Meteo 3: Chapter 7
Analyzing weather above Earth’s
surface
Read: pp. 251-277 (ignore
confluence)
Why Care about Upper-Air?
 Talked about surface pressure and what
winds we expect to see at the surface
 Lots of atmosphere not at the surface that
affects weather conditions at the surface!
 Need to have ways to observe and analyze
the atmosphere kilometers from the surface
Maps of weather above the surface
 Upper-air weather conditions plotted on maps
of constant pressure
 Data obtained from radiosondes: heights,
temperatures, wind speeds and directions,
humidity
Constant Pressure Surfaces
 Simplifies math
 All upper-air maps plotted
on constant pressure
surfaces
 Mandatory pressure
levels: pressures where
radiosondes always take
observations
 Pressure decreases faster
with height closer to
ground
A 500 mb chart- Height contours, isohypses,
or heights = isopleths of equal height
Density decreases with height, so a 150 mb
layer must be thicker higher in the atmosphere
A volume of air will become larger when heated
 Pressure decreases more rapidly with height in
cold air columns than warm air columns
Warm columns of air taller than cold columns
A 500 mb chart- Height contours, isohypses,
or heights = isopleths of equal height
Temperatures near the Surface
Forming upper-level troughs/ridges
 Cold air masses move south, warm air
masses move north…forming upper-level
troughs/ridges
Relationship between heights and pressure
 On a constant pressure map, a minimum in height
corresponds to a low pressure center on a constant height
surface on an altitude equal to that height
– Treat a center of low heights on a constant pressure surface as if it
were a center of low pressure
 On a constant pressure map, a maximum in height
corresponds to a high pressure center on a constant height
surface at an altitude equal to that height
– Treat a center of high heights on a constant pressure surface as if it
were a center of high pressure
 Height gradients α pressure gradients
A contour map of
pressure on a constant
height surface looks the
same as a contour map of
heights on a constant
pressure surface.
Therefore, the relationship
between the wind and
pressure fields is the same
as the relationship
between the wind and
height field.
Upper-level winds: Geostrophic approximation
 Forces on parcel: pressure (height) gradient
force….Coriolis force increasing in magnitude until it
becomes equal in magnitude but opposite in direction to
PGF…wind blows parallel to height contours with lower
heights to left
PGF
5400 m
5460 m
COR
5520 m
A 500 mb chart- with upper-air wind
observations
Geostrophic wind (wind aloft)
 Geostrophic wind: Wind that results due to a
balance between the height gradient and Coriolis
force
– Good approximation to wind above the ground
– There is some friction to throw this balance off, but its
effects are minimal at high altitudes
– Near the ground, friction causes wind to cross isobars
toward lower pressure at ~30º angle
Upper-level wind speed & Jet Stream
 Horizontal temperature gradients α height gradients AND
the larger the height gradient, the faster the wind speed
 Wind speeds increase with altitude up to about 250 mb
because height gradient on constant pressure surface
increases with altitude
Jet Stream
 When warm and cold air
masses collide, the
strongest winds occur just
below the tropopause (the
top of the troposphere,
about 250 mb)
 This fast flowing river of air
over mid-latitudes = midlatitude jet stream
 Subtropical jet?
250mb Heights and Wind Obs
More on jet streams
 Embedded in mid-latitude “westerlies”
 Only several hundred kilometers wide, thousands of km
long
 Discontinuous
 Sharp surface front underneath jet stream
 Moves south in winter, north in summer
 Stronger in winter than summer
 Jet Streak: Pocket of faster winds embedded in the jet
stream
– Located in regions with enhanced height gradients at ~ 250 mb
Summer and winter jets
high-amplitude pattern (major storms
possible); “meridional pattern”
low-amplitude pattern (no major
storms); “zonal pattern”