Air masses are classified based on their temperature and humidity characteristics. Broadly, the air masses are classified into polar and tropical air masses.
Both the polar and the continental air masses can be either of maritime or continental types.
The principal source regions of the earth may be classified according to the nature of the surface (land or water) and latitude of the region.
Thus the source regions are classified as under:
- Surface
- Continental
- Maritime
- Latitude
- Arctic (located in the high latitudes)
- Polar (between Arctic source regions and subtropical highs)
- Tropical (occupy subtropical high pressure belts)
- Equatorial (located around the equator between the trade winds of the northern and southern hemispheres)
The air masses may be classified as under:
Polar
-
- Continental Polar airmass (winter time) cPK
- Source regions: Central Canada and Siberia.
- Extremely cold, dry, stable airmass (coldest wintertime airmasses)
- Produce intense cold waves
- No clouds in these air masses.
- Continental Polar airmass (summer time) cPW
- Source regions: Central parts of high latitude continents. Example Central Canada
- Cool and dry airmasses
- Steep lapse rates.
- When cPK moves out to oceanic surface, it is modified into cPW air mass with haze, fog and low stratus clouds.
- Maritime Polar airmass (winter time) mPK
- mPK forms over open areas in the higher latitudes cool and moist few clouds in their source regions
- Extensive precipitation is produced when forced to ascend mountain barriers
- Lower layers moist and unstable and dry and cool in upper parts.
- Produce squally weather.
- Maritime Polar airmass (summer time) mPW
- Cool and moist in the lower parts, but dry aloft
- Temperature inversion is produced with moisture discontinuity Temperature slightly higher
- Continental Polar airmass (winter time) cPK
Tropical
- Continental Tropical air mass
- Source regions: Subtropical high pressure land areas
- High temperature and low moisture content.
- In United States, these air masses are only important in summer. They are both dry in winter and summer.
- In summer they are very hot
- Subsidence and stability found in the upper parts of these air masses in their source regions.
- If cT air mass is aloft over warm moist air at the surface, atmosphere becomes convectively unstable and violent thunderstorms and tornadoes are produced.
- Maritime Tropical air mass mT
- Warm and moist and highly unstable having convective unstability.
- Maritime Tropical air mass (winter time)
- Source regions: Warm oceans in both the hemisphere Warm moist and unstable air masses
- Steep lapse rate up to tropopause and moisture well distributed up to high levels.
- When these air masses are lifted over fronts or high mountains, they produce heavy Precipitation
- Maritime Tropical air mass (summer time)
- Source regions located in the belt of great semi permanent highs of the tropical oceans including the Caribbean Sea.
