A look at the five Basic Types of Air Masses that Determine the Uss Weather

A look at the five basic types of air masses that determine the US’s weather helps people better understand the upcoming weather forecast. First developed in 1920 by a Norwegian father and son team, Vilhelm and Jacob Bjerknes, understanding about air mass patterns help is better than taking someone’s word for its accuracy – or inaccuracy. Air masses and atmosphere have become very important in determining the weather, with air developing its mass through a mixture of atmospheric gases. According to NASA, “Weather is the state of the atmosphere at some place and time.” (NASA.gov)

“Efforts at incorporating numerical data on weather into mathematical formulas that could then be used for forecasting were initiated early in the century at the Norwegian Geophysical Institute. Vilhelm Bjerknes and his associates at Bergen succeeded in devising equations relating the measurable components of weather, but their complexity precluded the rapid solutions needed for forecasting. Out of their efforts, however, came the polar front theory for the origin of cyclones and the now-familiar names of cold front, warm front, and stationary front for the leading edges of air masses …” (University of St. Andrews, Scotland )

Air masses can be seen on any planet, at any given place or time, while involving a variety of weather formations. Air mass properties are dependent on the location of its formation, with high latitude air masses colder than low latitudes. Altogether, this forms the four basic types of air masses: cold and dry; cold and humid; warm and humid; and warm and dry.

What causes this are large groups or high volumes of air that are similar in humidity and temperature – called air masses. The National Weather Service Weather Forecast in Corpus Christi, Texas describes air masses “a large body of air that has relatively uniform temperature and humidity characteristics.”(NOAA.gov)  This huge group is referred to as air mass source regions, acquiring the properties of the land beneath it if the mass remains over it long enough. Spring brings the most violent weather in the United States, caused by the clashing of the continental polar air and the maritime tropical air. This is described on weather maps with a K or W attached to the two-letter air mass description (cT, mT, mP, cP). The W refers to warmer air over the cooler land, while the K refers to a colder air mass moving over a warmer land mass.  

~ There are five basic types of air masses that determine the US’s weather (USAToday):

Continental Arctic (cA) – this air mass consists of very low cold temperatures with low moisture levels. The northern Arctic is its source, with 24 hours of complete darkness setting record low temperatures. This winter air mass heads south from Canada to the United States in the winter. Continental Polar (cP) – the cP air mass is dry like the cA but not nearly as cold. The continental polar is associated with bringing pleasant weather in the summer to the northern areas. However, it dominates the United States winters because it develops further south than any of the continental masses. For this reason, the cP air mass has been known to develop during the summer, influencing the northern part of the US’s weather. Continental Tropic (cT) – developing over northern Mexico and the Desert Southwest in the summer, the cT air mass is exceptionally hot and dry. Record high temperatures peak in the Mississippi Valley and the Plains until it moves eastward. At that time, moisture will evaporate into the air – similar to the maritime tropical air mass.   Maritime polar (mP) – the mP air masses form over northern Atlantic and Pacific oceans, bringing cloudy, cool and moist weather to the United States. The area most influenced is the Northeast or Pacific Northwest. It can develop any time during the year, usually not as cold as the continental polar air masses.  Maritime Tropical (mT) – the mT air masses are usually seen across the eastern part of the United States, causing warm temperatures with lots of moisture. Originating over the Gulf of Mexico and the southern Atlantic Ocean, they can form any time but typically cover the United States during the summers. Hot and humid weather comes from this air mass, locating itself in the South and Eastern part of the country.

When science refers to idealized air masses, they speak of flat and uniform compositions – Siberia, central Canada, north and south oceans, and large deserts. Once one of these five types of air masses forms, it moves out of its source regions. This causes different conditions to develop in other parts of the country.

~ Fronts and air masses

Fronts are boundaries between two air masses, narrow transitions between differing temperatures and humidity. The fronts consist of cold, warm, stationary, and occluded fronts When a cold air mass replaces a warm air mass, the developing front is called a cold air front. The air behind the cold front is much colder and drier than what is in front of it. When a warm air mass replaces a cold air mass, the developing front is called a warm air front. A very warm and moist air mass is behind the warm front, while the air ahead of it is cooler and drier. The warm fronts are located on a weather map by a sold red line with semi-circles pointing the direction of the movement.  

The stationary front is one that stalls – it does not move. However, the air masses move parallel to its boundary. And a stationary front will bring about significant weather event, such as heavy flooding, considerable cloudiness, rain or snow. An occluded front has been known to move slowly – taking as much as several days to merge from a cold front into a warm front. The warm air lifts off the earth’s surface, merging with the cold front above it. A cold front moves faster than a warm front, with the winds in a low-pressure system gathering contrasting air masses to form the front. NASA refers to weather lows as the “chief weather makers of regions in the middle latitudes.”

~ World Meteorological Organization (WMO)

The world’s nations founded the WMO in order to observe the earth’s atmosphere and to share weather forecasts and data through a wide variety of instruments and weather stations – satellites, about 10,000 land-based observation stations (surface stations), ships, radar stations, airplanes, and weather balloons. This includes Canada’s Atmospheric Environment Service and the USA National Weather Station, working together to issue public advisories and weather forecasts. They also issue warnings and watches for winter storms, severe thunderstorms, floods, tornadoes or hurricanes through the movement of the air masses.

 ~ Conclusion

Today air mass and fronts are studied intensively as predictors of future weather. In order for an air mass to form, it must hover a large area and take on the water or land’s humidity and temperature. Certain locations favor air mass development – topographic and atmosphere conditions. North America is dominated by six or more air masses on a regular basis. Two originate in Alaska and central Canada, sweeping into the United States during the winter – heavy precipitation, cold temperatures and strong winds. They attack New England and the Great Lakes usually first.