OCR Text |
Show operation without complicated and special equipment. The degree of riming generally decreases with progress inland from the Pacific Coast, until the problem becomes much reduced-- although never eliminated- in the continental climate of the Rocky Mountains. Typical storm and temperature patterns produce in many areas a characteristic " rime 1ine", or altitude above which riming is much more common than it is below. Unfortunately this line often lies below the desirable site for an anemometer, but if it is possible to locate the anemometer at a satisfactory site in respect to the wind and still stay below the rime line, this will greatly diminish the problems from rime. From time to time specially formulated coatings have been proposed for wind sensors which would inhibit or prevent adhesion of rime. So far, none has come to our attention which in fact will perform this function, including the frequently suggested teflon. In the absence of a heat source to eliminate the rime by melt, the best coating for anemometer or wind vane is still a coat of high- gloss black paint ( see below under Paint). The black readily absorbs solar radiation, so the instrument surface is heated, the rime melts at the surface, and drops away. There is a definite advantage of a high- gloss surface over a dull or matte one. Black paint of course does not prevent rime formation at night or during storms with heavy overcast, but it does cause the instruments to shed the rime promptly as soon as the sun comes out. The only sure way to eliminate the rime problem is to provide enough radiant heat directed at the wind sensors to keep the rime melted off as fast as it forms. This method, using electric infrared lamps, has been used successfully in Colorado to keep wind sensors free of rime even during rather intense periods of riming. Presumably this same method will work at other sites where heavy riming is more frequent, but it obviously is limited to mountain tops equipped with electric power. Earlier experiments with propane- fired radiant heaters in the Sierras were only partially successful. Whatever type of heater is used, it is safe to expect that a large amount of heat will be needed to keep anemometers rime- free in the more severe conditions of a maritime climate. The tested heater system pictured in Figure 15 is recommended for anemometer installations wherever electric power is available and where riming is not severe. The three lamps are General Electric Type PAR 56 medium flood lamps, 120 volts, 300 watts. Each lamp is mounted in a General Electric Type S401 fixture with a Type S402 lens. These three fixtures are supported by pipe brackets at 120° intervals around a circle and 18" below the anemometer cups. The lamps are pointed upward so the radiant heat is directed at the anemometer cups. Exact details of the mounting brackets are 47 |