2.3.4 Sources of Error

One of the largest sources of error in any temperature system is due to solar radiation. Temperature sensors must be adequately shielded from the influences of direct or reflected solar radiation in order to provide representative measurements. A well ventilated shelter may be adequate for surface temperature measurements but would be impractical for levels higher than a few meters above ground. Tower-mounted sensors are generally housed in aspirated radiation shields. It is advisable to utilize motor driven aspirators to ensure adequate ventilation. Care should also be taken that moisture not be allowed to come in contact with the sensor or the inside surfaces of the radiation shield. In some sensors moisture will change the electrical properties of the sensor, causing error. In others, the evaporative cooling will cause the temperature reading to be too low. For temperature difference measurements, sensors should be housed in identical aspirated radiation shields with equal exposures.

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