托福official64听力lecture2 continental polar air mass原文解析+翻译音频

[00:00.00]Listen to part of a lecture in an earth science class.
[00:04.29]As you know from your reading, an air mass is, it's a large body of air that's got uniform properties in terms of temperature and moisture. [00:15.29]There are four types of air masses. And the one we will concentrate on today is the continental polar air mass. [00:24.42]Continental polar air masses originate form over continent near the poles over areas that are typically cold and dry. [00:34.81]And it's the polar air masses form the arctic in northern Canada that largely influence weather patterns in the United States, especially in the eastern, east of the Rocky mountains.
[00:48.78]Now, these continental polar air masses are not generally associated with heavy precipitation. [00:55.87]In fact, since they generally remain dry and cold throughout their track across north America, they bring the mostly clear skies and cooler temperatures we see throughout most of the eastern United States during the winter months. [01:10.94]However, when a polar air mass sweeps across the great lakes region in late autumn and winter, it can bring extremely heavy snow falls to that region. [01:28.98]We refer to the snow storms as lake effect snows, snows whose moisture comes from the great lake themselves.
[01:30.15]Let me explain how this happens. But first, here's a map of the region, You see Rochester on the southern shore of lake Ontario and Buffalo and the far eastern shore of lake Erie. [01:42.10]These two cities are among the snowiest cities in America, thanks largely to the lake effect. [01:48.94]Over one recent ten years period, Rochester and buffalo each received like twenty seven meters of snow. It's nearly ninety feet. [01:59.80]Okay, let's turn to the other pair of cities on this map, Thunder Bay and Marquette.
[02:06.38]As you can see, they're both on the shore of lake Superior. [02:09.84]But Marquette consistently gets two to three times more snow than thunder bay. Why? [02:17.12]Uh, basically what happens is that during the summer, bodies of water, especially ones as large as the great lakes, absorb huge amounts of energy, both from the sun and from the warm summer air that's passing over them. [02:32.76]So the lakes become these giant reservoirs of heat, whereas the land around the lakes, well, land doesn't store heat as effectively as water does. [02:44.78]As a result, during the cold seasons fall and winter, the land loses its heat quickly, the land gets cold fast, and as you go farther north, it gets very, very cold. [02:59.41]But the lake store heat more efficiently, so they stay warmer longer. [03:05.07]So eventually you get these significant temperature disparities between the lakes and the land that surrounds each one. [03:12.80]There can be about an eight degrees Celsius temperature difference at the southern great lakes, and up to about a seventeen degree disparity farther north, where the water is still warm. But the land is particularly cold.
[03:27.48]Then, here comes the dry continental polar air mass, cold air, moving southeastward across the fairly warm lakes. [03:38.14]Polar air masses generally move from the north and west to the south and east. [03:43.59]Okay, and warm air rises, right? so the warm, moist air from the lake rises up into the cold, dry air above it. [03:54.45]Then the difference in temperature inside the air mass creates instability and unstable and turbulent situation in the atmosphere. [04:04.32]As the warm air rises through the cold air, it cools down and condense, is forming clouds. [04:12.64]In the clouds, snowflakes form, and snowfall occurs over the lake and on the downward shores. [04:20.62]And because they're downwind, cities to the south and east of the great lakes will get more snow. [04:28.76]So because of their location, Marquette, Buffalo, and Rochester get buried each winter, while Thunder Bay on lake superior's northern shore doesn't get hit as hard.
[04:42.24]Oh, uh, I should mention that many scientists predict that lake effect snow storms will increase in frequency and intensity over the course of this century. [04:52.42]They pointed out that the average temperature of the great lakes surface waters increased during the last century, while ice cover decreased, so if this continues, which we certainly can't rule out, there will be less lake water freezing and more that evaporates into the continental polar air masses for longer periods of time each winter. [05:17.41]So winters around the great lakes will keep getting wetter, which means more snow, or does it? [05:26.44]Well, let me point out that if air temperatures, keep increasing in the next hundred years, polar air mass temperatures will too. [05:35.29]And winter will be much milder and get snow. Even though these areas get a lot of precipitations, the air would still have to be cold enough to support snow fall as opposed to rain.