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.
Where air masses converge, they form boundaries called "fronts". Fronts are identified by change of temperature based upon their motion. With a cold front, a colder air mass is replacing a warmer air mass.
A warm front is the opposite affect in that warm air replaces cold air. There is also a stationary front, which, as the name implies, means the boundary between two air masses does not move.
The motion of air masses also affects where a good portion of precipitation occurs. The air of cold air masses is more dense than warmer air masses. Therefore, as these cold air masses move, the dense air undercuts the warmer air masses forcing the warm air up and over the colder air causing it to rise into the atmosphere.
So, fronts just don't appear at the surface of the earth, they have a vertical structure or slope to them as well. Warm fronts typically have a gentle slope so the air rising along the frontal surface is gradual. With warm fronts, the gentle slope favors a broad area of rising air so there is typically widespread layered or stratiform cloudiness and precipitation along and to the north of the front. The slope of cold fronts, being much more steep forces air upward more abruptly.
This can lead to a fairly narrow band of showers and thunderstorms along or just ahead of the front. Therefore, an air mass that develops over northern Canada is called a continental polar air mass and is cold and dry.
One that forms over the Indian Ocean is called a maritime tropical air mass and is warm and humid. Air masses are classified on weather maps using two or three letters. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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Weather is influenced by latitude, altitude, and local and regional geography. It impacts the way people dress each day and the types of structures built. Explore weather and its impacts with this curated collection of classroom resources.
An atmosphere is the layers of gases surrounding a planet or other celestial body.
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