Air Masses Air Masses

• Large bodies of air characterized by similar temperatures and amounts of moisture at a given altitude • So large they can take days to move over an area causing constant weather • Can be 1600 km (~1000 mi) wide and several km thick

Movement of Air Masses

• A moving air mass carries temperature and moisture with it • As it moves, the characteristics of an air mass change – and so does the weather over the areas it passes Classifying Air Masses • Source region – the area over which an air mass gets its properties • Named by source region – Polar (P) air masses • Form at high latitudes • Are cold – Tropical (T) air masses • Form at low latitudes • Are warm Classifying Air Masses (cont.) • Also classified by surface – Continental (c ) air masses • Form over land • Dry air – Maritime (m) air masses • Form over water • Humid air Classifying Air Masses (cont.)

• Thus, there are 4 types of air masses – cP – continental polar – dry and cold – cT – continental tropical – dry and warm – mP – maritime polar - ? and ? – mT – maritime tropical - ? and ? • Well, actually there is a 5th – Arctic Polar (aP) – dry and cold Air Masses Continental Polar (cP) Air Masses • Begin in northern Canada, Alaska’s interior, & Siberia • Cold and dry in winter – clear skies and cold temps • Cool and dry in summer – days with cooler weather • Not usually associated with ↑ precipitation except for “lake effect snow”

Lake Effect Snow Maritime Tropical (mT) Air Masses • Warm, ↑ moisture content, unstable • Source of most precipitation in eastern part of US • The high temps and icky humidity found in FL, LA, etc. are caused by mT air masses

Rain storm over Florida keys Maritime Polar (mP) Air Masses • Some air masses affecting North America begin in Siberia – cP changes into mild, humid, unstable mP air – Brings low clouds & precipitation – Uplift  rain & snow on windward side of mountains Maritime Polar (mP) Air Masses • mP air masses also from North Atlantic off coast of eastern Canada • In winter, New England is on the northern or northwestern edge of a low-pressure system – Remember low pressure system cyclones rotate counterclockwise – This brings in mP air with snow and low temps – These are called “nor’easters” Continental Tropical (cT) Air Masses

• These don’t affect North America too much • Well, except in the southwestern US and Mexico • Sometimes they affect areas outside their sources causing – Hot temps and drought in the summer – Warm and mild temps in fall Arctic Polar (aP)

• Source region for Arctic (a) air is northern Canada, Siberia, and the Arctic • Colder than polar air masses • Develop over ice and snow covered ground • Dry Summary

• What is an air mass? How large are they? • How are air masses classified? • Which air masses are typically dry? • Which air masses are typically warm? • What causes lake effect snow?