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.
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 • 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 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
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.