What Is Humidity?

What is relative humidity?

Relative humidity is the most common way to measure humidity, expressed as a percentage. It's the ratio of the actual water vapor in the air to the maximum water vapor the air could hold at its current temperature, multiplied by 100. At 50% relative humidity, the air contains half as much water vapor as it could potentially hold. At 100% relative humidity, the air is saturated and any additional water vapor would condense out as dew, fog, or clouds. Other measures exist (absolute humidity, specific humidity, dew point), but relative humidity is the most commonly cited.

How does humidity affect how it feels outside?

Humidity dramatically affects how hot or cold the air feels. In hot weather, high humidity prevents sweat from evaporating efficiently, since the air already holds nearly all the water vapor it can. With less evaporative cooling, the body has trouble shedding heat, making hot humid days feel much hotter than the temperature alone suggests. The heat index combines temperature and humidity into a single 'feels like' temperature. In cold weather, dry air feels less cold than humid air at the same temperature because dry air conducts heat away less efficiently than humid air.

Why does cold air feel dry?

Cold air feels dry because cold air can hold much less water vapor than warm air. At freezing temperatures, even saturated air (100% relative humidity) contains very little water vapor. When that cold air is heated indoors, its absolute water content stays the same but its capacity to hold water increases dramatically, dropping relative humidity to very low values (often 20% or less). The dry indoor air causes static electricity, chapped skin, and respiratory irritation. Humidifiers are used in winter to add moisture back to indoor air.

When does dew or fog form?

Dew or fog forms when air cools to its dew point, the temperature at which the air becomes saturated (100% relative humidity) without any change in moisture content. The dew point is always equal to or below the current air temperature. As nighttime temperatures drop, air sometimes cools below its dew point, and water vapor condenses on surfaces (dew) or in the air as suspended droplets (fog). Higher initial humidity means higher dew point and easier formation of dew or fog. Weather forecasters often report dew point as a more useful humidity measure than relative humidity for predicting condensation.