Since winds near Earths surface flow mainly along the isobars with a slight drift toward lower pressure, it follows that the wind direction in the vicinity of a front must conform with the isobars. The arrows in figure 4-2-9 indicate the winds that correspond to the pressure distribution. 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 follow-ing rule: IF YOU STAND WITH YOUR BACK AGAINST THE WIND IN ADVANCE OF THE FRONT, THE WIND WILL SHIFT CLOCK-WISE AS THE FRONT PASSES. This is true with the passage of all frontal types. Refer back to figure 4-2-3.

NOTE: The wind flow associated with the well-developed frontal system is shown in figure 4-2-3, 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 directioncounterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite of backing is veering.

VEERING. Veering is a change in wind directionclockwise 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 4-2-9. 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

Figure 4-2-9.Types of isobars associated with fronts.

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 discontinuityno front; thus, no wind discon-tinuity). The classical picture of the variation in wind along the vertical through a frontal zone is shown in figure 4-2-10.

An example of a frontal zone and the winds through the frontal zone is shown in figure 4-2-11. On this sounding the upper winds that show the greatest variation above the surface layer are those between the 800- to 650-mb layers. This in-dication coincides closely with the frontal indica-tions of the temperature (T) and dew-point (Td) curves (see fig. 4-2-11). Since the wind veers with height through the layer, the front would be warm.

The vertical wind shift through a frontal zone depends on the direction of the slope. In cold fronts the wind backs with height, while in warm fronts the wind veers with height. At the surface the wind ALWAYS veers across the front, and the isobars have a sharp cyclonic bend or trough

Figure 4-2-11.Distribution of wind and temperature through a warm frontal zone. 

that points toward higher pressure. Sometimes the associated pressure trough is not coincident with the front; in such cases there may not be an appreciable wind shift across the frontonly a speed discontinuity.

Figure 4-2-10.Vertical distribution of wind direction in the vicinity of frontal surfaces.

Pressure

One of the important characteristics of all fronts is that on both sides of a front the pressure is higher than at the front. This is true even though one of the air masses is relatively warm and the other is relatively cold. Fronts are associated with troughs of low pressure. (A trough is an elongated area of relatively low pressure.) A trough may have U-shaped or V-shaped isobars.

How the pressure changes with the passage of a front is of prime importance when you are deter-mining frontal passage and future movement. The pressure changes associated with frontal passage are discussed under the various types of frontal systems in lessons 3 through 6.

Friction causes the air (wind) near the ground to drift across the isobars toward lower pressure. This causes a drift of air toward the front from both sides. Since the air cannot disappear into the ground, it must move upward. Hence, there is always a net movement of air upward in the region of a front. This is an important characteristic of fronts, since the lifting of the air causes conden-sation, clouds, and weather.

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