Temperature and Humidity Probe

Ambient temperature and relative humidity are measured at each site using Vaisala instruments. Most of the sites (31) use the model HMP-35A, which Vaisala discontinued manufacturing in 1998 and then supporting in 2003. The replacement was model HMP-45D (in use at CO11, ND01, NE01, TX21, TX41), which will be discontinued for manufacturing in 2010 and support in 2015. The replacement model is a HMP-155, which can be ordered with a configuration that will be a direct replacement for the HMP-45D without additional adapters. Specifications for each instrument are as follows:

  • HMP-35A  -20°C to +60°C [Pt-100 sensor]  0% to 100% RH [Humicap sensor]
  • HMP-45D  -40°C to +60°C [Pt-100 sensor]  0% to 100% RH [Humicap-180]
  • HMP-155  -80°C to +60°C [Pt-100 sensor]  0% to 100% RH [Humicap-180R]

The Vaisala instrument is connected to the vis-MFRSR datalogger, so its output is measured every 15 seconds and integrated into 3-minute averages, and its data is retrieved nightly in conjunction with the polling of the vis-MFRSR datalogger.

UVMRP does not maintain calibration of these instruments, as they are intended to provide only a relative indication of the weather conditions around the site. Indeed, the radiation shield and probe are not mounted in accordance with standard meterological convention. However, there is an adjustment potentiometer on the probe to compensate for higher than 100% humidity readings, and many sites have had this correction performed over time.