Climatology: Air Masses

What is it?

• Air mass  is a 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.
• they are associated with one or other wind belts.

Source Regions

• The homogenous surfaces, over which air masses form, are called the source regions.
• The main source regions are the high-pressure belts in the subtropics (giving rise to tropical air masses) and around the poles (the source for polar air masses).

Conditions for Formation of Air Masses

• The source region should be extensive with gentle, divergent air circulation (slightly at high pressure).
• Areas with high pressure but little pressure difference or pressure gradient are ideal source regions.
• There are no major source regions in the mid-latitudes as these regions are dominated by cyclonic and other disturbances.

Classification of Air Masses Based on Temperature and Humidity

1. Polar
   • Continental Polar Air Mass (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 Air Mass (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 Air Mass (winter time) mPK
     • It 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 Air Mass (summer time) mPW
     • Cool and moist in the lower parts, but dry aloft
     • Temperature inversion is produced with moisture discontinuity Temperature slightly higher
2. Tropical
   • Continental Tropical Air mass
     • Source regions: Subtropical high pressure land areas
     • High temperature and low moisture content.
     • In the 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, the atmosphere becomes convectively unstable and violent thunderstorms and tornadoes are produced.
   • 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.