Some terminology for directions
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The east-west direction is often called the zonal direction
­ zonal means parallel to latitude circles
The north-south direction is often called the meridional direction
­ meridional means parallel to longitude circles
Conventions for naming wind directions:
So for example, a wind blowing toward the east is.......
So what do the winds in the atmosphere typically look like (on average)?
North Pole
Equator
South Pole
West - East
Winds on the 500 mb surface as averaged over 1979-2001
colors: wind speed
So what do the winds in the atmosphere typically look like (on average)?
North Pole
Equator
South Pole
West - East
Winds on the 200 mb surface as averaged over 1979-2001
colors: wind speed
So what do the winds in the atmosphere typically look like (on average)?
200 mb
500 mb
1000 mb
North Pole (NP)
Equator (EQ)
South Pole (SP)
Winds in vertical cross-section as averaged over 1979-2001
green: westerly (from west to east)
blue: easterly (from east to west)
Jet Streams
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Clearly, the circulation in both hemispheres is dominated by
strong winds in the upper troposphere, between 30o and 60o
latitude (roughly)
As you've probably guessed, these strong wind bands are
called the jet streams
So why are the jet streams there in the first place?
So to summarize:
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Using nothing but the north-south temperature difference, we've
diagnosed that the winds at upper levels blow west to east.
This is our jet stream.
And since the temperature difference is mostly concentrated
between 30o and 60o (in both hemispheres), that's where the
jet stream is also.
If we repeated the argument for different pressure surfaces at
different heights, we'd find that the winds are strongest in the
upper troposphere, near the 200 mb level (roughly 10 to 12
km).
East
NP
EQ
West
3D view of relation between temperature gradients,
pressure surfaces and winds at upper levels.
Jet Streams
....so the jet streams are associated with temperature gradients
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For the weaker subtropical jet, the temperature contrast is mainly
limited to the upper troposphere---i.e., it doesn't extend to the
ground. On the other hand.....
The temperature contrast for the polar jet stream is deep,
extending through the entire troposphere.
­ The fronts we see on surface charts are typically
associated with the polar jet
­ So unless stated otherwise, when we refer to the jet
stream, we're usually referring to the polar jet
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Note that since the jet depends on temperature contrasts, it's
position migrates north and south with the seasons.
Average positions of the polar jet stream in
summer (orange) and winter (blue)
But on any given day, the low-level temperature gradient will
have some pattern, with the pattern changing every day.
Example temperature distribution at the ground, and.....
temperature
gradient
And whatever the temperature pattern is for that day, the jet
stream will have more or less the same pattern.
…..and the associated winds at 300 mb.
jet
stream