The dew point of a given body of air is the temperature to which it must be cooled to become saturated with water vapor. This temperature depends on the pressure and water content of the air. When the air is cooled below the dew point, its moisture capacity is reduced and airborne water vapor will c Contact online >>
The dew point of a given body of air is the temperature to which it must be cooled to become saturated with water vapor. This temperature depends on the pressure and water content of the air. When the air is cooled below the dew point, its moisture capacity is reduced and airborne water vapor will condense to form liquid water known as dew.[1] When this occurs through the air''s contact with a colder surface, dew will form on that surface.[2]
The dew point is affected by the air''s humidity. The more moisture the air contains, the higher its dew point.[3]
When the temperature is below the freezing point of water, the dew point is called the frost point, as frost is formed via deposition rather than condensation.[4]In liquids, the analog to the dew point is the cloud point.
If all the other factors influencing humidity remain constant, at ground level the relative humidity rises as the temperature falls; this is because less vapor is needed to saturate the air. In normal conditions, the dew point temperature will not be greater than the air temperature, since relative humidity typically[5] does not exceed 100%.[6]
The dew point depends on how much water vapor the air contains. If the air is very dry and has few water molecules, the dew point is low and surfaces must be much cooler than the air for condensation to occur. If the air is very humid and contains many water molecules, the dew point is high and condensation can occur on surfaces that are only a few degrees cooler than the air.[8]
A high relative humidity implies that the dew point is close to the current air temperature. A relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is maximally saturated with water. When the moisture content remains constant and temperature increases, relative humidity decreases, but the dew point remains constant.[9]
Increasing the barometric pressure raises the dew point.[10] This means that, if the pressure increases, the mass of water vapor per volume unit of air must be reduced in order to maintain the same dew point. For example, consider New York City (33 ft or 10 m elevation) and Denver (5,280 ft or 1,610 m elevation[11]). Because Denver is at a higher elevation than New York, it will tend to have a lower barometric pressure. This means that if the dew point and temperature in both cities are the same, the amount of water vapor in the air will be greater in Denver.
When the air temperature is high, the human body uses the evaporation of perspiration to cool down, with the cooling effect directly related to how fast the perspiration evaporates. The rate at which perspiration can evaporate depends on how much moisture is in the air and how much moisture the air can hold. If the air is already saturated with moisture (humid), perspiration will not evaporate. The body''s thermoregulation will produce perspiration in an effort to keep the body at its normal temperature even when the rate at which it is producing sweat exceeds the evaporation rate, so one can become coated with sweat on humid days even without generating additional body heat (such as by exercising).
As the air surrounding one''s body is warmed by body heat, it will rise and be replaced with other air. If air is moved away from one''s body with a natural breeze or a fan, sweat will evaporate faster, making perspiration more effective at cooling the body, thereby increasing comfort. By contrast, comfort decreases as unevaporated perspiration increases.
A wet bulb thermometer also uses evaporative cooling, so it provides a good measure for use in evaluating comfort level.
Discomfort also exists when the dew point is very low (below around −5 °C or 23 °F).[citation needed] The drier air can cause skin to crack and become irritated more easily. It will also dry out the airways. The US Occupational Safety and Health Administration recommends indoor air be maintained at 20–24.5 °C (68–76 °F) with a 20–60% relative humidity,[12] equivalent to a dew point of approximately 4.0 to 16.5 °C (39 to 62 °F) (by Simple Rule calculation below).
Lower dew points, less than 10 °C (50 °F), correlate with lower ambient temperatures and cause the body to require less cooling. A lower dew point can go along with a high temperature only at extremely low relative humidity, allowing for relatively effective cooling.
Devices called hygrometers are used to measure dew point over a wide range of temperatures. These devices consist of a polished metal mirror which is cooled as air is passed over it. The dew point is revealed by observing the loss of clarity in the reflection cast by the mirror. Manual devices of this sort can be used to calibrate other types of humidity sensors, and automatic sensors may be used in a control loop with a humidifier or dehumidifier to control the dew point of the air in a building or in a smaller space for a manufacturing process.
There are several different constant sets in use. The ones used in NOAA''s presentation[17] are taken from a 1980 paper by David Bolton in the Monthly Weather Review:[18]
These valuations provide a maximum error of 0.1%, for −30 °C ≤ T ≤ 35°C and 1% < RH < 100%.Also noteworthy is the Sonntag1990,[19]
Another common set of values originates from the 1974 Psychrometry and Psychrometric Charts.[20]
Also, in the Journal of Applied Meteorology and Climatology,[21] Arden Buck presents several different valuation sets, with different maximum errors for different temperature ranges. Two particular sets provide a range of −40 °C to +50 °C between the two, with even lower maximum error within the indicated range than all the sets above:
For every 1 °C difference in the dew point and dry bulb temperatures, the relative humidity decreases by 5%, starting with RH = 100% when the dew point equals the dry bulb temperature.
The derivation of this approach, a discussion of its accuracy, comparisons to other approximations, and more information on the history and applications of the dew point, can be found in an article published in the Bulletin of the American Meteorological Society.[22]
For example, a relative humidity of 100% means dew point is the same as air temp. For 90% RH, dew point is 3 °F lower than air temperature. For every 10 percent lower, dew point drops 3 °F.
The frost point is similar to the dew point in that it is the temperature to which a given parcel of humid air must be cooled, at constant atmospheric pressure, for water vapor to be deposited on a surface as ice crystals without undergoing the liquid phase (compare with sublimation). The frost point for a given parcel of air is always higher than the dew point, as breaking the stronger bonding between water molecules on the surface of ice compared to the surface of (supercooled) liquid water requires a higher temperature.[23]
The dew point is the temperature at which air is saturated with water vapor, which is the gaseous state of water.
When air has reached the dew-point temperature at a particular pressure, the water vapor in the air is in equilibrium with liquid water, meaning water vapor is condensing at the same rate at which liquid water is evaporating.
Dew point is closely linked to relative humidity, which is the ratio of the pressure of water vapor in a parcel of air relative to the saturation pressure of water vapor in that same parcel of air at a specific temperature. Relative humidity (RH) is expressed as a percentage.
The relative humidity is 100 percent when the dew point and the temperature are the same. If the temperature drops any further, condensation will result, and liquid water will begin to form.
Compared to relative humidity, dew point is frequently cited as a more accurate way of measuring the humidity and comfort of the air, since it is an absolute measurement (unlike relative humidity).
Most people are comfortable with a dew-point temperature of 60 degrees Fahrenheit (16 degrees Celsius) or lower. At a higher dew point of, for example, 70 F (21 C), most people feel hot or "sticky" because the amount of water vapor in the air slows the evaporation of perspiration and keeps the body from cooling.
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