NARRATOR: Listen to part of a lecture in an earth science class.

旁白：听一段地球科学课程中的讲座。

MALE PROFESSOR: 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.

男教授：正如你们从阅读中所知，气团是……它是一个具有统一温度和湿度特性的大气团。

There are four types of air masses, and the one we’ll concentrate on today is the continental polar air mass.

有四种类型的气团，我们今天要重点讨论的是大陆极地气团。

MALE PROFESSOR: Continental polar air mass is originate, er, form over a continent near the poles, over areas that are typically cold and dry.

男教授：大陆极地气团起源于极地附近的大陆，形成于通常寒冷干燥的地区。

And it's the polar air masses from the arctic and northern Canada that largely influence weather patterns in the United States, especially in the east of the Rocky Mountains.

正是来自北极和加拿大北部的极地气团在很大程度上影响了美国的天气模式，尤其是在落基山脉以东的地区。

MALE PROFESSOR: Now, these continental polar air masses are not generally associated with heavy precipitation.

男教授：现在，这些大陆极地气团通常与强降水无关。

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.

事实上，由于它们在穿越北美洲的过程中通常保持干燥和寒冷，它们带来了我们冬季在美国东部大部分地区看到的晴朗天空和较低的温度。

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.

然而，当极地气团在深秋和冬季横扫五大湖区时，它可能会给该地区带来极其大量的降雪。

We refer to the snow storms as lake effect snows, the snows whose moisture comes from the Great Lakes themselves.

我们将这些雪暴称为湖效应雪，其水分来自五大湖本身。

MALE PROFESSOR: 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 on the far eastern shore of Lake Erie.

男教授：让我解释一下这是如何发生的。但首先，这里是该地区的地图。你们看到罗切斯特位于安大略湖南岸，布法罗位于伊利湖的最东岸。

These two cities are among the snowest cities in America, thanks largely to the lake effect. Over one recent 10-year period, Rochester and Buffalo each received like 27 meters of snow. It's nearly 90 feet.

这两座城市是美国降雪最多的城市之一，这主要归功于湖效应。在最近的一个十年期间，罗切斯特和布法罗各自降雪量达到了约27米。这接近90英尺。

MALE PROFESSOR: Ok. Let's turn to the other pair of cities on this map, Thunder Bay and Marquette.

男教授：让我们转向地图上的另一对城市，桑德贝和马凯特。

As you can see, they're both on the shore of Lake Superior. But Marquette consistently gets two to three times more snow than Thunder Bay. Why?

正如你们所见，它们都位于苏必利尔湖的岸边。但马凯特的降雪量始终是桑德贝的两到三倍。为什么？

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.

基本上，夏季时，水体，尤其是像五大湖这样的大水体，吸收了大量的能量，既来自太阳，也来自经过它们的温暖夏季空气。

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. As a result, during the cold seasons fall in winter, the land loses its heat quickly.

因此，湖泊成为巨大的热量储存库，而湖泊周围的陆地则不然。陆地不像水那样有效地储存热量。因此，在寒冷的季节，即秋季和冬季，陆地迅速失去热量。

The land gets cold fast, and as you go farther north, it gets very, very cold. But the lakes store heat more efficiently, so they stay warmer, longer.

陆地迅速变冷，越往北走，天气变得非常非常冷。但湖泊更有效地储存热量，所以它们保持温暖的时间更长。

So eventually you get these significant temperature disparities between the lakes and the land that surrounds each one.

最终，湖泊和周围陆地之间会出现显著的温度差异。

There can be about an 8 ℃ temperature difference at the southern great lakes, and up to about a 17 degree disparity farther north, where the water is still warm, but the land is particularly cold.

在五大湖南部，温度差异可能约为8℃，而在更北的地方，温度差异可能高达17度，那里的水仍然温暖，但陆地特别冷。

MALE PROFESSOR: Then here comes the dry continental polar air mass, cold air, moving southeastward across the fairly warm lakes.

男教授：然后，干燥的大陆极地气团，冷空气，向东南方向移动，经过相对温暖的湖泊。

Polar air masses generally move from the north and west to the south and east. Okay. And warm air rises, right?

极地气团通常从西北向东南移动。好的，暖空气上升，对吧？

So the warm, moist air from the lake rises up into the cold, dry air above it.

因此，湖泊中温暖潮湿的空气上升到其上方寒冷干燥的空气中。

Then the difference in temperature inside the air mass creates instability, an unstable and turbulent situation in the atmosphere.

然后，气团内部的温度差异造成了不稳定性，即大气中的不稳定和湍流情况。

As the warm air rises through the cold air, it cools down and condenses, forming clouds. In the clouds, snowflakes form and snowfall occurs over the lake and on the downward shores.

当暖空气穿过冷空气上升时，它会冷却并凝结，形成云。在云中，雪花形成，降雪发生在湖面上和顺风的湖岸上。

Because they're downwind, cities to the south and east of the Great Lakes will get more snow.

由于它们是顺风的，五大湖以南和以东的城市会得到更多的雪。

So because of their location, Marquette, Buffalo and Rochester get buried each winter, while Thunder Bay on Lake Superiors, northern shore doesn't get hit as hard.

因此，由于它们的位置，马凯特、布法罗和罗切斯特每年冬天都会被大雪覆盖，而位于苏必利尔湖北岸的桑德贝则不会受到如此严重的雪灾。

MALE PROFESSOR: I should mention that many scientists predict that lake effect snow storms will increase in frequency and intensity over the course of this century.

男教授：我应该提到，许多科学家预测，本世纪湖效应雪暴的频率和强度将会增加。

They point out that the average temperature of the Great Lakes surface waters increased during the last century, while ice cover decreased.

他们指出，五大湖表面水的平均温度在上个世纪有所上升，而冰覆盖则减少。

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.

因此，如果这种情况继续下去，我们当然不能排除这种可能性，冬季湖泊冻结的水将减少，而蒸发到大陆极地气团中的水分将增加，时间也会更长。

So winters around the Great Lakes will keep getting wetter, which means more snow.

因此，五大湖周围的冬季将会变得越来越湿润，这意味着更多的雪。

MALE PROFESSOR: Or does it? Let me point out that if air temperatures keep increasing in the next hundred years, polar air mass temperatures will, too.

男教授：或者真是这样吗？让我指出，如果空气温度在未来一百年内继续上升，极地气团的温度也会上升。

And winners will be much milder and get snow. Even though these areas get a lot of precipitation, the air would still have to be cold enough to support snowfall as opposed to rain.

冬季将会变得更加温和，降雪也会减少。尽管这些地区降水量很大，但空气仍然必须足够冷才能支持降雪而不是降雨。