Refer to figures.
Air masses are classified according to moisture content, source or type and temperature using a 3-letter system:
First letter - moisture content:

• Continental (c) ⇒ dry
• Maritime (m) ⇒ moist

Second letter - source region or type (latitude/temperature):

• Equatorial. (E)
• Tropical. (T) ⇒ warm
• Polar. (P) ⇒ cold
• Arctic. (A)

Third letter: temperature, c(old) or w(arm)

Hence the 5 air masses affecting Europe are:

AIR MASS	SOURCE	CHARACTERISTICS
Arctic maritime, mAc	Polar ice cap. Only experienced during NH winter	Relative and absolute humidity increase significantly as the air moves down the Norwegian Sea and the air mass becomes very unstable.
Polar maritime, mPc	Northwest area of North Atlantic	stable, cold, absolute humidity low, relative humidity high
POLAR CONTINENTAL, cPc	SIBERIA (winter only) northern Russia	Stable, very cold and dry. it is typically colder than the Arctic maritime air, because the Polar ground is typically colder than the Arctic sea.
Tropical maritime, mTw	The Azores anticyclone	Warm, stable, absolute humidity high, RH high.
Tropical continental, cTw	N Africa/Middle East/SE Europe (SE Europe – primarily conterminous Balkan Peninsula)	warm, dry, stable.