Published: 2007-09-25 11:39:51
Updated: 2007-09-25 11:39:51
Posted September 25, 2007 11:39 a.m. EDT
MIKE MOSS SAYS: Barry, The trick here is that the relative humidity (RH) at the altitude where rain is being produced IS always 100% when rain occurs, but this altitude can be many thousands of feet above the ground, and the rain may fall into a layer of unsaturated air (air with less than 100% RH) at lower altitudes, including the layer of air in contact with the surface and therefore being measured by ground-based hygrometers. If the rain is fairly widespread and continues for some time, the surface relative humidity will often climb and may reach 100%, but it is also possible for drier air in the lower atmosphere to continue to feed into the area where rain is falling, preventing the humidity from reaching saturation.
There are many possible combinations here, including situations where the air near the surface is not only unsaturated, but is very dry and quite deep. In this case, the rain falling from above may completely evaporate before reaching the ground, in which case the precipitation is called virga. Or, most of it may evaporate, leaving only a few sprinkles or very light rain at the ground, with humidity well below 100%. Another factor that comes into play in these situations is evaporative cooling. When rain, sleet or snow falls from a saturated layer aloft into drier air near the surface, that drier layer will often cool sharply due to partial evaporation of the falling precipitation. In most cases, this simply means a cool-down due to some rain. In some wintertime scenarios, this evaporative cooling process (the intensity of which varies with both the intensity of precipitation and the initial degree of dryness of the low-level air) can mean the difference between receiving a cold rain, sleet, freezing rain or a wet snow at the surface, sometimes inducing multiple, rapid transitions between one precipitation type and another.