Finalizing the Contour Analysis 

After vertical consistency between levels is checked and any necessary adjustments made, the contours can be darkened in. This is usually done using felt-tip markers. High and low height centers are denoted by the letters H and L, respectively.

The most common upper-air pattern consists of alternating troughs and ridges. They form a series of waves which encircle each hemisphere. Occa-sionally, closed lows exist in the troughs and closed highs in the ridges (fig. 8-1-6). The lows are normally poleward and highs equatorward of the basic wind flow. All of these features reflect the vertical extent of features at lower levels, sea level in particular. Note the axes of the systems shown in figure 8-1-7. They are not vertical. As stated earlier, lows or troughs slope upward toward colder air, and highs and ridges slope toward warmer air. To determine the location of the colder and warmer air aloft, we do an isotherm (temperature) analysis for each level.

Figure 8-1-6.Common upper-air patterns.

Figure 8-1-7.Slope of surface pressure systems with height. Isotherm Patterns

Isotherm patterns at lower levels usually con-sist of tongues or pockets of warm and cold air that move across the map with reasonable con-tinuity from day to day. There is a correlation between isotherm patterns and contour (isoheight) patterns.

Dynamically, warm tongues in the troposphere are related to pressure ridges. This relationship is due to the sinking and adiabatic warming of air, created by horizontally converging air at a higher level in the atmosphere. At the same time the air in the troposphere is sinking and warm-ing, the convergence causes the air in the lower stratosphere to rise and cool. Intensifying upper-level ridges or highs are characterized by warming in the troposphere and cooling in the stratosphere. If a ridge builds (intensifies) and attains sufficient amplitude, the northern portion may cut off, forming a separate upper-level high.

Cold tongues are related to pressure troughs in the upper troposphere. This relationship is due to upper-level divergence and the resulting lifting and adiabatic cooling of air in the lower and middle troposphere. An additional consequence of this divergence is the sinking and adiabatic warming of air in the lower stratosphere. Deepen-ing lows tend to be associated with intensifying cold tongues in the troposphere and warm tongues in the stratosphere.

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