Other than the clouds floating by overhead, we rarely think about the movement of air. On a daily basis, however, large masses of air pass by us in the atmosphere. And unless they bring about sudden or severe weather, we don't pay them much attention. These air masses get "pushed around the globe" by wind. They transport warm, cool, dry, or humid conditions from place to place. As I'm sure you've noticed, some air masses can take several days to pass over an area. They stall out. But when the weather does finally change, we know that a new air mass has moved in.
Although the air masses I'm going tell you about concern North America, every continent will have similar patterns. Meteorologists call the place where these air masses are formed, their source regions. A maritime air mass forms over oceans or bodies of water. They bring humid air. They are designated by the small letter "m". Air masses that are formed over land are called continental air masses. They bring us dry air. A lower-case "c" is used to designate them. Weather events that happen along the front edge of these air masses are called "fronts."
The second part of an air masses name is taken from the latitude of its source region, which expresses its temperature. They are designated by a capital letter. Polar (P) - Polar air is cold and originates between 50° N/S and 60° N/S; Arctic (A) - Arctic air is extremely cold (so cold, it is sometimes mistaken for the Polar Vortex). It forms poleward of 60° N/S; Tropical (T) - Tropical air is warm to hot. It forms at low latitudes, generally within 25° of the equator; Equatorial (E) - Equatorial air is hot and originates along 0° (the equator). Since the equator is mostly devoid of land areas, there is no such thing as continental equatorial air—only mE air exists. Since it rarely affects North America, it is not included in this list. Are you with me so far? I find this stuff fascinating...I hope you do too!
From these categories we can come up with the five combinations of air masses that affect the weather on the North American Continent.
1. Continental Polar (cP) - This air mass is dry, cold, and stable. They form over the snow-covered interiors of Alaska and Canada. The most common forms of cP come in winter, of course, but they can be the source of weather even in the summertime. Because the air mass is cool, these summer cP air masses often bring much-needed relief from sizzling summer heat waves.
2. Continental Arctic (cA) - Just like the cP, the cA airmass brings cold and dry air, but since it is formed farther north, over the Arctic basin and Greenland ice cap, its temperatures are generally colder. And unlike the cP, these air masses only happen in wintertime....generally-speaking.
Unlike other North American air masses, you won't find any maritime (m) classifications in association with Arctic air. "While arctic air masses do form over the Arctic Ocean, this ocean surface pretty much remains ice-covered throughout the year. Because of this, even air masses that originate there tend to have the moisture characteristics of a cA air mass."
3. Maritime Polar (mP) - As the name implies, these are cold and moist, unstable air masses. Those affecting the US and North America originate over the North Pacific Ocean and Northwestern Atlantic Ocean. What makes these masses interesting is that since they are formed over an ocean which is typically warmer than land, mP air masses are usually milder than cP or cA air. "In winter, mP air is associated with nor'easters and generally gloomy days. In summer, it can lead to low stratus, fog, and periods of cool, comfortable temperatures."
4. Maritime Tropical (mT) - These masses are warm and very humid. Those that affect the US originate over the Gulf of Mexico, the Caribbean Sea, the western Atlantic, and the subtropical Pacific. Just as the mP air masses are unstable, so to are the mT air masses. Cumulus clouds, thunderstorms, and showers are associated with these masses. That's me, below, after I got caught in a summer shower, brought on by an mT air mass. In winter, mT air can lead to "advection fog." This is what happens when warm, humid air becomes chilled. It condenses as it moves over the colder land surface.
5. Continental Tropical (cT) - These air masses are hot and dry. They only impact weather in North America during the summertime. They are formed over Mexico and the southwestern U.S. These are masses tend to give us cloudless weather, even though they are unstable. Continental tropical air tends to have extremely low humidity. If one of these air masses "hangs around" an area for too long, they can cause a severe drought.
I hope this simplified explanation of air masses helps you to better understand the weather. While you might not want to rush out and become a meteorologist, I do think that having some understanding how weather works, can make life a little more enjoyable. And you get the added bonus of sounding like a "smarty-pants" the next time someone brings up the topic of weather!