I was an AF aviation weather forecaster for 12 years, then 15 years as a dropsonde systems operator with the AF Reserve Hurricane Hunters.
By: Randy Bynon , 2:59 PM GMT on January 29, 2007
The podcast for this lesson can be found at
In the last few lessons we've talked about how the earth is heated and the reason for our global circulation. We also talked about the primary properties of our atmosphere that help to make our weather. But the global circulation theories only explain how weather forms at the equator or at 30 and 60 degrees north. What about the rest of the globe? We get weather everywhere. Well, that's what we'll get into in this lesson.
If you recall, in our last lesson on global circulation, we talked about the semi-permanent pressure systems that are established in the high and low pressure bands around the globe near the Equator, 30N and 60N, systems such as the Bermuda High and the Aleutian Low. These systems are "semi-permanent" because they exist in the zones of rising or falling air that result from the 3-cell global circulation theory and those circulations never change other than to shift north or south during the change in seasons.
An air mass is essentially a large bubble of air, usually at least 1000 miles across, that sits stationary over an area that has a uniform surface (i.e. large uniform land area with no mountains, large body of water, or a large region of snow covered land) for a long period of time. The region has to have a fairly uniform and gradual temperature, moisture, and pressure gradient surface and aloft. We call these regions source regions. As a bubble of air sits over this source region long enough, it acquires the temperature and humidity characteristics of the region. What these characteristics are depends on the source region. Meteorologists assign labels to these characteristics. The moisture content is addressed first and abbreviated with a small letter. A dry air mass is considered Continental (abbreviated “c”). A moist air mass is Maritime (abbreviated “m”). Then temperature is addressed and abbreviated with a capital letter. A warm air mass is called Tropical (abbreviated “T”). A cold air mass is called Polar (abbreviated “P”). Some common source regions for Continental Polar (cP) air masses are Siberia, Canada, and Greenland. Common source regions for Continental Tropical air masses are the Sahara Desert, Australia, Southwest Asia, and the US desert SW. Maritime Polar air masses form over the North Atlantic, the North Pacific and the Antarctic Oceans. Maritime Tropical air masses form in the tropical Atlantic, Pacific, and Indian Oceans.
So let’s put all this together! An air mass that sits over a large snowfield, like the continent of Greenland, would most likely be a dry, cold air mass. We would call that a Continental Polar air mass (abbreviated “cP”). An air mass that sits over a warm ocean, like the tropical Pacific, would be a Maritime Tropical air mass (abbreviated “mT”).
So we have an air mass that has a particular set of temperature and humidity characteristics. What now? The air mass itself doesn’t really cause any weather. Afterall, all the weather characteristics of the air mass are uniform by nature. So how do they cause weather? Weather happens when the air mass starts to move. Any air mass forms because pressure gradients in the source region were light meaning there was little air flow in the region either at the surface or aloft. But those conditions don’t last forever. At some point, upper level flow at the jet stream level will pick up and begin to move the air mass. As soon as the air mass moves out of it’s source region, it begins to modify but the air mass is large enough that it will take time to significantly modify it’s characteristics. In the meantime, it’s running into air with different characteristics. The boundary where these air masses meets is where the weather usually occurs and we refer to these boundaries as fronts.
And fronts will be the topic for our next lesson!
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