4.2: Humidity

Absolute Humidity

Humidity measures the amount of water vapor in the air. The amount of water vapor in the air depends on the controls over evaporation discussed earlier. A meteorologist can express air humidity in several ways. Each humidity measurement is, to some degree, controlled by air temperature.

Absolute humidity is the weight of water vapor per unit volume of air, usually measured in units of grams of water vapor per cubic meter of air (g/m³). Absolute humidity is not often used to express the moisture content of air because it is sensitive to changes in both air temperature and atmospheric pressure. For instance, let's say a 1-cubic-meter parcel of air at the surface contains 2 grams of water. This gives us an Absolute Humidity of 2 g/m³. Now lift the parcel of air upwards into the atmosphere. As the air rises, the decrease in atmospheric pressure on the parcel allows it to expand and occupy more space. Let's say the parcel doubles in size due to uplift. Before rising, the absolute humidity was 2 g/m³. As the air doubles in volume, the new absolute humidity is 1 g/m³. In actuality, the parcel still contains the same amount of water, 2 grams. But given how absolute humidity is calculated, it appears that the amount of water in the air has decreased. Thus, instead of absolute humidity, we use a measure that is not sensitive to changes in air volume.

Specific Humidity

Specific humidity is the mass of water vapor in the air per unit mass of air, including the mass of water vapor. The units of measurement are grams of water vapor per kilogram of air (g/kg). Because weight is not significantly affected by temperature or atmospheric pressure, specific humidity is a much more useful measure of humidity. Another measure very similar to specific humidity is the mixing ratio. The mixing ratio is the weight of water vapor per unit weight of dry air. Because the atmosphere contains so little moisture by volume, the mixing ratio is virtually equal to the specific humidity.

Relative Humidity

Relative humidity is the most commonly used way of describing atmospheric moisture. It is the ratio of the amount of water vapor present in the air to the maximum amount needed for saturation at a given pressure and temperature. It is typically multiplied by 100 and expressed as a percent.

\[R H=\frac{water\ vapor\ content}{water\ vapor\ capacity} \times 100 \%\]

Relative humidity indicates how close the air is to saturation, not how much water vapor it contains. Regardless, we know that the air's water vapor saturation level depends on its temperature. We know that as air temperature increases, its water vapor capacity increases. That is, as the air temperature increases, it can keep more water in the vapor state. Relative humidity can also be expressed as a ratio of the actual vapor pressure to the saturation vapor pressure.

\[R H=\frac{actual\ vapor\ pressure}{saturation\ vapor\ pressure} \times 100 \% \]

Thus, air at 50 percent relative humidity contains half the amount of water vapor needed to saturate it. Air with 100 percent relative humidity is said to be saturated. Interestingly, relative humidity can exceed 100 percent. When that happens, air is said to be supersaturated.

Dew Point Temperature

The dew point is the temperature the air must be cooled to (at constant pressure) to reach 100 percent relative humidity, or, in other words, saturation. At this point, the air cannot hold any more water in its gaseous form. If the air were cooled even further, water vapor would condense into liquid water, typically as fog or precipitation. The dew point temperature is a good indicator of the amount of moisture in the air. High dew points indicate higher water vapor content, while low dew points indicate low water vapor content in the air. When the air temperature matches the dew point, the air is saturated, and the relative humidity equals 100 percent.

It is called the "dew point" because dew forms on surfaces when the air reaches saturation. When the dew point is below the freezing point of water, it is also called the frost point.

Wet-Bulb Temperature

Figure \(\PageIndex{1}\): A sling psychrometer consists of two thermometers. A small piece of cotton or fabric is wetted with distilled water and placed over one of the thermometer's bulbs. A handle on the other end allows the device to be whirled in the air until the wet bulb is completely dry. (Public Domain; CambridgeBayWeather via Wikimedia Commons)

Wet-bulb temperature is the lowest temperature that can be achieved if water evaporates within the air. In other words, it is the temperature the air needs to be cooled to for it to saturate. It isn't easy to calculate, but it is easy to measure. To measure the wet-bulb temperature, simply wrap a wet cloth around the thermometer's bulb. Typically, this thermometer is attached to an apparatus called a sling psychrometer, as shown in Figure \(\PageIndex{2}\). The handle at the end makes it easy to whirl it through the air, creating a lot of airflow over the thermometer's wet cloth. Evaporation from the wet cloth cools the temperature measured by one of the thermometers. The temperature measured by the dry bulb is called the dry-bulb temperature or current air temperature. The temperature measured by the wet bulb after evaporative cooling is called the wet-bulb temperature. The wet-bulb temperature is always lower than the air temperature (or dry-bulb temperature) when relative humidity is less than 100%. When the relative humidity is 100%, the wet-bulb temperature equals the air temperature.