heated. That air rises, forming clouds, rain, and sometimes thunderstorms. How would a moving air mass form an inversion? When a warmer air mass travels over colder ground, the bottom layer of air cools and, because of its high density, is trapped near the ground.

    In general, cold air masses tend to flow toward the equator and warm air masses tend to flow toward the poles. This brings heat to cold areas and cools down areas that are warm. It is one of the many processes that act towards balancing out the planet’s temperatures. Air masses are classified based on their temperature and humidity characteristics. Below are examples of how air masses are classified over North America.

    • Maritime tropical (mT): moist, warm air mass
    • Continental tropical (cT): dry, warm air mass
    • Maritime polar (mP): moist, cold air mass
    • Continental polar (cP): dry, cold air mass
    Source regions of common air masses. The arctic front is along the northern border of Alaska and Canada. The polar front is along the northern border of the United States. mT air mass in the Pacific Ocean near Baja California moving northeast. mP air mass in the Pacific Ocean near Canada and the Pacific Northwest moving east. cP air mass between the Arctic and Polar Fronts moving south. mP air mass in the Atlantic Ocean between the Arctic and Polar Fronts moving west. mT air mass in the Atlantic Ocean below the Polar Front moving northwest. cT air mass in the southwest moving northeast.
    Figure \(\PageIndex{1}\): Typical air masses over North America.

    Weather Fronts

    When two air masses meet, the boundary between the two is called a weather front. At a front, the two air masses have different densities, based on temperature, and do not easily mix. One air mass is lifted above the other, creating a low pressure zone. If the lifted air is moist, there will be condensation and precipitation. Winds are common at a front. The greater the temperature difference between the two air masses, the stronger the winds will be. Fronts are the main cause of stormy weather. There are four types of fronts: cold, warm, occluded and stationary. With cold fronts and warm fronts, the air mass at the leading edge of the front gives the front its name. In other words, a cold front is right at the leading edge of moving cold air and a warm front marks the leading edge of moving warm air.

    The symbol for a cold front is a blue line of equilateral triangles pointing up. The symbol for a warm front is a red line of semicircles pointing up. The symbol for an occluded front is a purple line of equilateral triangles and semicircles. The symbol for a stationary front is a line of red semicircles pointing up followed by blue equilateral triangles pointing down.
    Figure \(\PageIndex{2}\): Diagram depicting how weather fronts are drawn on weather maps.

    Cold Fronts

    Imagine that you are standing in one spot as a cold front approaches. Along the cold front, the denser, cold air pushes up the warm air, causing the air pressure to decrease. If the humidity is high enough, some types of cumulus clouds will develop. High in the atmosphere, winds blow ice crystals from the tops of these clouds to create cirrostratus and cirrus clouds. At the front, there will be a line of rain showers, snow showers, or thunderstorms with blustery winds. A squall line is a line of severe thunderstorms that forms along a cold front. Behind the front is the cold air mass. This mass is drier so precipitation stops. The weather may be cold and clear or only partly cloudy. Winds may continue to blow into the low pressure zone at the front. The weather at a cold front varies with the season.

    • Spring and summer: The air is unstable so thunderstorms or tornadoes may form.
    • Spring: If the temperature gradient is high, strong winds blow.
    • Autumn: Strong rains fall over a large area.
    • Winter: The cold air mass is likely to have formed in the frigid Arctic so there are frigid temperatures and heavy snows.
    Cold front moving to the right and forcing an air parcel up. Cold air pushes warm air up at a steep arching angle. Clouds and lightning form at the front.
    Figure \(\PageIndex{3}\): Cross section of a cold front and associated weather.

    Warm Fronts

    Along a warm front, a warm air mass slides over a cold air mass. When warm, less dense air moves over the colder, denser air, the atmosphere is relatively stable. Imagine that you are on the ground in the wintertime under a cold winter air mass with a warm front approaching. The transition from cold air to warm air takes place over a long distance so the first signs of changing weather appear long before the front is actually over you. Initially, the air is cold: the cold air mass is above you and the warm air mass is above it. High cirrus clouds mark the transition from one air mass to the other. Over time, cirrus clouds become thicker and cirrostratus clouds form. As the front approaches, altocumulus and altostratus clouds appear and the sky turns gray. Since it is winter, snowflakes fall. The clouds thicken and nimbostratus clouds form. Snowfall increases. Winds grow stronger as the low pressure approaches. As the front gets closer, the cold air mass is just above you but the warm air mass is not too far above that. The weather worsens. As the warm air mass approaches, temperatures rise and snow turns to sleet and freezing rain. Warm and cold air mix at the front, leading to the formation of stratus clouds and fog.

    Cross section of a warm front. Warm air rises over cold air at a low angle with stratus clouds at the front. In the cold air mass, snow falls, then transitions to rain. Fog occurs at the edge of the cold air mass against the warm air mass..
    Figure \(\PageIndex{4}\): Cross section of a warm front and associated weather.

    Occluded Fronts

    An occluded front usually forms around a low pressure system when a cold front overtakes a warm front. The occlusion starts when a cold front catches up to a warm front. The air masses, in order from front to back, are cold, warm, and then cold again. The Coriolis effect curves the boundary where the two fronts meet towards the pole. If the air mass that arrives third is colder than either of the first two air masses, that air mass slip beneath them both. This is called a cold occlusion. If the air mass that arrives third is warm, that air mass rides over the other air mass. This is called a warm occlusion. The weather at an occluded front is especially fierce right at the occlusion. Precipitation and shifting winds are typical. The Pacific Coast has frequent occluded fronts.

    Remember, a weather front is basically the boundary between two air masses of different densities. At the center of each air mass is typically an area of high pressure. This means that weather is typically sunny within air masses, but their temperatures could vary with the season and humidity could vary based on the source region of the air mass.

    An occluded front. Cold front on left catches up to warm front on right. Occlusion is slightly curved to the left at a low pressure center.
    Figure \(\PageIndex{5}\): Image of an occluded front in which a cold front overtakes a warm front.

    Now more often than not, these weather fronts are not isolated events. Often, they are part of a larger rotating system called a mid-latitude cyclone. This type of cyclone will be discussed later in this chapter, but as an introduction it is a low pressure system that is usually mixing warmer air from the south (in the Northern Hemisphere) and colder air from the north.

    Stationary Fronts

    At a stationary front, the air masses do not move. A front may become stationary if an air mass is stopped by a barrier, such as a mountain range. A stationary front may bring days of rain, drizzle, and fog. Winds usually blow parallel to the front, but in opposite directions. After several days, the front will likely break apart. When a cold air mass takes the place of a warm air mass, there is a cold front.

    References

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