LITMIR.BIZ

      FIGURE 3-2: The map shows the different air masses that affect weather in the continental United States and helps explain why the nation gets so much dramatic weather.

      LAKE-EFFECT SNOWS

      In late fall and early winter, people who live along the southern and eastern shores of the Great Lakes are very familiar with the effects of big expanses of water on the air. The continental polar air mass that brings cold, clear winter days to the Midwest has a very different look to it by the time it crosses the Great Lakes.

      Even after — or if — the lakes freeze over, still they affect the amount of snowfall in the region. Four main weather-generating processes are at work when the cold, dry Canadian air flows over the lakes and the regions along the southern and eastern shorelines.

      In the summer, the Great Lakes have a very different effect on the weather of the region. Because the water of the lakes is often cooler than the surrounding air this time of year, they can dampen thunderstorm activity.

Maritime tropical air masses from the Pacific sometimes sweep northeastward over California, and when they do, they can bring heavy rains and flooding. Similarly, maritime tropical air from out of the Gulf of Mexico and Caribbean Sea can fuel flooding storms as it moves up the Mississippi and Ohio valleys.

      Summer air masses

      The continental polar air that brought frigid misery during winter often has a welcome cooling effect over much of the United States in summer. Fair weather generally prevails over the Northern Plains and Midwest when continental polar air is around, occasionally reaching even the Gulf of Mexico with pleasant dry air.

      Maritime tropical air masses from the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico spend a lot of time over the eastern two-thirds of the United States during summer, especially the Southeast, bringing warm temperatures and high humidity.

      Continental tropical air forms over the big desert regions of northern Mexico and the U.S. Southwest in summer, keeping things hot and dry.

News from the Fronts

      During the most powerful storms of winter and summer you will probably find two air masses of very different qualities — different temperatures and humidities — crashing into one another. When upper winds and other conditions are right, this is where you will see a powerful mid-latitude cyclone with well-defined fronts that bring rain or snow over big sections of the country. In spring and summer, along these fronts can be long lines of severe thunderstorms that can form tornadoes when conditions are right — or wrong!

      It might look like a rainy or snowy mess from down below, but a front, the transition zone or line between two distinct air masses, has a certain shape or structure to it, depending on what’s going on.

      Cold fronts

When a cold front moves into space occupied by warmer air, it forces the warmer air to rise up and over the leading cold wedge. This is because cold air is denser, or heavier, than warm air and sort of bullies it out of the way. As the warm air rises, it mixes with the cold air and condenses into big clouds. Figure 3-3 shows what happens with the passage of a typical cold front.