Atmospheric humidity, which is the amount of water vapour or moisture in the air, is another leading climatic element, as is precipitation. All forms of precipitation, including drizzle, rain, snow, ice crystals, and hail, are produced as a result of the condensation of atmospheric moisture that forms clouds in which some of the particles, by growth and aggregation, attain sufficient size to fall from the clouds and reach the ground.
At 30 °C (86 °F), 4 percent of the volume of the air may be occupied by water molecules, but, where the air is colder than −40 °C (−40 °F), less than one-fifth of 1 percent of the air molecules can be water. Although the water vapour content may vary from one air parcel to another, these limits can be set because vapour capacity is determined by temperature. Temperature has profound effects upon some of the indexes of humidity, regardless of the presence or absence of vapour.
The connection between an effect of humidity and an index of humidity requires simultaneous introduction of effects and indexes. Vapour in the air is a determinant of weather, because it first absorbs the thermal radiation that leaves and cools Earth’s surface and then emits thermal radiation that warms the planet. Calculation of absorption and emission requires an index of the mass of water in a volume of air. Vapour also affects the weather because, as indicated above, it condenses into clouds and falls as rain or other forms of precipitation. Tracing the moisture-bearing air masses requires a humidity index that changes only when water is removed or added.
Different levels of humidity
Absolute humidity
Absolute humidity is the vapour concentration or density in the air. If mv is the mass of vapour in a volume of air, then absolute humidity dv is simply dv = mv/ V, in which V is the volume and dv is expressed in grams per cubic metre. This index indicates how much vapour a beam of radiation must pass through. The ultimate standard in humidity measurement is made by weighing the amount of water gained by an absorber when a known volume of air passes through it; this measures absolute humidity, which may vary from 0 gram per cubic metre in dry air to 30 grams per cubic metre (0.03 ounce per cubic foot) when the vapour is saturated at 30 °C. The dv of a parcel of air changes, however, with temperature or pressure even though no water is added or removed, because, as the gas equation states, the volume V increases with the absolute, or Kelvin, temperature and decreases with the pressure.
Specific humidity
The meteorologist requires an index of humidity that does not change with pressure or temperature. A property of this sort will identify an air mass when it is cooled or when it rises to lower pressures aloft without losing or gaining water vapour. Because all the gases will expand equally, the ratios of the weight of water to the weight of dry air, or the dry air plus vapour, will be conserved during such changes and will continue identifying the air mass.
Relative humidity
Relative humidity (U) is so commonly used that a statement of humidity, without a qualifying adjective, can be assumed to be relative humidity. U can be defined, then, in terms of the mixing ratio r that was introduced above. U = 100r/ rw, which is a dimensionless percentage. The divisor rw is the saturation mixing ratio, or the vapour capacity. Relative humidity is therefore the water vapour content of the air relative to its content at saturation. Because the saturation mixing ratio is a function of pressure, and especially of temperature, the relative humidity is a combined index of the environment that reflects more than water content. In many climates the relative humidity rises to about 100 percent at dawn and falls to 50 percent by noon. A relative humidity of 50 percent may reflect many different quantities of vapour per volume of air or gram of air, and it will not likely be proportional to evaporation.
Precipitation
Precipitation is one of the three main processes (evaporation, condensation, and precipitation) that constitute the hydrologic cycle, the continual exchange of water between the atmosphere and Earth’s surface. Water evaporates from ocean, land, and freshwater surfaces, is carried aloft as vapour by the air currents, condenses to form clouds, and ultimately is returned to Earth’s surface as precipitation. The average global stock of water vapour in the atmosphere is equivalent to a layer of water 2.5 cm (1 inch) deep covering the whole Earth. Because Earth’s average annual rainfall is about 100 cm (39 inches), the average time that the water spends in the atmosphere, between its evaporation from the surface and its return as precipitation, is about 1/40 of a year, or about nine days. Of the water vapour that is carried at all heights across a given region by the winds, only a small percentage is converted into precipitation and reaches the ground in that area. In deep and extensive cloud systems, the conversion is more efficient, but even in thunderclouds the quantities of rain and hail released amount to only some 10 percent of the total moisture entering the storm.
In the measurement of precipitation, it is necessary to distinguish between the amount—defined as the depth of precipitation (calculated as though it were all rain) that has fallen at a given point during a specified interval of time—and the rate or intensity, which specifies the depth of water that has fallen at a point during a particular interval of time. Persistent moderate rain, for example, might fall at an average rate of 5 mm per hour (0.2 inch per hour) and thus produce 120 mm (4.7 inches) of rain in 24 hours. A thunderstorm might produce this total quantity of rain in 20 minutes, but at its peak intensity the rate of rainfall might become much greater—perhaps 120 mm per hour (4.7 inches per hour), or 2mm (0.08 inch) per minute—for a minute or two.
Relation between humidity and precipitation
When it rains, it will increase the relative humidity because of the evaporation. The air where the rain is falling may not be completely saturated with water vapor. However, the longer it rains, the more the humidity will increase because of the air constantly drawing the water. The evaporation will cool the air and increase the absolute moisture content of the air locally. On a larger scale, rain will remove water vapor through air condensation and deposit it on the surface. This means that across larger volumes, the average relative humidity reduces through rain. There are a variety of factors that need to be taken into consideration, including:
- Amount of rainfall
- Temperature
- Volume of space
When the air is hotter, it will cause the water to evaporate faster, thus creating a higher level of humidity. If the air is cooler, the water will reduce the humidity level and actually make it seem cooler than the temperature outside.
There is also the matter of the dewpoint temperature, which talks more heavily about moisture and the amount of water vapor in the air. It is the temperature that the air must be cooled to in order for air to reach saturation. Dewpoint can vary from the mid-60s all the way up to the high 80s depending upon location and the time of year. It’s also important to remember that humidity levels will affect everyone differently. However, the dew point will remain the same.
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