Relative humidity (RH) is the ratio of the water vapor actually in the air to the amount the air could hold at its current temperature, expressed as a percent.
The physics of the ratio
The air's capacity to hold water vapor rises sharply with temperature. Because RH divides the actual vapor by that temperature-dependent capacity, warming the air lowers RH even when no vapor has left, and cooling it raises RH even when no vapor has arrived. Saturation occurs at 100 percent RH, which is also where the temperature equals the dewpoint.
This is why RH is best thought of as a distance-to-saturation measure rather than a moisture measure. Two locations at 60 percent RH can hold very different amounts of water vapor if one is at 45 degrees Fahrenheit and the other is at 90.
How forecasters use it
Forecasters read RH by layer. Near the surface, high RH favors fog, drizzle, and low ceilings. In the middle troposphere, roughly 700 to 500 mb, lower RH lets falling precipitation evaporate and cool the air, which strengthens downdrafts and can support strong outflow winds. In the upper troposphere, RH shapes cloud tops and anvil coverage.
For severe weather setups, forecasters often prefer to look at the whole vertical profile of RH on a sounding rather than a single-level value. A moist boundary layer under a drier middle troposphere is a common signature associated with strong downdrafts and hail-producing storms.
Where it misleads
A high RH does not mean a lot of moisture. Cool morning air at 95 percent RH can hold less total vapor than a warm afternoon at 40 percent RH. To compare moisture between times or places, use dewpoint or mixing ratio, which do not depend on temperature.
RH is also sensitive to small errors in the temperature reading, especially in dry air near saturation. Forecasters cross-check with dewpoint and, when available, direct moisture soundings before drawing conclusions about how close a layer is to producing clouds or precipitation.