8.6: Types of Air Masses and How They Form - Geosciences

8.6: Types of Air Masses and How They Form - Geosciences

An air mass is a large body of air with relatively uniform temperature, humidity, and pressure. Air masses move with the global atmospheric system and can change as the move over landmasses and oceans, picking up or loosing warmth and moisture as they move.

Types of air mass are classed by where they form:



Polar - source regions above 60° north and south:

Polar Maritime
(cold and moist)

Polar Continental
(cold and dry)

Temperate - between 25° and 60°N/S:

Temperate Maritime
(cool and wet)

Temperate Continental
(warm and dry)

Tropical - source regions within about 25° of the equator:

Tropical Maritime
(warm and wet)

Tropical Continental
(hot and dry)

As air masses move they change to match the attributes of the next region. For instance, if a polar (or Arctic) air mass moves south over the North American continent it will become warmer and dryer (becoming a temperate-continental air mass; see example in Figure 8.15). If it moves east over the Atlantic Ocean it may become warmer and pick up moisture and become a temperate-maritime air mass. When a maritime air mass moves over a large landmass it can loose its moisture, heat up, and become a continental air mass.

Air masses can move rapidly (if air pressure gradients are high). Air masses can control the weather for a relatively long periods ranging from days to months. They can also stagnate in one region causing long periods or rain or drought. Tropical storms and hurricanes can form in association with tropical-maritime air masses. Most weather occurs along around air masses at boundaries called fronts (discussed below).

Figure 8.15. Origin of air masses affecting North America's weather. Air masses move as air pressure gradients change over time.

A Year in Weather (2013)
NASA YouTube animation- a global mercator map showing storm systems around the world for a year starting in January, 2013. Note the tropical cyclones (typhoons) in the Eastern Pacific, the weather patterns in the Intertropical Convergence Zone, and the Antarctic circumpolar region.

8.6: Types of Air Masses and How They Form - Geosciences

Air masses form in "source regions" where there is little topography and relatively stagnant winds near the surface. The air mass takes on the properties of the surface of the source region (e.g., dry, hot, moist, etc.). It takes several days for an air mass to "form", so they generally form in areas of high pressure (light winds).

  • Continental air masses are characterized by dry air near the surface while maritime air masses are moist.
  • Polar air masses are characterized by cold air near the surface while tropical air masses are warm or hot. Arctic air masses are extremely cold.

These air masses originate over northern Canada and Alaska as a result of radiational cooling. They move southward, east of Rockies into the Plains, then eastward. Continental polar or continental arctic air masses are marked by surface high pressure, cold temperatures, and low dew points.

Some maritime polar air masses originate as continental polar air masses over Asia and move westward over the Pacific, collecting warmth and moisture from the ocean. Some mP air masses originate from the North Atlantic and move southwestward toward the Northeast States. The latter air mass generally is colder and drier than the mP off of the Pacific.

Some maritime tropical air masses originate in the subtropical Pacific Ocean, where it is warm and air must travel a long distance over water. These rarely extend north or east of southern California. Some maritime tropical air masses originate over the Gulf of Mexico and Caribbean Sea. They can be associated with fog and low clouds as they moves northward. In the spring and summer, this air mass accounts for the thunderstorms in the Great Plains and elsewhere.

Continental tropical air masses originate in northern Mexico. They are characterized by clear skies and negligible rainfall. If one moves into the Great Plains and stagnates, a severe drought can result.

Topography can play a crucial role in the modification of air masses. For example, the Rocky Mountains cause flow from the west to be lifted over the mountains. The originally mP air loses its moisture as it precipitates, leaving dry air to move eastward. Hence, mP air becomes cP air after it is forced over the Rockies.

Air Masses

The air masses in and around North America include the continental arctic (cA), maritime polar (mP), maritime tropical (mT), continental tropical (cT), and continental polar (cP) air masses.
Credit: NOAA

Air is not the same everywhere. In North America, for example, cold and dry air covering thousands of miles flows south from the Arctic, especially in winter, and warm moist air flows north from the Gulf of Mexico. These different types air are called air masses.

An air mass is like a team whose players are all wearing the same uniform. In this case, the players are air, not people. And the uniforms that they wear are the similar characteristics such as temperature and humidity. Like sports teams, when two air masses come together, there is often turbulence. The turbulence of the two air masses moving together can cause clouds and thunderstorms to form. The border between two air masses at the Earth’s surface is called weather front.

Air masses are given a two-part name that describes the humidity and temperature characteristics of the region where they form. The first part of an air mass’ name describes its humidity. Air masses that form over the ocean, called maritime air masses, are more humid than those that form over land, called continental air masses. The second part of the name describes the temperature of the air mass, which depends on the latitude where it formed. Air masses that form near the equator or in the tropics (equatorial or tropical air masses) are warmer than air masses that form in polar areas or uin the Arctic (polar or arctic air masses).

The word that describes humidity (maritime or continental) is paired with the word that describes temperature (equatorial, tropical, polar or arctic). For example, if an air mass forms over a tropical ocean, it is called maritime tropical. If an air mass forms over land in the far north it is called continental polar.

An air mass can change as it moves into different environments. For example, if a continental polar air mass moves into warmer areas and over the ocean the air will warm and moisture may evaporate from the ocean surface into the air, adding humidity.

Cold and warm air masses usually come together in middle latitudes areas such as the United States, where they form weather fronts and can produce massive storms.

Cold Fronts

When cold air replaces warm air, a cold front results. As warm air rises and cools,its water vapor condenses with cloud formation. The rain resulting from cold fronts is short lived and heavy, generally affecting a distance of about 50 miles as the front moves on. Cold fronts blow over areas faster than other types of fronts, producing some of the most violent thunderstorms that move with the front while maintaining their intensity. They are often associated with a line of strong thunderstorms, a squall line, parallel to the front and moving ahead of it and leaving cooler weather behind with clear blue skies. On a weather map, the cold front symbol is usually a blue line with triangle pips pointed in the direction of the front's travel.

Air mass

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Air mass, in meteorology, large body of air having nearly uniform conditions of temperature and humidity at any given level of altitude. Such a mass has distinct boundaries and may extend hundreds or thousands of kilometres horizontally and sometimes as high as the top of the troposphere (about 10–18 km [6–11 miles] above the Earth’s surface). An air mass forms whenever the atmosphere remains in contact with a large, relatively uniform land or sea surface for a time sufficiently long to acquire the temperature and moisture properties of that surface. The Earth’s major air masses originate in polar or subtropical latitudes. The middle latitudes constitute essentially a zone of modification, interaction, and mixing of the polar and tropical air masses.

Air masses are commonly classified according to four basic source regions with respect to latitude. These are Polar (cold), Arctic (very cold), Equatorial (warm and very moist), and Tropical (warm). In the United States the major air mass types are typically continental Polar, maritime Polar, continental Tropical, and maritime Tropical.

Continental Polar (cP) air usually forms during the cold period of the year over extensive land areas such as central Asia and northern Canada. It is likely to be stable and is characteristically free of condensation forms. When heated or moistened from the ground with strong turbulence, this type of air mass develops limited convective stratocumulus cloud forms with scattered light rain or snow showers. In summer strong continental heating rapidly modifies the coolness and dryness of the cP air mass as it moves to lower latitudes. Daytime generation of cumulus clouds is the rule, but the upper-level stability of the air mass is usually such as to prevent rain showers.

Maritime Polar ( mP) air masses develop over the polar areas of both the Northern and the Southern hemispheres. They generally contain considerably more moisture than the cP air masses. As they move inland in middle and high latitudes, heavy precipitation may occur when the air is forced to ascend mountain slopes or is caught up in cyclonic activity (see cyclone).

The continental Tropical ( cT) air mass originates in arid or desert regions in the middle or lower latitudes, principally during the summer season. It is strongly heated in general, but its moisture content is so low that the intense dry convection normally fails to reach the condensation level. Of all the air masses, the cT is the most arid, and it sustains the belt of subtropical deserts worldwide.

The maritime Tropical ( mT) is the most important moisture-bearing and rain-producing air mass throughout the year. In winter it moves poleward and is cooled by the ground surface. Consequently, it is characterized by fog or low stratus or stratocumulus clouds, with drizzle and poor visibility. A steep lapse rate aloft in regions of cyclonic activity ensures the occurrence of heavy frontal and convective rains. In summer the characteristics of the mT air mass over the oceans and in zones of cyclonic activity are basically the same as in winter. Over warm continental areas, however, the air mass is strongly heated so that, instead of fog and low stratus clouds, widely scattered and locally heavy afternoon thunderstorms occur.

This article was most recently revised and updated by John P. Rafferty, Editor.

Effects of Thunderstorms

Thunderstorms have wide-ranging effects on human life, including electrocution, shock, and even worse, deaths. They normally have a significant effect on the weather over an expansive area, with energy generated at the rate of at least 10,000,000 kilowatt-hours on an average.

They normally lead to local atmospheric instability, and just a single thunderstorm can produce lightning, catastrophic flooding, tornadoes, very strong winds, and hail. In fact, lightning is listed as one of the deadliest occurrences of a storm.

When a thunderstorm happens, animals would get hurt, buildings could be destroyed, and lighting may burn the vegetation, leaving animals with inadequate food. In light of the fact that thunderstorms are dramatic events, they can lead to psychological and emotional complications.

If you have a chronic illness, powerful emotions can sometimes result in the development of physical symptoms. They may aggravate an asthma attack, for instance, or make it more difficult to manage an arthritis flare.

It is also worth noting that thunderstorms have some positive effects. For instance, lightning creates the ozone layer from oxygen. It is the ozone layer that makes life possible on the earth’s surface. Otherwise, we couldn’t survive the sun’s ultraviolet radiation.

Moreover, lighting creates nitrate from nitrogen. Nitrate is a fertilizer essential for plants to grow and thrive on earth.

Bottom line

Thunderstorms are an amazing phenomenon, which are of three main types. They can occur virtually anywhere and are normally the beginning of some of other deadly storms such as tornadoes and hurricanes. About 16 million thunderstorms occur across the globe every year, with about 100,000 of them experienced in the U.S. alone. About 10 percent of the thunderstorms reach severe levels. It is also worth noting that at any given time, there are about 2,000 thunderstorms in progress.

Photo by: boboshow

About Sonia Madaan

Sonia Madaan is a writer and founding editor of science education blog EarthEclipse. Her passion for science education drove her to start EarthEclipse with the sole objective of finding and sharing fun and interesting science facts. She loves writing on topics related to space, environment, chemistry, biology, geology and geography. When she is not writing, she loves watching sci-fi movies on Netflix.

How Do Winds Form?

Winds are formed by moving air masses that begins right from the sun’s radiation. When the sun hits the land, the heat is absorbed variedly on the surface of the earth. This is because of the differences in land surface cover such as water bodies, valleys, plains, vegetations, mountains, cloud cover and desert regions.

As the sun’s radiation unevenly heats the land, the air above the surfaces warms up and starts to rise because it becomes less dense. As the air rise, low atmospheric pressure is created. As a result, the air with cooler temperatures sink and the sinking continues to create higher atmospheric pressure.

This creates what is commonly called convectional currents. Convectional currents result when lighter air masses move up because of higher temperatures and are in turn replaced by cooler heavier air masses and the processes repeat itself again and again. Thus, this is the process that leads to the formation of winds.

The strength of the winds depends upon the energy of the convectional currents, that is, the higher the energy, the faster and more violent the winds, and vice versa. The air warms faster over land surfaces since the land tends to retain heat. On the contrary, the air warms at a slower rate because the sun’s radiation is slowly cooled by the cold water.

Fog Types

Radiation Fog: This fog forms when all solar energy exits the earth and allows the temperature to meet up with the dew point. The best condition to have radiation fog is when it had rained the previous night. This help to moisten up the soil and create higher dew points. This makes it easier for the air to become saturated and form fog. However, the winds must be light less than 15 mph to prevent moist and dry from mixing.

Precipitation Fog: This is fog that forms when rain is falling through cold air. This is common with a warm fronts but it can occur with cold fronts as well only if it's not moving too fast. Cold air, dry at the surface while rain is falling through it evaporates and causes the dew point to rise. This saturation forms fog.

Advection Fog: This type of fog forms from surface contact of horizontal winds. This fog can occur with windy conditions. Warm air, moist air blows in from the south and if there is snow or cool moisture on the ground it will come in contact with the warm, moist winds. This contact between the air and ground will cause the air blowing in to become cool. Then dew point rises and creates high humidity and forms fog.

Steam Fog: This type of fog is commonly seen in the Great Lakes but can be seen on any lake. This forms during the fall season. As summer ends, water temperatures don't cool right away but air temperature does. As a mass of dry, cold air moves over a warmer lake the warm lake conducts warm, moist air into the air mass above. This transport between the lake and air evens out. This corresponds to the second law of thermodynamics and this law state "any two bodies that come into contact, the system will become equilibrium state." Steam fog does not become very deep but enough to block some of the sunlight.

Upslope Fog: This fog forms adiabatically. Adiabatically is the process that causes sinking air to warm and rising air to cool. As moist winds blow toward a mountain, it up glides and this causes the air to rise and cool. The cooling of the air from rising causes to meet up with the dew point temperature. Fog forms on top of the mountains.

Valley Fog: Valley fog forms in the valley when the soil is moist from previous rainfall. As the skies clear solar energy exits earth and allow the temperature to cool near or at the dew point. This form deep fog, so dense it's sometimes called tule fog.

Freezing Fog: Freezing fog occurs when the temperature falls at 32°F (0°C) or below. This fog produces drizzle and these tiny droplets freeze when they come into contact with an object. But at the same time there is sublimation going on.

Ice Fog: This type of fog is only seen in the polar and artic regions. Temperatures at 14 F (-10°C) is too cold for the air to contain super-cooled water droplets so it forms small tiny ice crystals.

8.6: Types of Air Masses and How They Form - Geosciences

Hurricanes are the most awesome, violent storms on Earth. People call these storms by other names, such as typhoons or cyclones, depending on where they occur. The scientific term for all these storms is tropical cyclone. Only tropical cyclones that form over the Atlantic Ocean or eastern Pacific Ocean are called "hurricanes."

Whatever they are called, tropical cyclones all form the same way.

Tropical cyclones are like giant engines that use warm, moist air as fuel. That is why they form only over warm ocean waters near the equator. The warm, moist air over the ocean rises upward from near the surface. Because this air moves up and away from the surface, there is less air left near the surface. Another way to say the same thing is that the warm air rises, causing an area of lower air pressure below.

Air from surrounding areas with higher air pressure pushes in to the low pressure area. Then that "new" air becomes warm and moist and rises, too. As the warm air continues to rise, the surrounding air swirls in to take its place. As the warmed, moist air rises and cools off, the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean's heat and water evaporating from the surface.

Storms that form north of the equator spin counterclockwise. Storms south of the equator spin clockwise. This difference is because of Earth's rotation on its axis.

As the storm system rotates faster and faster, an eye forms in the center. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye.

If you could slice into a tropical cyclone, it would look something like this. The small red arrows show warm, moist air rising from the ocean's surface, and forming clouds in bands around the eye. The blue arrows show how cool, dry air sinks in the eye and between the bands of clouds. The large red arrows show the rotation of the rising bands of clouds.

When the winds in the rotating storm reach 39 mph, the storm is called a "tropical storm." And when the wind speeds reach 74 mph, the storm is officially a "tropical cyclone," or hurricane.

Tropical cyclones usually weaken when they hit land, because they are no longer being "fed" by the energy from the warm ocean waters. However, they often move far inland, dumping many inches of rain and causing lots of wind damage before they die out completely.

Tropical cyclone categories:

The two GOES satellites keep their eyes on hurricanes from far above Earth's surface&mdash22,300 miles above, to be exact! (Learn more about this kind of orbit.)

These satellites, built by NASA and operated by the National Oceanic and Atmospheric Administration (NOAA), save lives by helping weather forecasters predict and warn people where and when these severe storms will hit land.

Fronts and Pressure

At the completion of this section, you should be able to discuss why fronts are located in troughs and discuss trends in sea-level pressure associated with a frontal passage.

Now that you've learned about the circulations around high- and low-pressure systems, we're going to tie that new knowledge in with some topics that we covered previously in order to help you better see the big picture. For starters, let's review a couple of key definitions:

  • Air Masses are large blobs of air with horizontal dimensions of several hundred to a couple of thousand miles within which temperatures and moisture (dew points) at the surface (or at any other arbitrary altitude) are fairly uniform. In other words, temperature and moisture gradients within an air mass are small. Several types of air masses exist, and are named based on their source regions (which determine their temperature and moisture characteristics).
  • Fronts are boundaries that separate contrasting air masses. Since fronts lie at the edges of contrasting air masses, not surprisingly, fronts lie in zones with large gradients in temperature and dew point. The types of fronts we discussed previously are cold fronts, warm fronts, and stationary fronts.

So how are air masses, fronts, and the pressure pattern related? For starters, recall how air masses get their characteristics. In order for a large chunk of air to acquire the temperature and moisture characteristics of the underlying surface of the earth, it must stay over a given source region long enough for land or water to modify the overlying air. For this process to occur, it stands to reason that surface winds must be generally light. Broad regions of light winds are often found surrounding centers of surface high pressure, thus high-pressure systems mark the centers of air masses.

To see what I mean, check out the analysis of sea-level pressure from January 12, 1982. Note the strong high pressure system over Siberia, a region renowned as a source region for continental-Arctic (cA) air masses. Now, compare the pressure gradient around the high's center to the gradient around the low-pressure system centered over the Sea of Japan. Clearly, the pressure gradient associated with the high is much weaker than the pressure gradient around the center of low pressure, which translates to very light winds around the Siberian high. Those light winds allow the snow-covered, frigid ground to modify the overlying air and create a bone-chilling continental-Arctic (cA) air mass (the fact that northern Siberia, at latitudes above the Arctic Circle, tallies 24 hours of darkness each day during the heart of winter certainly helps).

So, if the "meteorological center" of an air mass is marked by a center of high pressure, then pressure must naturally decrease as you move toward the periphery of the air mass. When two air masses meet, the boundary must be a region of lowest pressure (because as you cross the boundary, pressure will start to increase again toward the center of another high). I think the schematic on the right, showing two opposing air masses and their high-pressure systems provides a helpful visual. Clearly, the transition zone between the two air masses must lie in a region of relatively low pressure.

Of course, the boundaries that separate contrasting air masses are called fronts, which leads us to the following conclusion: fronts lie in troughs of low pressure. Now, not all surface troughs coincide with fronts, but the bottom line is that fronts naturally exist in elongated regions of low pressure (troughs). Also recall that surface troughs are regions of wind shifts and surface convergence. Therefore, since fronts lie in troughs, then it also stands to reason that shifts in wind direction and surface convergence occur along fronts.

Wind shifts along fronts are also supported by the notion that a front is a boundary between opposing high pressure systems. Notice in the diagram below that the flow of air associated with the two high-pressure systems is divergent -- spreading outward away from a center of high pressure. Along the stationary front (alternating blue barbs pointing toward warmer air and red circles directed toward cold air) that marks the boundary between the air masses, winds from markedly different directions meet. Ultimately, the stationary front lies just on the warm side of the large temperature gradient associated with the frontal zone (right).

The shift in wind direction, surface convergence, and the fact that fronts lie in troughs all have consequences for the weather you may experience when a front passes. To see what I mean, let's turn to the morning of February 14, 2015, when a cold front was passing through northwestern Ohio. You can see the location of the front on the 15Z surface analysis, and note how the front lies essentially between different areas of high pressure (contrasting air masses) similar to our schematics above. If we zoom in on the analysis, it becomes more apparent that the cold front also lies in a southwestward bulge in the isobars, which marks a surface trough. Also note the dramatic wind shift across the front -- southwest winds ahead of the front in eastern Ohio, compared to north-northwest winds behind it in northwestern Ohio, Indiana, and Michigan.

When the front passed Findlay, Ohio around 15Z, the change in weather was notable, as you can tell from the graphs below. The top graph plots temperature, dew point, and relative humidity from 04Z on February 14, 2015 through 05Z on February 15. I've marked 15Z with a vertical black line. At 15Z, both temperature and dew point (purple and green traces, respectively) started to decline as colder, drier air arrived behind the cold front (so temperatures started dropping, even though it was 10 AM local time). Winds (as marked on the station models below the top graph) shifted from southwesterly and west-southwesterly ahead of the front to northwesterly after the front passed.

Finally, the bottom graph shows sea-level pressure, and pressure reached a minimum around the time the cold front passed at 15Z. That should make sense to you since fronts lie in troughs. Pressures steadily decreased until the front (and its trough) arrived, then pressure began increasing after the front passed (and the trough moved away with it). You can also tell that the frontal passage came with some snow (note the station model symbols for snow beneath the top graph), thanks in part to surface convergence and rising air in the vicinity of the front.

So, understanding the circulation of air around highs and lows, and how the pressure pattern ties in with air masses and fronts allows us to understand a lot of weather events that we experience! But, I've just scratched the surface here. We're just getting started in discussing the various types of weather (clouds, rain, snow, thunderstorms, etc.) that we experience every day. Specifically, this lesson has laid an important foundation, and we'll be applying many of its concepts during our look at low-pressure systems and the wide variety of weather that they bring (in the next lesson).