Airmass is a large volume of air in the atmosphere that is mostly uniform in terms of temperature and moisture. Air masses cover many hundreds or thousands of square miles and adopt the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions.
If the source region is the water, the dew point will be higher and the water mass is considered maritime. If the source region is over land, the air will be relatively dry, so the air mass is considered continental.
There are four temperature characteristics defining the airmass source region, from the coldest to warmest
- Arctic or antarctic
Therefore the air mass that develops over northern Canada is called a continental polar air mass and it is cold and dry. One that forms over the Indian ocean is called a maritime tropical air mass and is warm and humid.
Airmass is classified in the weather maps using two or three letters
- A lower case latter describes the amount of moisture in the airmass. An a for maritime (moist) c for continental (dry)
- An upper case latter describes the heat of the airmass. E for equatorial, T for tropical, M for monsoon, P for polar, A for arctic or antarctic, and S for superior – which means a unique situation with dry air formed by a powerful downward motion of the atmosphere
- A lower case latter describes the relationship between the airmass and the earth; k signifies that the air mass is colder than the ground below it while w describes an airmass that is warmer than the ground below it.