| OCR Text |
Show The principal problem with recording precipitation gages is securing a catch of snow in the orifice representative of what is falling on the ground. In a sheltered forest glade in a valley, where wind action is minimum, the catch is usually good, the only serious difficulty being caused by canopying at the orifice. As a general rule, good protection from wind is provided when the angle from the top of the gage to the top of surrounding trees or other obstructions is about 30 degrees. This angle should not exceed kS degrees. Such shelter by trees will assure a representative catch in the gage. Whenever the gage is exposed at a site where appreciable storm winds blow, it is necessary to provide a wind screen ( A38) around the gage or a snow fence around the sampling site. The screen consists of a ring about kG" in diameter from which are suspended freely- swinging metal leaves. The gage should be mounted concentric with the ring and with the top of the orifice about 1/ 2" below the tops of the leaves. The screen still does not solve all the catch problems in high winds, but it helps. The recording precipitation gage has to be mounted, together with its wind screen, in a location which will keep it above the snow surface even in the deepest snow years. Normally this will be on top of the instrumentation tower described below under Installation and Maintenance. At locations where the snow study plot is more than 2000 feet below avalanche starting zones and is not representative of precipitation there, it becomes necessary to use a telemetered precipitation gage. A regulated power supply, potentiometric recording gage, and a single- channel analog recorder ( A29a) are used in such situations. Total cost of this system is about $ 1000 plus cable. Details on instruments and circuitry for the remote precipitation gage are available on request from the Rocky Mountain Forest and Range Experiment Station in Ft. Collins, Colorado 80521. The present state of technology in recording precipitation gages has not reached the point where these gages can be left alone and unattended during periods of heavy snowfall if an accurate precipitation record is needed. As better equipment is developed, or better means are devised for using present equipment, supplementary instructions will be issued. 2k |
