Moisture

The term most frequently used to express the amount of moisture in the air is relative humidity (RH). The relative humidity is the ratio of the actual amount of water vapor in a sample of air compared to the total amount of water vapor the same sample can hold before condensation begins (i.e., it becomes saturated with water vapor) at a given temperature and pressure. As the term suggests, RH only gives us a relative sense of the amount of moisture in the air, not the actual amount. On the east coast, for example, the temperature may be 95 degrees Fahrenheit (35 degrees Celsius) with a relative humidity of 90 percent, while in the western desert, the temperature may be similiar with a relative humidity of only 20-30 percent. Clearly, there is more water vapor in the air on the east coast, but in either case the relative humidity does not tell us how much is in the air. Absolute humidity and specific humidity are measures of the actual amount of water vapor in the air. The absolute humidity is the mass of the water vapor per unit volume of air. Does this sound familiar? The absolute humidity is actually the density of the water vapor in the air, while the specific humidity is a ratio of the mass of the water vapor in a sample of air to the total mass of the sample. Similar to the specific humidity is the mixing ratio (w). The mixing ratio is the mass of the water vapor in a sample of air divided by the mass of the dry portion of the air (rather than the total mass of the sample as in the case of specific humidity). The mixing ratio is often recorded in grams of water vapor per kilogram of dry air (g/kg). The mixing ratio of saturated air is called the saturation mixing ratio (w_s).

An interesting thing about humidity is the manner in which it is measured. It would be difficult and time consuming, if not impossible, for meteorologists to measure out a volume of air, separate the water vapor from the dry air and weigh them separately. Fortunately, there are quicker and easier methods. The dew point temperature (T_d) is a direct measurement that is taken to determine the amount of moisture in the air. The dew point temperature is the temperature to which a sample of air must be cooled for condensation to occur or to make the sample saturated. This measurement is easy to make. You simply take a sample of air and cool it until you first begin to see condensation. Though the dew point temperature does not tell us directly how moist the air is, as we shall see a little later, we can use the dew point temperature to find out how much water vapor is in the air.

Another measurement of temperature that is related to humidity is the wet-bulb temperature (Tw). Instead of measuring the amount of water vapor already present in the air, the wet-bulb measures the amount of water vapor that is not present in the air. More specifically, it measures the amount of energy necessary to evaporate water into the air. The wet-bulb accomplishes this by measuring the temperature drop over a surface of water as a result of evaporating water into the air. A wet wick is placed over the end of a thermometer, and the thermometer is whirled around in the air. The dryer the air, the more evaporation that will take place. Evaporation requires energy. When a water molecule evaporates and breaks the molecular bond that holds it to the water surface, the body of water loses that energy along with the molecule. This results in the cooling of the body of water. The wet-bulb thermometer measures that drop in the temperature. The wet-bulb depression is the amount the temperature drops on the wet-bulb thermometer as a result of evaporation. In saturated air, the air temperature, dew point temperature, and wet-bulb temperature are equal. In unsaturated air, dew point is always the lowest and the air temperature is the highest. The wet-bulb temperature always occurs between the other two.

Much of what has been discussed regarding moisture and humidity suggests that the amount of moisture the air can hold is variable. Generally, the warmer the air is, the greater the potential the air has for holding water. The reason for this is that the average speed of the molecules in warmer air is greater than in cooler air. Warmer air is less dense and the molecules are spread out. When air cools, the average movement of all the molecules is slowed, including the water vapor molecules. Water vapor molecules have a strong molecular attraction to each other. Therefore, as the temperature cools and molecular motion slows, the natural molecular attraction between the water vapor molecules becomes greater than the kinectic energy they possess, causing the water vapor to condense. Condensation is obviously significant in that it eventually produces precipitation, but that is not all. When the formation of droplet begins, the nucleus of each droplet is a solid particle. In many cases that solid particle is a smoke particle or some other pollutant, which may eventually fall out of the cloud in rain droplets or some other precipitation. Condensation also releases latent heat. This heats the surrounding air, causing it to be less stable. If the air is rising, the release of latent heat may prolong its ascent because the additional heat energy keeps it warm and rising, extending the process of convection.