First of all let's talk about Airmass . an Airmass is a large body of air with fairly uniform temperature and moisture content . it may be several houndred miles across and usually forms where air remains stationary or nearly stationary at least for several days . during this time the airmass takes on the temperature and moisture properties of the underlying surface that determine its stability and known as Source Region . an ideal source regions is a large area with fairly uniform geoghraphy and temperature . a source region is usually located where air tends to stagnate . the best areas for airmass development are in the regions where atmospheric circulation has caused the buildup of semipermanent areas of high pressure . some of excellent areas are snow and ice covered polar regions , tropical oceans and large deserts . the middle latitudes are poor source regions because of the strong westerly winds and the continual mixing of polar and tropical airmasses .
Note : about airmasses maritime means moist , continental means dry , polar means cold , tropical means hot .