Met Mailbag is your chance to have a Weather Plus Meteorologist answer your weather question. Each Thursday, our NBC Weather Pulse Blog will publish the answers to questions you send us. This week's question was answered by NBC Weather Plus Meteorologist Elise Finch.

Question: What is the jet stream?
Submitted by Margot B., California

Answer: Well Margot, jet streams are fast-moving currents of air in the upper levels of the atmosphere.

These streams of air were once thought to be similar to jets of water...so they were named jet streams!

But it's probably better to think of them as relatively narrow rivers of air that flow in a wavelike manner about seven to eight miles above the earth's surface.

Jet streams are caused by temperature differences between air masses, which means that jet streams divide colder air to the north from warmer air to the south.

During the summer, for example, the mid-Atlantic jet is typically located in northern Canada.

That position allows warm air from the south to move north and warm things up.

During the winter months, the jet stream dives south and positions itself in the southern-most portion of the United States, which allows cold air from Canada to sink south.

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