The surface relative humidity (SFC RH) is calculated as the actual amount of moisture in the air divided by the maximum amount that can be in the air at a particular temperature. Two ways of calculating RH would be to take the mixing ratio of dewpoint divided by the mixing ratio of the temperature or taking the actual vapor pressure divided by the saturation vapor pressure.
When the air is saturated, the temperature is at the dewpoint or very close to it. When complete saturation occurs in the atmosphere, the RH will be near 100%. After ample time passes, precipitation will saturate the air, which results in the RH increasing to near 100%. Because of this, often the public associates precipitation events with a relative humidity of 100% and then erroneously reverses this association.
In actual practice, surface relative humidity is almost meaningless in determining if precipitation will occur or not. The lift and instability release that produces precipitation occurs aloft and not specifically at the surface. Often precipitation will reach the surface with an initial surface RH well less than 50%. To determine if precipitation is likely, synoptic vertical velocity and instability release potential is studied along with assessing column RH and soundings.
Column RH is the average RH within a deep vertical column of air. Column RH can be used to assess how close a deep layer of vertical air is to saturation. Column RH is given in FOUS data for three layers and on the graphical synoptic models. A popular column RH is the 850 mb to 500 mb column average RH. If the column RH is near saturation and ample lift occurs, precipitation is likely. Even the initial column RH can be well below saturation and precipitation occurs in the near term since part of the column could be saturated and part of it well unsaturated. Simply, lifting a saturated layer of the troposphere produces clouds and hydrometeors (hydrometeors if lift is adequate). The hydrometeors will reach the ground as precipitation if they do not evaporate beforehand.
Although an extended precipitation event at the surface will produce a surface RH near 100%, the initial surface RH value itself is nearly meaningless in determining if precipitation will occur in the first place.
There are many mornings when the RH is near 100% and the skies are clear. Radiational cooling at night can drop the temperature to the dewpoint near the surface. In fog events, the RH will be near 100% and no precipitation occurs. When precipitation reaches the surface, the RH will increase if the air is not saturated (especially if it is initially well below saturation). If the precipitation is light, the RH still might not rise to near 100%. It takes precipitation a certain amount of time to saturate the air. The lighter the precipitation, the longer it will take the air to reach saturation. Light snow has a difficult time saturating initially low RH air. Light precipitation may take several hours to saturate the air.