We’ve all experienced weather of all types, from cold, dry winter days, to hot, humid summer days. In general, these patterns are the product of large- scale atmospheric motions, both vertically and horizontally. In this writing, I want to talk a little about the types of large-scale air masses that contribute to our weather, without regard to small-scale influences, i.e., sea breezes and other “localized” effects.
Air masses that we come under the influence of in varying degrees and time spans range from “Maritime Tropical” to “Continental Arctic.” Let’s see just what these air masses are made of, and where they come from.
Maritime tropical air masses typically have temperatures ranging from 72-86 degrees F. In general, these air masses range from marginally stable to very unstable. It is in these air masses that moisture is abundant, and rains and thunderstorms are plentiful. The source regions for these air masses (at least for the ones that affect us) are the tropical and subtropical oceans. Our severe weather outbreaks, i.e., thunderstorms and tornadoes, occur when this kind of air mass is in place, and other parameters, i.e., air masses intrude.
The hot and dry air masses of the desert southwest sometimes move east, but we rarely feel the effects here. These air masses are referred to as “Continental Tropical.” Temperatures typically range from 86-110 degrees F. These air masses can be very unstable, however, there is little moisture to work with to produce rain and thunderstorms. The source regions, of course, are the subtropical deserts. Also note, these air masses are poorly developed during the winter months.
An air mass that doesn’t get a lot of attention around here, but we do feel every so often, is called “Maritime Polar.” These air masses form over the oceans poleward of 45-50 degrees latitude. The temperatures of these air masses range from 32-50 degrees F during the winter months, and 36-57 degrees F during the summer months. They are generally stable, and most often impact the Pacific Northwest and the northern half of the country. They can bring copious rains and snows to Washington and Oregon.
During the winter months, the very cold air masses moving south are called “Continental Arctic” air masses. The source regions of these air masses that affect the U.S. are the Arctic Basin and interior North American north of 55 degrees latitude. The source region temperatures of these air masses range from –31 to –67 degrees F (actual temperature!). These air masses are VERY dry and stable. It is in these air masses, by the way, that the most beautiful deep blue skies are witnessed. That has to do with the highly dense nature of the air molecules in such cold air. Also, it is the intrusion of these air masses that interact with the Gulf Stream and upper-level energy that develop nor’easters.
The last air mass to discuss is the “Continental Polar” type. These form in the interior of continents, between 45 and 55 degrees latitude. They range in temperature from –4 to –31 degrees F in winter, and 41-59 degrees F in summer. They are basically stable air masses, but in summer can become somewhat unstable and produce some “cool” day thundershowers. These air masses don’t affect us, instead staying generally in the northern portions of North America.