What is Humidity?
Humidity is defined as the amount of water vapor (gaseous phase of water) in the air. It is a key indicator of the likelihood of dew, frost, fog, and precipitation. The maximum amount of water vapor that can be held in air is affected by temperature; the higher the temperature, the greater the amount of water vapor the air can hold before reaching saturation.
Humidity is often discussed in terms of absolute humidity and relative humidity. The value for absolute humidity is returned as part of the secondary results of the calculation above, but it is relative humidity that is widely used in everyday life weather forecasts and is used as part of the calculation of dew point temperature.
Absolute Humidity
Absolute humidity is the measurement of the actual water content in the air, typically in units of grams per cubic meter (g/m³). It is calculated by dividing the total mass of water vapor by the volume of the air. Given the exact same amount of water vapor in the air, the absolute humidity does not change with the temperature if the volume is fixed.
Relative Humidity
Relative humidity compares the current ratio of absolute humidity to the maximum humidity for a given temperature, and expresses this value as a percentage. The higher the percentage, the higher the humidity.
Because warmer air can hold more water, relative humidity is affected by both temperature and pressure. Given the exact same physical amount of water vapor, there will be a higher relative humidity in cool air than there is in warmer air.
A relative humidity of 100% indicates that the air is completely saturated. This means that, given the current temperature conditions, water vapor in the air cannot increase further. 100% relative humidity is also the point at which dew begins to form.
What is Dew Point?
Dew point is defined as the temperature to which a given volume of air (at a constant atmospheric pressure) must be cooled for it to become saturated with water vapor, causing condensation and the formation of dew. Dew is the condensed water that a person often sees on flowers, cars, and grass early in the morning.
- Dew point varies depending on the amount of water vapor present in the air, with more humid air resulting in a higher dew point than dry air.
- The higher the relative humidity, the closer the dew point is to the current air temperature. At 100% relative humidity, the dew point is exactly equivalent to the current temperature.
- In cases where the dew point is below freezing (0°C or 32°F), the water vapor turns directly into frost rather than dew (often called the "frost point").
Comfort and Health Implications
While perception varies between people, higher dew points are generally uncomfortable because the increased humidity inhibits the proper evaporation of sweat, making it more difficult for a person's body to cool down. Conversely, lower dew points (very dry air) can also be uncomfortable, causing skin irritation, chapped lips, cracking, as well as drying out a person's respiratory airways.
The US Occupational Safety and Health Administration (OSHA) recommends that indoor air temperatures be maintained between 68-76°F with a relative humidity of 20-60% for optimal comfort.
Aviation and Dew Point
Dew point is also heavily considered in general aviation to calculate the probability of potential issues such as carburetor icing as well as the formation of ground fog. Pilots monitor the "temperature-dew point spread"; when the two numbers converge closely, fog is highly likely to form.