Wave Movement

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Wave Movement

We’ve discussed the relationship between waves and their associated temperature patterns. That is, long waves have cold troughs and warm ridges, while short waves have the opposite. The location and strength of warm and cold air in relation to troughs and ridges give an indication of their present and future movement. Locating and determining the strength of waves is done through a comparison of the isotherm and contour patterns on constant-pressure charts. The isotherms are considered either in phase or out of phase with pressure troughs and ridges. In phase, the thermal troughs and ridges coincide with the pressure troughs and ridges. Figure 8-3-3, cases A through C illustrate the in-phase relation-ships. The only difference in the three in-phase patterns is the amplitude of the isotherms as compared to that of the contours.

When the isotherms and contours are in phase and parallel (have the same amplitude), the wave stagnates, because there is no temperature advection taking place across the wave. When the isotherm amplitude is less than the amplitude of the contours (case B), colder air is advected into the west side of the trough and warm air into the eastern side. Where the temperatures are falling (in this case, the western side), the pressure should fall. Since systems move from high to low pressure, we would expect such a wave to move west (retrograde).

When the isotherms have a greater amplitude than the contours have (case C), warm air is advected into the western side of the trough and colder air into the eastern side. Therefore, pressures fall in advance of the trough and rise behind it. When this occurs, the trough moves slowly eastward toward the falling pressures (is slowly progressive).

The last two cases deal with out-of-phase isotherm-contour relationships and are associated

Figure 8-3-3.—Isotherm-contour patterns.

with short waves. See figure 8-3-3, cases D and E. These isotherms are classified as being 90° or 180° out of phase. If they are in phase, the coldest air is in the troughs and the warmest air in the ridges. For isotherms to be classified as 180° out of phase, the exact opposite must occur. When 180° out of phase, the waves are fast moving, and their speeds exceed that of the gradient wind within them. When 90° out of phase, the waves move with a speed equal to that of the gradient wind within the wave. In both out-of-phase cases, the temperature advection taking place is significant and the pressure changes are great downstream.

In all the above relationships, the effects of convergence, divergence, and dynamic deepening and filling on temperature are not taken into account. Therefore, these rules are used only to approximate a wave’s vector movement.

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