It may look like a map of the art museum or the subway, but it’s actually a map of the weather. Actually, those colored shapes provide information about weather conditions, and specifically air masses. An air mass is a large section of air with a fairly consistent humidity and temperature.
Air Mass Definition
An air mass is a large portion of air with relatively uniform temperature, humidity, and other characteristics that it takes from the water or land below it. Air masses can be up to thousands of miles across and may reach miles into the atmosphere.
4 Types of Air Masses
Experts classify air masses based on temperature and humidity. Air masses can be further categorized based on whether they occur over water or land.
The 4 types of air masses that impact North America most commonly are maritime tropical (mT), continental tropical (cT), maritime polar (mP), and continental polar (cP). We’ll discuss these 4 types of air masses more below.
(There are also less common types of air masses that form in regions outside North America and are seldom seen in the US.)
Source Regions and Air Mass Characteristics
Meteorologists classify air masses by where they come from, one of four “source regions.” These regions are typically flat and large with formations that are consistent—such as deserts or oceans, not hills and mountains.
This is because wind speeds need to stay low enough to let the air take on the characteristics of the region for a mass to form. Once a mass forms, wind can move the mass of air into a new region, and this creates weather or even storms as the characteristics of the mass clash with new humidity and temperature conditions.
Temperature affects air pressure. Denser, colder air has a higher pressure, while less dense, warmer air has less pressure. For this reason, air masses that are tropical, or warm, have low air pressure. Air masses that are polar, or cold, have high air pressure.
Predictably, the humidity of an air mass depends on whether it forms over land or water. Maritime air masses tend to be humid, as they form over oceans as water evaporates. Continental air masses tend to be drier, as they form over land where there is less exposure to moisture from large bodies of water.
Polar Air Masses
Polar air masses form between 50 and 60 degrees latitude. Although they can form over water, Siberia and Northern Canada are common sources of these cold, dry air masses. Because they are extremely dry, polar masses have few clouds. Meteorologists refer to these air masses with a capital “P.”
Arctic Air Masses
These extremely cold air masses form around the polar regions and are usually referred to as “A,” (Arctic) and “AA” (Antarctic).
Tropical Air Masses
Tropical air masses are warm or even hot, as they form within 25 degrees latitude of the equator. These masses can develop over water or land, and are abbreviated with a “T.” Source regions include northern Mexico, the Gulf of Mexico, and the southwestern United States. As these air masses move across land, they typically cool rapidly, resulting in storms and precipitation.
Equatorial Air Masses
Equatorial air originates along the equator at 0 degrees and is extremely hot. There is no continental equatorial air since the equator is mostly devoid of land areas, and these masses rarely affect the US.
Continental Air Masses
Continental air masses develop either north or south of the equator, between 25 and 60 degrees latitude. These air masses are dry, as they form over large land areas. Meteorologists represent this with a lowercase “c.” The lowercase is used because temperature is considered the more important factor.
Maritime Air Masses
A maritime air mass is a type of air mass that forms over large bodies of water, such as oceans or large lakes. These air masses are typically characterized by high humidity and moderate temperatures.
Maritime air masses are formed when moist air from the ocean surface is lifted upwards by the wind, creating a layer of cool, moist air near the surface. As the air continues to rise, it cools further and the moisture it contains condenses, forming clouds and precipitation.
Maritime air masses are typically associated with mild, rainy weather. When they move over land, they can bring precipitation and cloud cover, resulting in cool, overcast conditions. However, they can also bring warm, moist air to coastal regions, leading to warmer, more humid weather.
Maritime air masses can also influence the weather in inland areas when they interact with other air masses. For example, a maritime air mass moving over a warm, dry continental air mass can cause thunderstorms and severe weather. Similarly, a maritime air mass moving over a cold polar air mass can cause heavy snowfall and blizzard conditions.
From there, meteorologists combine the temperature and origin/humidity characteristics to classify a given air mass. The four most common in North America are these.
Large air masses that form over central and northern Canada and Alaska are continental polar air masses. Continental polar air masses are the hallmark of North American winters, bringing cold, clear, dry air with them. Their effects are milder in summer, although clashes between maritime tropical air masses moving north and continental polar air masses move south can cause storms.
Mostly in summer, these hot, dry air masses form and bring heat to the dry Southwestern regions and northern Mexico. Continental tropical air masses occasionally push to the northeast, and bring dry, hot weather to the Great Plains, but in general they cover less area than other air masses.
A far greater factor along the West Coast, the icy cold waters of the North Pacific and North Atlantic oceans push cool, humid masses of air to form. These can bring rain, fog, and cool temperatures to the coastal areas even in summer.
In summer’s heat, warm, humid masses of air form over tropical oceans such as the Atlantic, and in the Gulf of Mexico. These maritime tropical air masses move first into the southeastern United States, bringing hot, humid weather, thunderstorms, and rain. Then these air masses move northeast, changing weather in the central and eastern US until they die out.
In the west, the Pacific Ocean generates maritime tropical air masses that affect West Coast weather. They lose moisture as they cross the coastal mountain ranges, again dying out. In winter, maritime tropical air masses can bring heavy rain or snow, especially in the American southwest.
Other Air Masses
So, do continental arctic (cA), maritime arctic (mA), or maritime equatorial (mE) masses exist? (We already said continental equatorial masses do not.)
Similar to cP air, continental arctic air is also cold and dry, but it is even colder, forming farther north over the Greenland ice cap and Arctic basin. Typically an air mass reserved for wintertime, continental arctic air can bring very low humidity and bitter cold.
However, unlike the other types of North American air masses, there is no classification for maritime arctic air. Air masses do form over the Arctic Ocean, but because the surface of the water has usually remained covered in ice, these air masses retain the humidity characteristics of a continental arctic air mass.
Maritime equatorial air does exist, but rarely affects the US.
We hope this primer on the 4 types of air masses has been useful to you! And the next time you see a weather map, you’ll know what’s coming.