Jetstreams

[Vcl]

Could someone please give me a simple explanation as to why jetstreams form at certain times of the year and what causes their formation. THanks in advance.

[AltoAdige]

The reason Jetstreams form is because of the tropopause. The tropopause is at its lowest at the poles and at its highest at the equator. If i remember its around 25,000feet at the poles 60,000+ at the equator...well anyway between the poles and the equator as the tropopause altitude changes, there are areas where the tropopause may dip steeply, for example at 40 deg. N it may be at 39,000 feet while at 39 deg. N it may be at 31,000 feet, then at 38 deg. N it will be at 41,000.....try to picture this one in your head :-) Now those altitudes...say they are like this running west to east....they act like a river( the middle of the river being a low altitude while its sides being at a very high altitude........this funnels the wind and the coriolis does the rest.
As to why the jetstream changes season to season, im not sure but i always remember SSW....Strong South Winter, thats how the jetstream is in the winter.

Alto Adige

[Checkboard]

It's mostly to do with the general circulation in the atmosphere.

If you were to cut the Earth in half, and look at the edge, you would see the hot air near the equator rising into the atmosphere, then spreading out towards the high latitudes (i.e. travel north or south) before sinking back towards the Earth and heading back to the equator. This is the "Tropical Cell".

At the Poles, you would see the cold air sinking, then spreading out to head along the ground towards the equator before warming up and rising to return to the poles at altitude. This is the "Polar Cell".

Between these two cells is the "Sub-Tropical Cell", which is driven like a cog between the Tropical and Polar Cells.

Now as you would know from a sea level pressure chart (like the ones on the TV weather), the closer the isobars are, the stronger the wind speed is. The (Geostrophic) wind speed is controlled by pressure difference, and a high pressure difference (close isobars on the sea level chart) drive strong winds. With me so far?

Now cold air is dense compared to warm air, so if you have a column of cold air next to a column of warm air (both at the same pressure at sea level) then the higher up the columns you go, the greater the pressure difference will be between the two, as the pressure will be dropping faster in the cold column than the warm one. This means that at the boundary of the cells mentioned above (which have significantly different temperatures/densities) there is a large pressure difference at altitude - which powers a very strong wind.

Between the Polar and the Sub tropical cell, you get the Polar jetstream, and between the Tropical and Sub-tropical cells you get the Sub-tropical jetstream. The jetstreams move north and south as the sun moves north and south with the seasons (relatively speaking). Also, as the land and the sea heat up at different rates, the position and speed of the jets is effected by the various land masses around the Earth.

The jetstreams obey Buys Ballot's Law, and so run from west to east in both the Northern and Southern hemisphere.

[RadarContact]

This might help:

http://www.usatoday.com/weather/wwind0.htm

Or just do a quick search on "Jet streams" in your favourite search machine :)