Читать книгу Weather For Dummies - John D. Cox - Страница 45

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.

 The dry air absorbs moisture from the lake, which fuels the snowfall and gives the air more latent heat energy that is released as it condenses into a cloud.

 Flowing over the warmer lake water, the temperature of the cold air rises, and as it warms, the air lifts higher into the sky and becomes more unstable.

 After blowing over the smooth surface of the lake, the air plows into the uneven surface of the land, slows down in the friction, and piles up, rising farther into the sky.

 And the cold air gets another lift from the hills and higher terrain it encounters above the far shores.

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.