Figure 4-29.—Vertical distribution of wind direction in the vicinity of frontal surfaces.

From this it can be seen that a front is a wind shift line and that wind shifts in a cyclonic direction. Therefore, we can evolve the following rule: if you stand with your back against the wind in advance of the front, the wind will shift clockwise as the front passes. This is true with the passage of all frontal types. Refer back to figure 4-22. NOTE: The wind flow associated with the well-developed frontal system is shown in figure 4-22, view E. Try to visualize yourself standing ahead of each type of front depicted as they move from west to east. The terms backing and veering are often used when discussing the winds associated with frontal systems. BACKING.—Backing is a change in wind direction—counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite of backing is veering. VEERING.—Veering is a change in wind direction—clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere. The opposite of veering is backing. The speed of the wind depends upon the pressure gradient. Look at figure 4-28. In view A, the speed is about the same in both air masses; in views B and C, a relatively strong wind is followed by a weaker wind; and in view D, a weak wind is followed by a strong wind. An essential characteristic of a frontal zone is a wind discontinuity through the zone. The wind normally increases or decreases in speed with height through a frontal discontinuity. Backing usually occurs with height through a cold front and veering through a warm front. The sharpness of the wind discontinuity is proportional to the temperature contrast across the front and the pressure field in the vicinity of the front (the degree of convergence between the two air streams). With the pressure field constant, the sharpness of the frontal zone is proportional to the temperature discontinuity (no temperature discontinuity—no front; thus, no wind discontinuity). The classical picture of the variation in wind along the vertical through a frontal zone is shown in figure 4-29. An example of a frontal zone and the winds through the frontal zone is shown in figure 4-30. 4-29 AG5f0429 COLD AIR ^COLDAIR WARM AIR N W S E Figure 4-29.—Vertical distribution of wind direction in the vicinity of frontal surfaces. AG5f0430 MBS 500 600 700 800 900 1000 -30 -20 -10 0 Td T FRONTAL ZONE Figure 4-30.—Distribution of wind and temperature through a warm frontal zone.