The three types of humidity – relative, absolute and specific – are different ways of measuring the water vapor component of air at a particular geographic location. However, the word humidity has evolved in common usage to represent what it measures; it is often used to mean the water content itself and not just its measure.
Relative humidity is the most well known as it’s the type used in weather forecasting. This is because it is the measurement most indicative of the likelihood of fogs forming or precipitation (rain, hail, sleet or, snow) falling. Relative humidity is a comparison of actual water vapor content in the air against the maximum possible, the saturation vapor pressure of water for the current air temperature; the higher the temperature, the higher the possible maximum. For example, if the temperature is 21 degrees Celsius (70 degrees Fahrenheit), then the maximum concentration of water vapor the air can hold is 2.54 percent. Or in other words, in any given volume of air, a maximum of 25,400 parts out of every million (ppmv) can be water molecules (H2O). If, in fact, the water vapor content is 12,700 ppmv, then the relative humidity is 50%, because the air contains half of the water vapor it could hold.
Absolute humidity measures the water vapor content in absolute terms; how much there is, without any regard for or concern about the maximum amount the air could carry. It is measured in terms of mass of water per volume of air, and is therefore also known as volumetric humidity. While any mass and volume units could be used, in the US, pounds per cubic foot is common, and elsewhere, grams per cubic meter. In chemical engineering it is preferable to measure the mass of the water against the mass of the rest of the air (called dry air) in the given volume, rather than the volume itself. This is referred to as the mixing or humidity ratio and is usually measured in pound per pound units in the US, or kilogram per kilogram elsewhere.
Specific humidity is very similar to the humidity ratio, but instead of being the ratio between the water vapor and dry air, it is the ratio between the water vapor and all the air, including the water vapor. As with the humidity ratio it is measured on a mass to mass basis, with pound per pound or kilogram per kilogram units.
The amount of water vapor in air varies quite dramatically. The absolute humidity varies from next to nothing in dry deserts like the middle of the Sahara, through 0.1% at the poles, one to two percent in the spring and fall seasons of temperate regions, to five percent or more on hot, humid days, wherever they may occur. Evaporation is always the cause of rising absolute humidity. But if evaporation is occurring slower than the temperature is rising, while the water content of the air will be increasing, the relative humidity may be decreasing. The increase in water vapor is not keeping up with the increasing capacity of the warming air to hold water vapor.
Evaporation from bodies of liquid water on the Earth’s surface, both the flowing streams and rivers, and the comparatively still lakes, seas and oceans, is the primary source of the air’s water vapor. As stated earlier, the warmer the air the more water vapor it can contain, so the majority of evaporation occurs in the tropics. That is why hurricanes and cyclones always form in that region.
The second largest source is transpiration by plants, an evaporative process that enables the transport of nutrients from the roots to the upper parts of the plant. Water evaporates through stomata in the leaves, that function like valves, resulting in a lower pressure that sucks the water up in the same way we might suck a drink through a straw. To continue the flow the plant must continue to transpire water vapor into the surrounding air. Some plants grow very deep root systems to access ground water. This mechanism can be self-sustaining for forests in what would otherwise be dry areas. As humanity have found to their detriment at times; removing forests doesn’t necessarily result in farm or pasture lands, but can lead to deserts instead. The Four Corners region of the US is a prime example: a reasonably forested area, that was felled for building materials and to clear lands for maize, became scrub desert incapable of supporting the local nation of ancient Pueblo Dwellers within about 300 to 400 years.
Minor sources include the evaporation of animal sweat for cooling purposes and both animal and plant respiration. Human industrial processes have been an increasing source over the last few hundred years.
The primary cause for decreasing absolute humidity is cooling temperatures. As the temperature decreases the saturation vapor pressure lowers and liquid water will condense in the form of microscopic droplets once the relative humidity has reached 100%. This may result in clouds forming, or dew or frost on the ground. Paradoxically, as the absolute humidity of the air goes down it may feel wetter to us. This is because the relative humidity is increasing towards 100% and we are most comfortable with a relative humidity of 50%.
Precipitation can reduce both the absolute and relative humidities. On hot, humid, summer days, a brief summer shower can fall and make everything much more comfortable, for a short time after anyway. Air temperature cools with altitude, so saturated hot air rising produces micro-droplets of liquid water as it cools. This can result in light falls or summer showers of cool water droplets. As these droplets pass through the air closer to ground level they cool the air, lowering the saturation vapor pressure and thus causing liquid water to condense and join the falling drops. At the end of the shower some of the water vapor has been removed from the air closer to the ground, a reduction in the absolute humidity. The air temperature warms to the previous level almost immediately, raising the saturation vapor pressure back to what it was, but the water vapor content has been reduced, so the relative humidity is also reduced. Unfortunately, evaporation soon raises the relative humidity again, and we soon feel hot and sticky once more.
While our discomfort when the relative humidity is high or low is a noticeable effect, the primary effect of humidity is our planet’s weather. The molecular mass of water vapor is 18.02, lower than that for the average of the other components based on their respective percentages; dry air having a molecular mass of 28.98. What this means is that the higher the water vapor content (humidity) of air, the lower its mass, and air pressure is directly proportional to its mass. This is where the high and low pressure readings shown in weather forecast maps comes in. The higher the absolute humidity and therefore the water vapor content of the air, the lower the overall air pressure and vice versa. Low pressure regions have a high water vapor content and high pressure regions have a low water vapor content. The lower the water vapor content of the air, the less likely it is to result in precipitation.
This also impacts on airflow or wind. Although the tidal influences of the moon, and to a lesser extent the sun, influence air movement much as they do the oceans, air movement is predominantly from high pressure areas to low pressure areas. As the water vapor content of air increases, the air becomes “lighter” and rises as higher pressure air pushes into the low pressure area. The rising air spirals, because it is above a curved rather than flat surface; in tropical regions this can result in the spiraling form of hurricanes or cyclones when the disparity between the low and high pressure areas is particularly great. This tends to be self-perpetuating, and often increasing, while over the water surface of oceans or seas, so that these storm systems do not break down until over land masses.