Isobaric Topography Method

a method of graphically representing the pressure, temperature, humidity, and wind in the troposphere and stratosphere through the use of isobaric contour charts, or constant-pressure charts, compiled from data obtained by radiosonde probing of the atmosphere (seeSYNOPTIC WEATHER CHART). The method is used in the analysis of atmospheric processes and in weather forecasting.

The geopotential φ = gz is used as a measure of height in constructing isobaric contour charts, where φ is expressed in dynamic, or geodynamic, meters and z is expressed in geometric meters. The quantity φ represents the work done when a unit mass of air is lifted in the field of gravitational force g from an initial level with pressure p₀ to a height z with pressure p_i. The dynamic meter is the work done when a unit mass of air at sea level at a latitude of 45° is lifted 1 m. In calculations of the geopotential, the value of the acceleration of gravity g for any latitude and up to a height of 30 km is taken as constant and equal to 9.8 m/sec². In order to express the position of an isobaric surface in units of work by the same number as that used for the surface’s geometric height z, the concept of geopotential height H = z was introduced. The following formula may be used to calculate geopotential heights:

H₂ – H₁ – 67.44T_vm log(p₁/p₂)

where H₁ and H₂ are the geopotential heights at the lower and upper levels, p₁ and p₂ are the pressures at these levels, and T_vm is the mean virtual temperature of the layer of air bounded by the levels H₁ and H₂.

If the height of some isobaric surface is reckoned from sea level, the geopotential is called absolute; if the height is measured relative to a lower isobaric surface, the geopotential is said to be relative. Thus, the absolute geopotential of an isobaric surface depends on the pressure at sea level and the mean virtual temperature in the layer of air between sea level and the isobaric surface. The relative geopotential depends only on T_vm, since the pressure at the lower and upper levels is taken as constant.

A chart showing values of the absolute geopotential, air temperature, humidity, wind direction, and wind velocity on a given isobaric surface is called an absolute isobaric contour chart, and a chart with relative geopotential data is called a relative isobaric contour chart.

On absolute isobaric contour charts, the contour lines connecting equal values of the geopotential are drawn usually at intervals of 40 geopotential meters. These lines represent the intersections of an isobaric surface with level surfaces. Since isobaric surfaces have a concave shape relative to the earth’s surface in cyclones and a convex shape in anticyclones, cyclones and anticyclones on such charts are regions with closed contour lines. Cyclones have low values of the geopotential at the center of the region, and anticyclones have high values at the center. The distance between adjacent contour lines is proportional to the pressure gradient and, consequently, to the wind velocity; this distance decreases with increasing wind velocity. The wind direction is approximately parallel to the contour lines, and the wind blows in such a way that in the northern hemisphere the low pressure value is to the left and the high pressure value is to the right.

On relative isobaric contour charts, which characterize the average temperature field between two isobaric surfaces, the cold and hot regions are also outlined by contour lines. The locations of sources of cold air most often coincide with cyclones and troughs, and the locations of sources of warm air most often coincide with anticyclones and ridges.

The combined analysis of absolute and relative isobaric contour charts, along with the analysis of surface weather charts, permits establishment of the vertical structure, origin, movement, and evolution of pressure fields and makes possible determination of the rate of transfer of heat and moisture at various heights. The positions of jet streams are found from the density of the contour lines on absolute isobaric contour charts, and the positions of atmospheric fronts are found from the density of the contour lines on relative isobaric contour charts. It is possible to predict the development of atmospheric processes and prepare weather forecasts on the basis of this type of analysis.

The principles of the isobaric topography method were developed by V. F. K. Bjerknes in 1912. The practical application of the method in the weather services of various countries became possible with the development of atmospheric radiosonde networks. The regular compilation of isobaric contour charts in the USSR was begun in 1938.