Final answer:
The three letters in an air mass abbreviation indicate the moisture content, temperature, and stability of the air mass.
Step-by-step explanation:
The three letters that specify the source region of an air mass, such as cPk, indicate the temperature and moisture characteristics of the air mass. The first letter represents the moisture content: c for continental (dry) air mass and m for maritime (moist) air mass. The second letter represents the temperature: P for polar air mass, T for tropical air mass, and A for arctic air mass. The third letter represents the stability of the air mass: k for cold air mass, w for warm air mass, and c for cool air mass.