Distinguish between the different ways of expressing atmospheric humidity and explain how each works.

Humidity is the amount of water vapor in the atmosphere. Only water vapor contributes to humidity. Condensation and precipitation lower humidity because they remove water vapor from the atmosphere.

As we have seen, evaporation from the oceans accounts for most of the water vapor in the atmosphere. The most humid air is found in the tropics near warm oceans, where evaporation rates are high (Figure 3.8). In tropical rainforests, however, most of the humidity comes from transpiration by plants, not evaporation. Together, evaporation and transpiration create a single process of moving water to the atmosphere, called evapotranspiration.

Hygrometers of several types are used to measure humidity. Sling psychrometers, for example, are large, hand-held instruments that measure humidity using thermometers. Human hair is also used to measure humidity. Humidity changes the length of a hair, and its length can be calibrated to give a precise humidity value. Electronic sensors measure humidity by measuring electrical conductivity across a metal plate. They are used in radiosondes (weather balloons) because they are small, light, and can track rapid changes in humidity. Radiosondes transmit these data in real time to a ground-based receiving station. Finally, satellites remotely sense humidity in the atmosphere from high above Earth. They have the advantage of providing continuous and hemispheric coverage.

As the saying goes, “It’s not the heat, it’s the humidity.” Heat waves are usually associated only with high temperatures, but high humidity also plays an important role in the effects of heat waves on people. As we saw in Section 2.2, the heat-index temperature is the temperature perceived by people as a result of atmospheric humidity coupled with air temperature (see also Appendix 5). When sweat evaporates from our bodies, it carries body heat away and provides cooling relief. High atmospheric humidity slows evaporation, sometimes causing our bodies to overheat.

Heat waves are the number one meteorological killer in the United States: Each year, about 400 people lose their lives because of heat waves. When a heat wave gripped the Chicago and Milwaukee areas in 1995, over 750 people died. In August 2003, as many as 70,000 people died in Europe as a result of exceptionally high heat-index temperatures there. In France nearly 15,000 people died as heat-index temperatures reached as high as 41°C (104°F). Again in 2010 and 2012, record-setting heat gripped the United States and Europe. According to the Intergovernmental Panel on Climate Change, heat waves caused by high temperatures and high humidity may become more frequent as the average temperature of the atmosphere continues to rise (see Section 6.4).

Many different units of measure are used to express humidity. All measures of humidity gauge how much water vapor is in the air. Here we will examine four of these measures: vapor pressure, specific humidity, relative humidity, and the dew point.

Vapor Pressure

Vapor pressure expresses the water vapor content of air by the amount of pressure it creates. Vapor pressure is the portion of air pressure exerted exclusively by molecules of water vapor.

In Section 1.2, we learned that the weight of the atmosphere exerts 1 kg of pressure on every square centimeter (1 kg/cm² or 14.7 lb/in²) of all objects at sea level. A millibar (mb) is a measure of atmospheric pressure that is conceptually identical to kilograms per square centimeter. Both units refer to the force exerted by air molecules as they push against a surface. Millibars are commonly used by meteorologists and other scientists because they are finer units that can express small changes in atmospheric pressure. (A hectopascal or hPa is the metric equivalent of a millibar.)

The average weight of the atmosphere at sea level exerts 1013.25 mb of pressure, and a portion of this pressure is caused by molecules of water vapor. Imagine stuffing a water bottle into a backpack already filled with books. The extra “pressure” exerted by the water bottle in addition to the pressure exerted by the books and other items in the backpack would be equivalent to vapor pressure.

Evaporation and condensation occur continuously throughout the atmosphere, but net condensation occurs only when air is saturated. The vapor pressure at which saturation occurs, called the saturation vapor pressure, varies with air temperature. As shown in Figure 3.9, for every 10°C increase in air temperature, saturation vapor pressure approximately doubles. In other words, as air temperature increases, more water vapor must be added for saturation to occur.

Specific Humidity

Another measure of humidity evaluates the water vapor content of an air sample by mass. Specific humidity is the water vapor content of the atmosphere, expressed in grams of water per kilogram of air (g/kg):

If, for example, there are 10 g of water vapor in a kilogram of air, then the specific humidity is 10 g/kg. Specific humidity changes only when water vapor is added to or removed from the atmosphere. Condensation lowers specific humidity, and evaporation raises it.

Relative Humidity

Relative humidity is a common way of expressing humidity that you may already be familiar with. Relative humidity (RH) is defined as the ratio of water vapor content to water vapor capacity, expressed as a percentage.

Temperature and relative humidity are inversely related (they change in opposite directions). Warm air has a greater water vapor capacity than cold air. Therefore, if the air temperature rises, the water vapor capacity of the air also rises. In terms of its water vapor capacity, the air will have proportionally less water vapor as the temperature increases (as long as no water vapor is evaporated into the air so the level of water vapor remains constant). It may sound right to say that warm air can “hold” more water vapor than cold air—but the air is not “holding” water vapor because water vapor is the air. Instead, it is more accurate to say that warm air has a higher water vapor capacity.

If the temperature of air is lowered, the water vapor capacity of the air decreases. As cooling continues, the air will eventually become saturated. When air is saturated, the relative humidity is 100%. When the relative humidity is 100%, clouds and precipitation may form. Figure 3.10 summarizes the general relationship between temperature and relative humidity graphically.

The following examples use water vapor content (expressed as vapor pressure) to illustrate quantitatively how relative humidity changes. First, we can express relative humidity using this formula:

We will apply this relative humidity formula to an air parcel, defined as a body of air with uniform humidity and temperature.

Imagine a parcel of air at 30°C with a vapor pressure of 22 mb. At this temperature, the saturation vapor pressure is 42 mb (refer back to Figure 3.9). With these numbers, we can calculate the relative humidity (RH) of the air parcel:

If we lower the temperature of the air parcel to 25°C, the relative humidity will increase:

Notice that the 22 mb vapor pressure in the air parcel did not change because we did not add or remove water vapor from the atmosphere. Only the air temperature changed. Crunch the Numbers further explores how temperature changes the relative humidity of air.

Polar air often has high relative humidity because it is cold. Warm tropical air also has high relative humidity because immense amounts of water are evaporated from warm tropical oceans. The high vapor pressure of tropical air creates high relative humidity even when the temperature is high, as shown in Figure 3.11.

Changes in vapor pressure also affect relative humidity. Increases in evaporation resulting in increases in relative humidity happen in many different atmospheric situations outside of the tropics as well. The two examples in Picture This show how air can become saturated by raising the vapor pressure.

The Dew Point

Have you ever wondered why the outside of a container of an ice-cold beverage becomes wet? The water on the outside of the container is condensate (or dew), which forms because the cold liquid lowers the temperature of the air surrounding the container to the dew point (or dew-point temperature): the temperature at which air becomes saturated (Figure 3.12).

The air parcel we considered became saturated when its temperature was lowered to 19°C. For that parcel of air, therefore, the dew point was 19°C. The dew point is a reliable indicator of humidity because in unsaturated air, the dew point does not change as air temperature changes.

Dry air has a low dew point, and humid air has a high dew point. Once dew points are above about 20°C (70°F), the humidity becomes uncomfortable for people. Figure 3.13 shows the geographic pattern of humidity in the United States using dew points.