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Drawing Isotherms

Isotherms (lines of equal temperature) are usually drawn at 5°C intervals. They are drawn as solid red lines. They are sketched according to reported temperatures. Use their previous posi-tions (past history) as a guide. Isotherms are labeled in the same manner as contours—at the end of each line and in a small break near the top of closed loops.

In regions of sparse data an estimate of the isotherm pattern becomes necessary. Integrating past history (heights and temperatures) with the latest surface-pressure pattern aids greatly in determining the pattern aloft. As we discussed earlier, temperatures at selected points are fre-quently used in extrapolating the heights of various upper levels. Because wind is the advect-ing mechanism of warm and cold air, you should be aware of its effect and how it impacts the ther-mal pattern.

WIND-TEMPERATURE RELATION-SHIP.— Wind determines the pattern isotherms take and their speed of movement. Their speed is somewhat less than the wind speed. The fac-tors working against the advection of isotherms with the speed of the wind are as follows:

1. The addition or subtraction of heat.

2. Adiabatic changes due to lifting or sub-sidence. 

As warm air is advected toward a cold region, it tends to be cooled, and cold air advected toward warm regions tends to be warmed. In the case of cold air spreading southward over warmer regions, it tends to sink (being more dense), which results in adiabatic warming. At the same time, instability results. Both processes slow the advance of the cold air. Cold air moving offshore over warm ocean currents can be slowed by as much as 50 percent. When warm air is advected over cold regions, it tends to be lifted. This results in adiabatic cooling. The lifting process is more important at intermediate levels aloft; however, both lifting and subsidence work against the advection of isotherms with the speed of the wind.

The temperature pattern should explain tem-perature changes at individual stations. Because wind is the advecting mechanism, when there is a change of wind with height, the isotherm pat-tern must be adjusted. The isotherms are oriented to conform to the shear vector between the isobaric layers where the wind changed. The shear vector is derived from the wind speed and direc-tion at the top and bottom of the layer.

WIND SHEAR.— The vectoral rate of change of wind with respect to altitude is called vertical wind shear. It is determined by taking the vector difference between the reported wind at the top and bottom of the layer and dividing by their ver-tical separation. On the other hand, the vector difference between geostrophic winds at two levels is called the thermal wind. The thermal wind is not a wind that actually blows in the atmosphere; rather, it is the vector difference between winds at two levels. For example, if the 700-mb geostrophic wind is southwest at 50 knots and the 500-mb geostrophic wind is west at 80 knots, the vector difference, or thermal wind, is from the northwest at 58 knots. See figure 8-1-8.

In that example, the mean isotherms between 500 mb and 700 mb would be oriented in a north-west to southeast direction, with colder air to the north. In general, thermal winds parallel mean isotherms in a given layer, with colder air to the left as you look downstream (the direction toward which the wind is blowing). The spacing of the isotherms is made to conform to the magnitude of the shear vector. The greater the shear vector, the closer the spacing of the isotherms (tighter gradient) and the more rigorously the direction of the isotherms conforms to the direction of the shear vector.

Figure 8-1-8.—Thermal wind between two pressure surfaces.

Over the oceans, computed shear vectors are of considerable value in drawing isotherms. Speeds of less than 10 knots may not be significant, but when the shear vector exceeds this value, it is of particular use in the analysis. In the free atmosphere the vertical wind shear is largely controlled by the temperature field. In areas where no temperature data is available, an isotherm analysis can still be carried out provided that the variation of wind with height is known. The basic relationships for the Northern Hemi-sphere are summarized as follows:

1. If wind speed increases with increasing height but does not change direction, contours and isotherms are parallel, with cold air to the left facing downstream.

2. If wind speed decreases with increasing height but does not change direction, contours and isotherms are parallel, with cold air to the right facing downstream.

3. If there is no change in wind speed or direction with height, the air temperature is uniform throughout the layer.

4. If the wind veers with increasing height, the isotherms cross the contours in such a way that advection of warmer air takes place.

5. If the wind backs with increasing height, the isotherms cross the contours in such a way that advection of colder air takes place.

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