It is well known that fog will form only when the relative humidity is 100 percent, but what about when it is raining? Must the dew point temperature equal the air temperature when it is raining?
Rick Roellig, Lisle
Fog can form when the humidity is less than 100 percent, but it generally is in the range of 95 to 100 percent when fog is present. Occasionally, though rarely, the relative humidity will be 100 percent with no fog. The dew point temperature is the temperature to which air must be cooled in order for its relative humidity to be 100 percent (because as air is cooled, its ability to hold water vapor decreases. Chilling air therefore increases its relative humidity if its moisture content remains unchanged.) Rain often occurs when the relative humidity is below 100 percent (but it is always 100 percent in the clouds where condensation is occurring.)