| Relative Humidity |
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Formulaically expressed as: : Equivalently, it is the ratio of the mass of water per unit volume of gas and the mass of water per unit volume of a saturated gas. The numerators of these ratios are the two ways of expressing Absolute Humidity . The following graph compares Dew Point (maximum humidity in red) to 50% relative humidity (green line halfway between zero and the dew point across the range of temperatures). IN SIMPLE WORDS The warmer air is, the more water vapor it can "hold." Dew point is a measure of how much water vapor is actually in the air. Relative humidity is a measure of the amount of water in the air compared with the amount of water the air can hold at the temperature it happens to be when you measure it. Now, let's see how dew point and relative humidity work. Imagine, that at 3 p.m. you measure the air's temperature at 30 degrees and you measure its humidity at 9 grams per cubic meter of air. What would happen if this air cooled to 10 degrees with no water vapor being added or taken away? As it cools to 10 degrees the air becomes saturated; that is, it can't hold any more water vapor than 9 grams per cubic meter. Cool the air even a tiny bit more and its water vapor will begin condensing to form clouds, fog or dew - depending on whether the air is high above the ground, just above the ground, or right at the ground. Back at 3 p.m., when we made the measurements, we could say that the air's dew point is 10 degrees C. That is, if this particular air were cooled to 10 degrees at ground level, its humidity would begin condensing to form dew. How about relative humidity? At 3 p.m. the air has 9 grams of water vapor per cubic meter of air. We divide 9 by 30 and multiply by 100 to get a relative humidity of 30% In other words, the air actually has 30% of the water vapor it could hold at its current temperature. Cool the air to 20 degrees. Now we divide 9, the vapor actually in the air, by 17, the vapor it could hold at its new temperature, and multiply by 100 to get a relative humidity of 53% (rounded off). Finally, when the air cools to 10 degrees, we divide 9 by 9 and multiply by 100 to get a relative humidity of 100% - the air now has all the vapor it can hold at its new temperature. OTHER IMPORTANT FACTS A gas in this context is referred to as saturated when the vapor pressure of water is at the equilibrium vapor pressure for water vapor; liquid water (and ice, at the appropriate temperature) will fail to lose mass through evaporation when exposed to saturated air. It also corresponds to the possibilility of Dew or Fog forming, within a space that lacks temperature differences among its portions, for instance in response to decreasing temperature. Fog consists of droplets of liquid. (Even though these droplets may be so small as to fall imperceptibly slowly through the mixed gas we call air, this behavior is too different from that of water vapor to reflect it in the same scale. This explains the restriction of relative-humidity discussions to 100% and below.) The statement that relative humidity can never be above 100%, while a fairly good guide, is not absolutely accurate, without a more sophisticated definition of humidity than the one given here. An arguable exception is the Wilson Cloud Chamber which uses, in nuclear physics experiments, an extremely brief state of " Supersaturation " to accomplish its function. For a given Dewpoint and its corresponding Absolute Humidity , the relative humidity will change inversely with the Temperature . This is because the partial pressure of water increases with temperature – the principle behind everything from Hair Dryer s to Dehumidifier s. Due to this changing partial pressure of water vapor in air as temperature changes, the water content of air at sea level can get as high as 3% at 30 °C (86 °F), and no more than about 0.5% at 0 °C (32 °F). This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during Winter , reflected by dry Skin , Itch y Eye s, and persistence of Static Electric charges. Even with saturation (100% humidity) outdoors, heating of whatever outside air comes indoors raises its moisture capacity, reflected in decreased relative humidity and increased evaporation rates from moist surfaces. Similarly, during summer in humid climates a great deal of water condenses from air cooled in air conditioners. Warmer air is cooled below its dewpoint and that water condenses. This phenomenon is the same as that which causes the outside of a cup containing an ice-cold drink to get wet. Water vapor is a lighter gas than dry air, so humid air will tend to rise through drier air at the same temperature. This phenomemon is a mechanism behind Thunderstorms , since as the humid rising air also becomes colder as it rises due to Adiabatic cooling, and as the air cools past its dew point, water vapor condenses into small droplets (which may form clouds). They will further condense into larger water droplets if certain conditions are met, for example, the presence of small particles (seeds). Should the droplets become too large for the updraft to lift, they will fall down as rain. Relative humidity is often mentioned in Weather Forecasts and reports, as it is an indicator of the likelihood of Precipitation , dew, or fog. In hot Summer Weather , it also increases the Apparent Temperature to Human s (and other Animal s) by preventing the Evaporation of Perspiration from the skin. This effect is calculated in a Table , resulting in the Heat Index or Humidex . A device used to measure humidity is called a Hygrometer , one used to regulate it is called a Humidistat , or sometimes Hygrostat . (These are Analogous to a Thermometer and Thermostat for temperature, respectively.) SEE ALSO EXTERNAL LINKS |
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