slightly below freezing. This is a cold shock to the Gulf Coast but a testimony to
air mass modification.
One well-appreciated instance of air mass modification is lake effect snow. In
early winter the Great Lakes of the United States and Canada are not yet frozen
over. Polar and Arctic air masses stream over the lakes. These cold air masses do
not contain much moisture but have high relative humidities. As the air passes
over a few tens of kilometers of lake surface, the air mass gains water vapor and
saturates. Downwind of the lakes, the arrival of the air over land initiates a small
bit of lifting that cools the air lower than its dewpoint temperature. Condensation
and the precipitation processes begin and make copious amounts of snow without
the presence of a winter storm.
A simple air mass classification scheme considers the surface and latitude over
which air passes. For instance, consider the differences in air types that can be gen-
erated over the great Antarctic ice sheets versus the tropical reaches of the Indian
Ocean. Air masses having their origins in these places will provide vastly different
weather as they progress over a location.
Maritime Tropical air (mT) is a product of tropical and subtropical oceans. It is
warm at any time of the year and associated with a high amount ofhumidityand
latent heat. Its air is a prolific bringer of precipitation into the middle latitudes as it
is transported away from its tropical origins. This air mass is usually unstable and
it has numerous summer thunderstorms. Ironically, it is this air mass that provides
most of the moisture falling as snow in the middle latitudes. The maritime Tropical
air is drawn intomiddle latitude cyclonesand lifted and mixed with polar air.
Continental tropical air (cT) is a hot, dry air mass associated with the subtropi-
cal deserts of the world. It produces the hottest temperatures on the planet while
not containing enough water vapor to produce significant precipitation. Record
summertime temperatures areassociated with the circulation of this air into the
middle latitudes. Although this air mass is unstable in the first few hundred meters
above the surface because of extreme heating of the surface, thought of as a whole,
continental Tropical air has great stability because of the subsidence of air into the
subtropical high.
Continental Polar air (cP) has source regions on the continents of the upper
middle latitudes and, with the maritime Polar air mass, continental Polar is behind
most of the cold fronts passing through the middle latitudes. This air is cold, dry,
and usually stable. Because of these characteristics, continental Polar air is not
associated with significant precipitation.
Maritime polar air (mP) has its source regions over the oceans of the upper
middle latitudes. It is moister and milderthan continental Polar air. Landfalling
middle latitude cyclones fueled by maritime Polar air can bring prolific precipita-
tion to mountainous coasts of the upper middle latitudes.
Air Masses 9