Jet Streams Flashcards
What is the definition of a jet stream?
A horizontal, high speed (60+kts), and tubular current of air in the vicinity of the tropopause.
How does a jet stream form?
When there are high temps in one place pushing the tropopause up and cold temps in another place pulling the tropopause down, the tropopause literally snaps and warm air rushes into the stratosphere, cools, and sinks back down. Then it warms and rises, lather rinse repeat.
Then because of the coriolis force, it gets dragged to the right (from west to east ((the polar jet stream does))) as warm air rises, cools, falls etc. This creates a tube of corkscrew air.
What are the characteristics of a jet stream?
- Width - few hundred mile
- Depth - 2 to 6 miles
- Altitude - 10 to 30,000ft (depends on season/geographic area since the jet stream will be around the tropopause, which changes height)
- At least 60kts
- Always located on the warm air side and 2000-3000ft below the tropopause
- Ragged, windswept cirrus clouds are an indication of very strong winds or jet streams and always form on the warm side of the jet stream
- If you are flying with a tailwind in a jet core, the cirrus will always be on your right hand side
How do the winds on either side of the jet stream change?
Cold side: Steeper isotach gradient/winds drop off faster, more wind shear
Warm side: less wind shear
Another diagram of how a jet stream forms
** There should be a break between the Polar and Ferrel Cells but they didnt put one in for some reason
What kind of movement does seasonal changes in temperature produce in the jet streams?
In the winter, with colder temps, particularly over the polar and ferrel cells, the tropopause lowers. So jet streams will be at a lower altitude in the winter than in the summer.
Also the polar jet will sink and spread out towards the warmer ferrel cell. Ferrel cell will sink a bit and spread into the warmer hadley (equator) cell. So in winter, jets will move south towards the equator. In the summer, the reverse will happen.
How do the speeds change in a jet stream with seasonal changes?
In winter jet streams are faster because of bigger pressure and temperature differentials
Another diagram of jet stream formation
How do temperatures in the jet stream core behave?
They more or less are stable/stay the same
Which is the most important jet stream in the northern hemisphere?
The polar jet stream, and influences our weather significantly
Diagram of jet stream core
If the isotachs are spaced closer together than __NM on the 250mb chard, you can expect…?
If the isotachs are less than 90NM apart you can expect enough horizontal shear to create significant amounts of CAT
Vertical shears in in a jet stream varies from…
5kt/1000ft up to 20kt/1000ft
Where is CAT (clear air turbulence) most likely to occur in a jet stream?
In an upper trough on the poleward side of the stream
If you are flying in the jet stream with a tail wind, which direction should you turn to get out of a turbulent area?
To the right
Common jet stream turbulence locations
If approaching a jet stream core from the side to pass through it what should you do to avoid vertical turbulence?
Watch the OAT.
- If the temp rises, climb to get out of turbulence
- If the temp drops, descend
- If the temp doesn’t change, do either
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