Source:http://www.islandnet.com
Below are direct quotes from the website:
Often, the conflict between air masses is so intense that great cyclonic weather systems develop along the frontal boundary to lash the surface beneath with high winds, rapidly changing temperatures and precipitation of all varieties. The differences among air masses was likely first recognized when humans realized that major changes in weather had recognizable, repeatable patterns: e.g., cold, dry conditions come from the north; hot and dry or hot and humid weather from the south. With the advent of regular weather observations across large regions of the continents and weather/climate records from locations around the globe, meteorologists began to see repeatable patterns that showed large bodies of air could be distinguished by their temperature and humidity levels. Air masses acquire their characteristic temperature and moisture (or absolute humidity) signature from the source regions over which they are born. Mid-latitude oceans can be source regions under certain conditions because their surfaces have very uniform characteristics. From the characteristic properties picked up in their breeding ground, air masses are designated as hot or cold, wet or dry.
Wet air masses are consider to be air masses forming over the oceans, and dry air masses, those forming over the continents. Equatorial air masses are all considered to be wet because much of the land area under the equatorial zone is covered in tropical rainforests that can add as much moisture to the air as the equatorial oceans. All arctic (or antarctic in the Southern Hemisphere) are considered dry because there is little evaporation into them from the frigid polar oceans and their temperatures are so low that even at saturation, the absolute humidity is very low.

Polar Maritime (mP)

Very cool because of the high latitude but not cold, due to moderating influence of the sea and the warm ocean currents at these latitudes.

Moderately moist because of the cool temperature, but not as dry as polar continental air because of evaporation from the water surface.