2.5 Precipitation

Precipitation data, although primarily used in wet deposition modeling, are also used for consistency checks in data review and validation. The two main classes of precipitationmeasuring devices suitable for meteorological programs are:

• the tipping bucket rain gauge and 
• the weighing rain gauge. 

Both types of gauge measure total liquid precipitation. Both types of gauge may also be used to measure the precipitation rate, but the tipping bucket is preferable for that application. A third type, the optical rain gauge, has not yet been adequately developed for widespread use.

The tipping bucket rain gauge is probably the most common type of instrument in use formeteorological programs. The rainfall is collected by a cylinder, usually about 8 to 12 inches in diameter, and funneled to one of two small "buckets" on a fulcrum. Each bucket is designed to collect the equivalent of 0.01 inches (0.3 mm) of precipitation, then tip to empty its contents and bring the other bucket into position under the funnel. Each tip of the bucket closes an electrical contact which sends a signal to a signal conditioner for analog and/or digital recording. These are fairly reliable and accurate instruments. Measurement errors may occur if the funnel is too close to the top of the cylinder, resulting in an underestimate of precipitation due to water splashing out of the cylinder, especially during heavy rainfall. Underestimates may also occur during heavy rainfall because precipitation is lost during the tipping action. Inaccuracies may also result if the tipping bucket assembly or the entire gauge is not leveled properly when installed. Tipping buckets are generally equipped with heaters to melt the snow in cold climates, however, the total precipitation may be underestimated due to evaporation of the frozenprecipitation caused by the heating element. It would be preferable for the heater to be thermostatically controlled, rather than operate continuously, to avoid underestimation due to evaporation that may also occur during periods of light rain or drizzle. Underestimation of precipitation, especially snowfall, may also result from cases where the gauge is not adequately sheltered from the influence of the wind. A wind shield should therefore be used in climates thatexperience snowfall. Strong winds can also cause the buckets to tip, resulting in spurious readings.

The weighing rain gauge has the advantage that all forms of precipitation are weighed and recorded as soon as they fall into the gauge. No heater is needed to melt the snow, except to prevent snow and ice buildup on the rim of the gauge, alleviating the problem of evaporation of snow found with the heated tipping bucket gauge. Antifreeze is often used to melt the snow in the bucket. However, the weighing gauge requires more frequent tending than the tipping bucket gauge, and is more sensitive to strong winds causing spurious readings. The weight of precipitation is recorded on a chart mounted on a clock-driven drum for later data reduction. Weighing systems are also available which provide an electrical signal for digital processing.

2. PRIMARY METEOROLOGICAL VARIABLES 
 2.1 Wind Speed
      2.1.1 Cup Anemometers
      2.1.2 Vane-oriented and Fixed-mount Propeller Anemometers 
      2.1.3 Wind Speed Transducers 
  2.2 Wind Direction 
      2.2.1 Wind Vanes  
      2.2.2 U-V and UVW Systems  
      2.2.3 Wind Direction Transducers 
      2.2.4 Standard Deviation and Turbulence Data  
  2.3 Temperature and Temperature Difference 
      2.3.1 Classes of Temperature Sensors  
      2.3.2 Response Characteristics  
      2.3.3 Temperature Difference 
      2.3.4 Sources of Error 
  2.4 Humidity  
      2.4.1 Humidity Variables  
      2.4.2 Types of Instrumentation  
  2.5 Precipitation 
  2.6 Pressure  
  2.7 Radiation  
  2.8 Recommendations