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

dhVg

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

L

H

Pressure Changes Aloft

L H

Thermal Wind Component Directed into board

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