Transcript Upper Level Winds

Upper Level Winds
Lessons 22,23,24
Upper Level Winds
Upper wind
• Upper winds are driven by pressure
differences aloft.
• Upper level pressure gradients are produced
by the horizontal temperature distribution in
the upper atmosphere.
• These temperature differences aloft are
referred to as:
– Thermal Gradients, resulting in
– Thermal Wind Components.
500 mb Contours
Contour
Lines
Contour Charts
• Constant Pressure Charts
• Contour lines are lines of constant
height.
• Strength of upper wind is:
– inversely proportional to distance between
the contour lines.
– The closer the distance between the
contours the stronger the upper wind
Contour Charts

h
Vg 
d
Vg = geostrophic wind speed.
 h = change in height of contour
lines.
d = distance between contours.
Upper Air (Contour) Charts
• Isotherms of mean temperature not usually
drawn.
• Contour charts are drawn for standard
pressure levels
–
–
–
–
700 mb.
500 mb.
400 mb.
300 mb. Etc.
Upper Air Charts, cont’d
• The strength of the upper wind is directly
proportional to the slope of the isobaric
surface.
• Lines of constant height above the surface
are plotted on the charts
– These lines are called CONTOURS and are
plotted for a predetermined height change.
– Upper winds blow along the contours.
Upper Air Charts, cont’d
• A contour line is also referred to as an
ISOHYPSE, i.e. a line of constant height
Pressure Changes Aloft
1000 mb msl Pressure
Pressure Changes Aloft
H
L
Pressure Changes Aloft
Thermal Wind
Component Directed
into board

L
H
Thermal Wind Component
• The thermal wind component:
– strength is directly proportional to the magnitude
of the Thermal Gradient.
– is directed along isotherms of mean temperature.
– in the NH the direction is such that the cold air
mass is on the left and on the right in the SH.
– usually increase in strength with height.
Thermal Wind Components
• The vector difference between between
winds at different levels is dependent on:
– the horizontal distribution of MEAN
temperature in the layer.
Upper Geostrophic Wind
• Is the vector
sum of the
lower
geostrophic
wind and the
thermal
wind
component
for the layer.
Warm Air
Average Tropospheric Thermal
Wind Component
Average Stratospheric Thermal
Wind Component
500 mb Contour Chart
H
General character of Upper
Winds
• Below the tropopause
– Cold air is to the north in N. hemisphere
and to S. in southern hemisphere
– Therefore thermal wind components are
westerly.
– Upper winds with height become more
westerly increasing up to the tropopause.
Upper Winds (cont.)
• Above the tropopause (stratosphere).
– The temperature distribution is reversed.
– Warmer air is above the poles and colder
is aloft at the equator.
– Thermal wind components are easterly.
– Upper winds therefore decrease with
increasing height and may become
easterly at very high altitudes.
Global Upper
Atmospheric
Circulation
Upper
Wind
Flow
Average upper winds - January
Average upper winds - July