What does relative humidity express?

Relative humidity is a measure that expresses the ratio of the current amount of water vapor present in the air to the maximum amount of water vapor that the air can hold at a given temperature, often presented as a percentage. This concept is crucial in meteorology as it informs us about the moisture content in the air compared to its capacity for holding moisture.

At warmer temperatures, air can hold more moisture, leading to varying relative humidity percentages across different temperatures even if the actual water vapor content remains constant. This measurement plays a significant role in weather forecasting and understanding atmospheric conditions, as higher relative humidity can lead to discomfort and influence weather phenomena such as cloud formation and precipitation.

Other options do not accurately encapsulate the definition of relative humidity. For instance, temperature at high altitudes or pressure exerted by water vapor does not directly describe the relative measure of actual vapor present compared to capacity. Similarly, absolute density of water content does not relate directly to the concept of relative humidity, which is fundamentally about ratios rather than absolute quantities.