1. What is SURFACE RH?
Relative Humidity at the surface (or any pressure level) is the percent that the air is saturated. 0% is perfectly dry air while 100% represents complete saturation.
2. How is SURFACE RH determined?
The RH is the mixing ratio divided by the saturation mixing ratio and given in a percent. Another way to find the same value is the take the vapor pressure divided by the saturation vapor pressure and give in a percent.
3. Operational significance of SURFACE RH:
a. Dew or frost will occur when the temperature drops to the dewpoint over night (RH = 100%).
b. RH is generally lowest in the afternoon when temperatures are warmest. RH is generally highest in the early morning hours when temperatures are coolest.
c. Climatologically, RH tends to be lower in dry climates since there is not as much vegetation to evaporate and transpire moisture into the air. Climatologically, RH tends to be higher in highly vegetated climates.
d. RH tends to be lower on average during the course of a day in which high pressure is overhead; higher when low pressure is overhead.
*depends on magnitude of temperature and dewpoint change
a. The RH is NOT the dewpoint itself divided by the temperature itself (i.e. 50 F / 100 F = 50%). The mixing ratios or vapor pressures of the temperature and dewpoint needs to be determined before being able to calculate RH.
b. A surface RH of 100% does not always mean it is raining or going to rain. The surface RH is often near 100% on mornings in which a clear night preceded it. This is because on clear nights the temperature cools toward the dewpoint. Fog events can also produce an RH of 100% but no rain. Even when it does rain or snow the RH may not rise to 100% if the precipitation does not last long enough or is not heavy enough to evaporate and saturate the surrounding air.