[chris (OP)]

Dear all

I have a challenge to place before you:

I need a quick, simple and efficient way to explain the jet stream to a young audience.

Who can help?

[jeffreyH]

They are narrow streams of air. Several of which are in the northern and southern hemisphere. They are located in the tropopause. This is the boundary between the troposphere and the stratosphere. This region lies between 5.6 and 11 miles up depending upon where you are on the surface of the earth.

[evan_au]

Jetstreams are strong high-altitude winds, which take a wandering path around the Earth's poles. They sometimes affect the weather we experience on the ground, triggering floods, droughts and other extreme weather events. Jetstreams must be taken into account when planning the paths of airliners, and can cause the clear-air turbulence that is sometimes experienced by jets.

Jetstreams also occur on other planets - but especially beautiful are the close-up color videos taken by space probes around Jupiter.

Where do jetstreams come from?
When air heats up from the Sun, it becomes lighter (we say "less dense") and the hot air rises.
- This air flows towards the Earth's poles, where it cools down, becomes more dense, and falls back down towards Earth's surface, then flows back towards the equator. The cycle then repeats, forming large cells of air circulation.
- There are 3 main circulation cells on Earth's northern hemisphere (and a lot more on Jupiter).
- At the boundary of these cells, rising hot air occurs next to falling cool air, and there is a lot of turbulence. The edges of these cells are continually moving around.
- The Jet stream of fast-moving air occurs at the edges of these circulation cells, and moves north and south with the changing boundaries of these cells. This sometimes brings the jetstream over more populated areas, and other times keeps it near the poles, where not so many people live.
 
See: https://en.wikipedia.org/wiki/Jet_stream
NASA video (2 minutes):

[alancalverd]

Sadly, there isn't a satisfactory simple explanation, but a logical partial one can help.

Start with a tall glass of warm water and pour some cold water into it. Feel the outside of the glass. The cold water sinks to the bottom because it is denser (you can get away with "heavier" for a very young audience) than warm water. Air does the same thing: cold air sinks under warm air.

Now the north pole (sorry, Evan and fellow antipodeans, you'll have to multiply everything by -1!) is cold, so the air above it is dense, and the tropics are warm, so tropical air is less dense. Cold air gradually oozes southwards near the ground, pushing the warm air upwards, and at high altitude there is a movement of warm air northwards.

Now carefully peel an orange with a knife. Take a slice 1 cm wide around the "pole", and another 1 cm slice around the temperate latitudes, The temperate slice is a lot longer. Imagine that the atmosphere is like orange peel, and the orange is rotating at the same rate as the earth. A point at 80 deg latitude only travels about 3 cm in 24 hours, but a point at 45 deg latitude travels about 10 times as far in the same time. So the air that is moving northward at high altitude, is also travelling eastward  at about 300 mph when it starts to move north, and that 300 mph westerly wind is the origin of the jet stream.

It's actually a lot more complicated because symmetry demands that there are 6 circumferential, stable circulatory cells, and differential heating between land and sea produces local regions of high and low pressure that distort them, so the high altitude winds (above 30,000 ft) tend to wander about somewhat in response to changes in the weather systems underneath, and also move the weather systems around the planet.

[evan_au]

sorry, Evan and fellow antipodeans, you'll have to multiply everything by -1!

Large-scale air currents are fairly symmetrical between Northern and Southern hemispheres (but there will be local differences due to mountain chains, etc).
- But since the greatest population density is in the Northern hemisphere, the Northern populations are more affected by the vagaries of the Northern jetstream
- And long-distance airline routes between points in the Northern hemisphere are more likely to run into it, as the shortest path tends to take them over the North Pole.
- There aren't many heavily-populated areas in the areas where the southern jetstream travels - it's mostly the Southern Ocean.

However, ocean currents are strongly affected by the distribution of land masses, so the Southern Ocean basically circulates around Antarctica, almost unobstructed.