Relative Humidity
The amount of moisture in air compared to the maximum it can hold at that temperature. Higher humidity makes heat feel more oppressive.
The ratio of water vapor present in air to the maximum amount of water vapor that air can hold at a given temperature, usually expressed as a percentage. Higher relative humidity means air is closer to saturation; lower humidity means air is drier. It differs from absolute humidity, which measures actual water vapor mass regardless of temperature.
What this means in real life
On a hot summer day, the air might feel sticky at 80% relative humidity, but the same moisture level on a cold winter morning would feel dry at 30% relative humidity, because cold air holds less water vapor overall.
What it isn’t
It is not the absolute amount of water in the air. Two locations can have identical absolute moisture but very different relative humidity if their temperatures differ—a common source of confusion when comparing weather in different climates.
Commonly misused online
People often treat it as a direct measure of 'how wet' the air is, ignoring temperature. Posts claiming '90% humidity is unbearable everywhere' miss that the same percentage feels different at 70°F versus 95°F.