Download Chapter 12: METEOROLGY 12.1 Air masses Weather is the short

Chapter 12: METEOROLGY
12.1 Air masses
Weather is the short term variations in the Earth’s atmosphere. It can change within minutes, hours, days, weeks,
months or years. Climate is the long term average variations in weather for a particular area. Scientists use 30 year
averages to define climate for a particular area.
Air over a warm surface can be heated by conduction. The heated air rises because it is less dense than the surrounding
air. When this happens over thousands of square miles, for days or weeks, it is called an air mass. An air mass is a large
volume of air that has the same characteristics as the source region over which it forms.
Types of air masses: There are 5 types of air masses that influence weather in North America.
Tropical air masses: The origins of maritime tropical air are tropical bodies of water. In summer, they bring hot,
humid weather to the eastern 2/3 of North America. The southwestern US and Mexico are a source of continental
tropical air which is hot and dry, especially in summer.
Polar air masses: Maritime polar air masses form over the cold water of the North Atlantic and North Pacific.
The one that forms over the North Pacific primarily affects the west coast of the US bringing heavy rains in the winter.
Continental polar air masses form over the interior of Canada and Alaska. In winter, these air masses can carry frigid air
southward. In the summer, cool dry continental air masses bring relief to hot, humid weather.
Arctic air masses: Earth’s ice and snow covered surfaces above 60o N latitude in Siberia and the Arctic Basin are
the source regions of arctic air masses. During part of the winter, these areas receive almost no solar radiation but
continue to produce thermal energy. Because of this, they can become extremely cold and bring frigid temps in winter.
Air mass modifications – Air masses do not stay in one place forever. They move, transferring thermal energy from one
area to another. When the air mass moves over land or water that has different characteristics than its own, the air
mass can acquire some of the characteristics of that land or water. When this happens, it will exchange thermal energy
and or moisture with the surface over which it travels.