OCR Text |
Show areas of deep winter snow cover, provision should be made to raise the instrument shelter up a pole or the side of a tower to maintain this spacing as the snow accumulates. ( See Figure k.) If this is not possible, an elevated site at least six feet above the maximum expected snow depth should be chosen. It is better to have the sensor too high than too low. In the case of remote-sensing temperature recorders ( Al8 and Figure 5), the sensor and shield can be located on a support attached to the building but extending at least six feet above the roof line and well away from chimneys or other heat sources. This does not meet clima-tological specifications, but is satisfactory for snow and avalanche observations and offers the convenience of having the recorder inside the building for easy inspection. The standard instrument shelter, large enough to hold a complete thermograph as well as thermometers or other instruments, is the U. S. Weather Bureau louvered wooden shelter ( Al9). This provides adequate ventilation and reasonable radiation shielding. For maximum effectiveness ( and for preservation as well), it should be kept painted inside and out with a good grade of dead white paint. The normal base is adequate in areas of shallow snow, but provision must be made to raise it the prescribed distance above accumulating deep snow. Location at a windy site assures good ventilation and may reduce or eliminate the accumulation problems; however, too much wind causes trouble from drifting snow and instrument vibration. The shelter should stand in the open well away from trees, buildings or other large objects. The principal temperature errors in these instrument shelters are due to radiation heating during a calm, sunny day when there is not enough air motion to ventilate the temperature sensors. This normally is not a serious problem during mountain winters. The shelter should be mounted with the access door facing north; this minimizes radiation errors from direct sunlight when the door is opened to inspect or service the instruments. Maximum, minimum and present air temperatures are read from suitable thermometers mounted in the standard shelter described above. The dial- type max- min thermometer ( A21 a & b) is less susceptible to breakage and easier to read and reset than the regular mercury and alcohol maximum and minimum thermometers ( A21c), but the latter are more accurate. There is little difference in the cost. The dial thermometer should be mounted so that the sensor bulb underneath ( or behind) is exposed to free air circulation and is away from the wall or floor of the shelter. It should not touch the thermograph or other instruments. The mercury and alcohol thermometers must be mounted in a standard Townsend support ( B20) for proper use. 13 |