Why You Should Care
As the graphic illustrates, the weather is very affected by the jet stream, and knowing where it is can help you understand what's going on with the weather and, therefore, grant you great social success at parties. An example*:
Beautiful Member of the Desired Sex (BMDS): Awful weather we're
having, huh.
You: I wouldn't know; I never go outside.
BMDS: Well, it just got really cold all of a sudden, and it won't stop raining.
You: Don't worry. It'll start raining again in a couple of days, and after
that it'll be extremely warm.
BMDS: Really? How do you know?
You: I understand... THE JET STREAM!!!!!!1
BMDS: The... what?
You: The jet stream! Here's how it works! (You explain it to BMDS.)
BMDS: Oh, oh, how could I have lived so long without you, my love? I will
dump my current significant other right away and devote the rest of my existence
to you! Oh, heavens!
*Results not typical.
As will be made clear in the following section, the jet stream separates the cold polar air from the warmer subtropical air, so that the opposite sides of the jet stream have very different temperatures. It's cold to the north of it and warm to the south of it, so when you see a figure of the jet stream, if you can figure out which side of it you're on, you can tell whether you're in the cold part or the warm part. When you hear about a cold front or a warm front, that often refers to the bends of the jet stream and their direction of motion. One feature of this is that since warm air can hold a lot more water vapor in it than cold air, and warm air is less dense than cold air at the same pressure, when a front moves in, the warm air tends to rise above the cold. When the warm air goes up, it cools down, and since, as I said, warm air holds more water vapor than cold air, the water vapor condenses into liquid water droplets, also known in the field as "clouds". Eventually the water droplets in these "clouds" become bigger and fall to the earth as liquid water, known to experts as "rain". Thus, we often see rain near fronts, and depending on where you live, if you have cold weather for a while and it starts to rain, one possibility is that it will soon be warm weather.
Additionally, the jet stream serves as a storm track. When storms appear there, they follow the jet stream until they end, traveling from west to east (that's westerly flow, remember). If you're in Boston and your friend from New York tells you that it's raining torrentially there, most likely you should take your umbrella because that rain will probably come over you. The jet stream also aids in forming cold rotating storms around low pressure centers known as extratropical cyclones, and those, especially in the winter, like to bring snow, rain, and cold winds. While the jet stream sits at the top of the troposphere (the part of the atmosphere where weather happens), it can carry with it storms that affect even the surface.
You know what else happens at the height of the jet stream? Airplanes! Planes can use the jet stream to fly from west to east more quickly, or they can avoid it when flying east to west so that they don't go more slowly. You can't really see it very well here, but the pictures there are so pretty that they're worth a look.
But why is there even a jet stream? Ah, we'll see!