WIND DIRECTION AND SPEED.—Ship wind reports are generally reliable, but those from land stations are often affected by local topography, making them unrepresentative. Wind in the friction layer is much more subject to local influences over land than over water. The principal influences are terrain, vegetative cover, and local heating and cooling. In many places terrain orientation restricts air move-ment to certain directions. Frictional drag varies with vegetative cover, and local heating deforms pressure patterns, causing surface isobars to become nearly parallel to the surface isotherms. For all of these reasons, a chart of winds above or near the top of the friction layer is indispensable to accurate surface analysis over land areas. Except in very mountainous areas, the winds 2,000 feet above the surface (not sea level) have been found adequate.
The effects of local heating and cooling onpressure patterns are most marked along coastlines and lakeshores and result in com-bining the representative wind with the sea or land breeze component. Small islands also often show evidence of this effect. An esti-mate of the representative wind can be obtained by subtracting the sea or land breeze com-ponent vectorially from the (nonrepresentative) observed wind or by checking wind reports at higher levels.
Local convective activity can affect surfacewinds to a marked degree because of the extreme coinciding convergence and divergence necessary to maintain or compensate for the vertical motion.
Table 7-1-3 shows the reliability of winddirection and speed over land surfaces.
Table 7-1-3.—Reliability of Wind Data
All the factors discussed in table 7-1-3 affect
All the factors discussed in table 7-1-3 affectboth the speed and direction of the wind. There is further important correlation between these two properties of air motion: direction is most representative when speed is 10 knots or greater. Unless there is reason to believe that it also is unrepresentative, pressure should be given precedence over wind direction in drawing isobars in areas of light winds.
PRESENT WEATHER.—The weather occur-ring at a station is most likely to be representative if it lasts an appreciable length of time; however, even intermittent precipitation can be represent-ative if scattered throughout an air mass. Once the possibility of a reporting error is eliminated, you’ll have to assume the reported present weather is representative.
Smoke, dust, sand, haze, and fog are oftenpurely local phenomena, and hence can be unrepresentative. However, fog and dust can be characteristic of a large part of an air mass, par-ticularly advection fog and the dust storms of the southwestern United States.
The time of observation, with respect to thediurnal maximum of various types of weather, should also be considered.
CLOUDS.—High and middle clouds are usually more representative than low clouds or clouds with great vertical development. As in the case of present weather, the diurnal variation of the latter types is important in the evaluation of their representativeness. A good example of unrepresentative low clouds is west coast stratus, which rarely extends inland for any appreciable distance.
VISIBILITY.—Only restricted risibilities are likely to be unrepresentative. Such risibilities are associated with unrepresentative present weather discussed previously. Reduced risibilities are not likely to be the result of errors of judgment due to the existence of accurate distance markers and instruments at land stations. At sea, visibility may be quite unrepresentative due to the absence of such markers or instrumental means.
Based on the geographical location and airmass over an area, certain weather conditions are expected. Your ability to recognize errors and unrepresentative data permits you to fine tune the analysis. Errors should be corrected. Unrepresent-ative data is not in error; however, if you don’t recognize it as unrepresentative and adjust your analysis accordingly, the analysis will be in error in the region.
GENERAL SURFACE ANALYSIS PROCEDURES
There are several factors which will influence the order in which you will conduct your analysis. These factors include delayed reports, type of mapand impending weather, availability of past history and upper air charts, time required for completion, and your skill and experience. In general, you and the plotter will carry out the various steps and elements of the analysis in a manner similar to the following outline:
1. Complete the Chart Identification Block.
2. Make corrections to the previous analysisthat result from late or additional reports.
3. Indicate in ink (or yellow pencil) at leastthree previous positions of all high- and low-pressure centers which are expected to be found on the current chart. Also, indicate at least two previous positions of all fronts which are expected to be carried forward to the current chart.
4. Try to delineate fronts before drawingisobars. This is often difficult for the new analyst and is not always possible, especially when a front is weak or diffuse. In this case, sketch in the isobars first, especially in the areas where the front is most probable.
5. Draw isobars or continue isobaric analysisto delineate highs, lows, and other features of the pressure pattern.
6. Color in the fronts, keeping in mind thatthe color of the front is determined by the instan-taneous motion of the cold air mass.
7. Label the pressure systems; lows with ared L near the center and highs with a blue H.
8. Label air masses, if appropriate.
9. Color in all present and past weatherareas, as appropriate.
10. Draw isallobars. This is optional anddepends upon local policy.
The order may be varied to conform to local requirements and other factors as mentioned above. The individual procedures for analyzing the various elements of a surface chart are presented in the lessons that follow.