 The Bergeron classification is the most widely accepted form of air mass classification . Air mass classification involves three letters . The first letter describes its moisture properties , with c used for continental air masses ( dry ) and m for maritime air masses ( moist ) . The second letter describes the thermal characteristic of its source region : T for tropical , P for polar , A for arctic or Antarctic , M for monsoon , E for equatorial , and S for superior air ( dry air formed by significant upward motion in the atmosphere ) . The third letter is used to designate the stability of the atmosphere . If the air mass is colder than the ground below it , it is labeled k . If the air mass is warmer than the ground below it , it is labeled w . Fronts separate air masses of different types or origins , and are located along troughs of lower pressure . 
