What does the term 'relative humidity' measure?

The term 'relative humidity' specifically measures the moisture content in the air relative to its saturation capacity. This means it indicates the amount of water vapor present in the air compared to the maximum amount of water vapor the air can hold at a given temperature. When relative humidity is at 100%, the air is fully saturated with moisture, and any additional moisture could lead to condensation. This measurement is crucial in understanding fire behavior because higher humidity levels can reduce the likelihood of fires igniting and spreading, while lower levels can increase fire danger.

In contrast, measuring the temperature of the air, the amount of rainfall, or the temperature difference between day and night does not provide any information about how much moisture the air contains relative to what it could potentially hold, which is the key concept behind relative humidity.