NOTE: This Concept for the Day is a repeat of that which appeared in the previous week's Thursday Weather Summary.
Frequently, you may hear of a cold air mass moving southward from Canada or of Gulf (of Mexico) air dominating the Midwest. What is an air mass, and how can we detect the presence of an air mass on the surface weather map? Air masses are large volumes of air that are relatively uniform in their horizontal temperature and humidity characteristics. They are generally identifiable by the "Highs" or "H's" written on maps which represent the centers of large high pressure regions. These centers of high pressure are at the same time often locations of the centers of air masses.
Air masses form when air resides over a land or water surface with fairly uniform surface properties for several days or more. The air gradually assumes the temperature and moisture characteristics of that underlying surface. Sooner or later, these broad expanses of "conditioned" air begin to move. Masses of air from northern latitudes are usually cold and termed "polar" while those from southerly latitudes are warm or "tropical". If the surface of the source region were land, the air would probably be relatively dry or "continental", whereas ocean air would be more humid or "maritime".
Coterminous U.S. weather is mainly prone to continental polar air masses from central Canada, maritime polar air masses from the northern Pacific Ocean, or maritime tropical air masses from the Gulf of Mexico and their transitions. While rare, continental tropical air would develop over the desert Southwest and the Mexican Plateau.
For an additional explanation of air masses and their properties, call up the Thursday's optional Supplemental Summary Information .
To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Chapter 1 Progress Response Form, under section B. Daily Summary (found in Week 1 section of Part B and also on DataStreme Homepage).