Actual Versus Geostrophic Wind

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Actual Versus Geostrophic Wind

As discussed earlier, actual winds blow across isobars toward lower pressures. The angles of cross-isobar flow vary based on the friction created by the underlying surface. The ocean’s surface causes across-isobar angle of 10° to 20°, and that’s as close as we can get to geostrophic flow. For plotting purposes, the wind directions you give to computed geostrophic wind speeds over water should reflect a cross-isobar angle of 20°. Over land, this angle should increase to 35°. Actual wind speeds are also affected by fric-tion, but the amount of reduction is virtually impossible to compute. Because friction is much greater over rough terrain, geostrophic wind should only be computed for regions having relatively flat terrain. Even then an adjustment to the speed is required because of friction. Over land, use two-thirds of the geostrophic wind speed. For example, a wind scale reading of 36 knots would be adjusted for use over land to 24 knots –2/3 of 36 = 24.

Isobaric spacing computed over land areas must also be adjusted. Take the true wind speed plus one-third. Use the wind scale with this total. For example, a true wind of 24 knots requires that an additional 8 knots be added (1/3 of 24 = 8) before going to the wind scale.

When mountain ranges separate colder air on one side from warmer air on the other, isobaric spacing is much closer (packed) over the range. Along coastlines in the winter, when continen-tal arctic or polar air moves offshore over much warmer water, the isobars will pack over the water. It is very important to remember this in the absence of ship reports. The greater the temperature contrast between the air mass and the water, the tighter the gradient (spacing). At sea, more weight should be given to the reported winds than to pressures in drawing isobars. However, care must be taken so as not to disregard real and important pressure data. Isobars kink at fronts. The kink always points toward high pressure.

Where two highs or two lows exist side by side (a col), two isobars with the same pressure must occur along the axis between them; along the axis connecting a high and a low, two isobars cannot have the same value.

ISOBARIC PATTERNS

Frequently, reference is made to various isobaric patterns. Some of the basic patterns are described below and shown in figure 7-2-6.

Figure 7-2-6.—Basic isobaric patterns.

Ridge (or Wedge)

A ridge is an elongated area of relatively high pressure. The wind circulation is essentially an-ticyclonic in the Northern Hemisphere. Ridges are usually areas of fair weather. They are found between two distinct low-pressure areas.

The col is a region between two highs and two lows. It is characterized by relatively low pressure with calm or light winds. The weather in this region is extremely variable. Sometimes the col marks a locality of fine weather, while at other times severe thunderstorms are experienced.

Trough

A trough is an elongated area of relatively low pressure. The isobars of a trough may be either U-shaped or V-shaped. U-shaped troughs contain no fronts while V-shaped troughs are associated with fronts.

SUMMARY OF RECOMMENDATIONS FOR DRAWING ISOBARS

1. Choose an area within which to start, preferably an area with many reports and moderate winds.

2. Choose your isobar interval (4 mb or 2 mb) and select the initial value to draw.

3. Sketch this isobar by drawing down the wind until it runs off the chart or encloses a pressure system (forms a loop).

4. Sketch in all remaining isobars. Use in-termediate isobars where necessary.

5. Label every isobar.

6. Smooth (harden in) isobars to eliminate analysis irregularities.

The manner in which you do isobaric analysis will be based on your preference and local needs. The above list combines requirements and sug-gestions to get you started. The true learning comes with practice. With each chart, you will improve your understanding of the subject and become more proficient.

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