نطق دوم

Lecture 2:

Listen to part of a lecture in an Earth science class.

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

Continental polar air masses originate, form over a continent near the poles, over areas that are typically cold and dry, and it’s the polar air masses from the Artic and northern Canada that largely influence weather patterns in the United States, especially in the eastern, east of the Rocky Mountains.

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

However, when a polar air mass sweeps across the Great Lakes Region in late autumn and winter, it can bring extremely heavy snowfalls to that region. We refer to these snow storms as “lake-effect snows”, snows whose moisture comes from the Great Lakes themselves.

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, these two cities are among the snowiest city in America, thanks largely to the lake effect. Over one recent 10 years period, Rochester and Buffalo each received like 27 meters of snow. It’s nearly 90 feet.

Okay, let’s turn to the other pair of cities on this map: Thunder Bay and Marquette.

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?

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. 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 and winter, the land loses its heat quickly. 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. So eventually, you get these significant temperature disparities between the lakes and the land that surrounds each one. There can be about an eight degree Celsius temperature difference at the southern Great Lakes and up to about a seventeen degree disparity farther north where the water’s still warm but the land is particularly cold.

Then here comes the dry continental polar air mass, cold air, moving southeastward across the fairly warm lakes. Polar air masses generally move from the north and west to the south and east. Okay, and, warm air rises, right? So, the warm moist air from the lake rises up into the cold dry air above it. Then the difference in temperature inside the air mass creates instability, an unstable and turbulent situation in the atmosphere. 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. And because they’re downwind, cities to the south and east of the Great Lakes will get more snow. So because of their location, Marquette, Buffalo and Rochester get buried each winter while Thunder Bay on Lake Superior northern shore, doesn’t get hit as hard.

Oh, I should mention that many scientists predict that lake-effect snowstorms will increase in frequency and intensity over the course of this century. They point 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 a longer periods of time each winter. So winters around the Great Lakes will keep getting wetter which means more snow.

Or does it? Let me point out that, if air temperatures keep increasing in the next hundred years, polar air mass temperatures will too and winters will be much milder. And to get snow, even though these areas gets a lot of precipitation, the air would still have to be cold enough to support snowfall,as opposed to rain.

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