An air mass is a very large body of air having similar temperature and moisture content throughout its reach. While lacking a fixed size, air masses typically cover thousands of square kilometers or miles, sometimes even stretching over the majority of a country or region. Of the four major types of air masses, one in particular tends to influence the weather of the Pacific coast more than the others.
Air Masses: The Facts
Meteorologists generally divide air masses into four major categories: continental tropical, continental polar, maritime tropical, and maritime polar. Occasionally, they may use a fifth category, continental arctic, to describe extreme bone-chilling cold. Air masses develop over large areas with flat, uniform composition, such as oceans or vast plains, and take on the temperature and humidity characteristics of those regions. Air masses forming over land are dry, while those developing over oceans are more humid. Similarly, air masses developing near polar regions contain cold air, while those near tropical regions contain warm air. Air masses don’t remain hovering over the area in which they developed; they migrate to other areas, propelled by the atmospheric forces. As they move, they take on characteristics of their new environment—for instance, a polar air mass becomes warmer the farther south it travels.
Maritime polar air masses, abbreviated mP on weather maps, tend to dominate the Pacific coast region. These air masses form over a cold ocean current in the northern Pacific Ocean. Although mP air masses originate at northerly latitudes, they do not feature as cold of air as continental polar air masses. The prevailing westerly winds carry them eastward to the Pacific coastline, where they deliver relatively moist, cool air. As maritime polar air masses move southward and inland, they interact with the drier, warmer air already in place, modifying their impact. Maritime polar air masses have the capability to produce cloudy, damp, or foggy conditions, with fog more likely near the shoreline.
Additional Significant Air Masses
The southern portion of the Pacific coast—particularly southern California—can be influenced by maritime tropical air masses, abbreviated mT. These warm, moist air masses originate in the subtropical Pacific Ocean. They may bring fog or low clouds with them. Thanks to the prevailing westerlies, continental air masses almost never influence the weather on the Pacific Coast. On rare occasions, however, if the wind blows from the southeast, dry continental tropical air may make its way from Mexico to southern California’s Pacific coast.
The mountains lining the Pacific coast cause the region to experience very different weather than interior regions of the West. When maritime air masses travel inland, the air is forced upward over coastal mountain ranges, where it produces precipitation, hence losing its moisture content and taking on characteristics more typical of continental air masses. Sometimes, this change is apparent in just a few short miles, depending on the proximity of the mountains to the sea. The coastal ranges essentially comprise a geographic barrier, which explains why the weather of areas nestled along the Pacific coast usually differs so greatly from the weather of areas east of the mountains.