What is dew point?

Question

Is dew point is temperature under a shadow?

Answer 1

THE DEW POINT TEMPERATURE is the temperature to which the air must be cooled to reach saturation (assuming air pressure remains the same). The dew point is a direct measure of the amount of moisture present in the air, and directly affects how you feel... or in other words... measures the amount of humidity in the air. Remember, the temperature never drops below its dew point, but can drop to it. Generally, we start to feel some discomfort when the dew point gets to or just above 60 degrees.

Answer 2

The "dew point" or "dewpoint" of a given parcel of air is the temperature to which the parcel must be cooled, at constant barometric pressure, for water vapor to condense into water, called dew. When the dew point temperature falls below freezing it is called the frost point as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition.

Answer 3

Dew Point is measured in degrees. (F or C) and it is the temperature where water vapour condenses out of air. Where the temperature is close to the dew point, we see fog. Where the temperature is close to the dew point at ground level, we see dew on the grass. To calculate the dew point you need to know the humidity (measured with an hygrometer) as it relates to the temperature (as measured with a thermometer),

Answer 4

Dew point is the temperature where the air (at it's current moisture content) will reach 100% humidity. Colder air can't hold as much moisture content, so when you lower the temperature, you increase the relative humidity until it reaches 100% and the water vapor in the air condense out as dew.

Answer 5

Dew point is the temperature at which the humidity is 100%, or the temperature at which the air is saturated with water vapor.

When the humidity is high, the dew point is also high.
When the humidity is low, the dew point is also low.

Dew point is a temperature.

Answer 6

The dew point temperature under a shadow is directly proportional to the angle of the dangle, multiplied then subtracted by the the heat of the meat, for men anyway. No one has come up with a theory for women yet, they are the puzzles of the universe, dewly speaking of course.

Answer 7

'Dew point' is the temperature at which water vapor begins to condense out of the air.

Doug

Answer 8

dew point is the temperature the air would have to drop to pr reach to be completley saturated...high dew points in the summer mean very humid, muggy conditions...like here on the Atlantic Seaboard in the summer!!

Answer 9

The "dew point" or "dewpoint" of a given parcel of air is the temperature to which the parcel must be cooled, at constant barometric pressure, for water vapor to condense into water, called dew. When the dew point temperature falls below freezing it is called the frost point as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition. The graph to the right shows the maximum percentage of water vapor that can exist in air at sea level across a range of temperatures. Note that with higher temperatures the equilibrium partial pressure of water vapor increases thus more water evaporates. The behavior of water vapor does not depend on the presence of air. The formation of dew would occur at the dew point even if the only gas present were water vapour.

At a given barometric pressure, independent of temperature, the dew point indicates the mole fraction of water vapor in the air, or, put differently, determines the specific humidity of the air. If the barometric pressure rises without changing this mole fraction, the dew point will rise accordingly. Reducing the mole fraction will bring the dew point back down to its initial value. In the same way, increasing the mole fraction after a pressure drop brings the dew point back up to its initial level. For this reason, the same dew point in New York and Denver (which is at a much higher altitude) will imply that a higher fraction of the air in Denver consists of water vapor than in New York.

On the other hand, at a given temperature but independent of barometric pressure, the dew point indicates the absolute humidity of the air. If the temperature rises without changing the absolute humidity, the dew point will rise accordingly. Reducing the absolute humidity will bring the dew point back down to its initial value. In the same way, increasing the absolute humidity after a temperature drop brings the dew point back up to its initial level. Coming back to the New York - Denver example, this means that if the dew point and temperature in both cities are the same, then the mass of water vapor per cubic meter of air will also be the same in those cities.

The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. If the relative humidity is 100%, the dew point is equal to the current temperature. Given a constant dew point, an increase in temperature will lead to a decrease in relative humidity. It is for this reason that equatorial climates can have low relative humidity, yet still feel humid.

Humans tend to react with discomfort to high dew points. Those accustomed to continental climates often begin to feel uncomfortable when the dew point reaches between 15 and 20 °C (59 to 68 °F). Most inhabitants of these areas will consider dew points above 21 °C (70 °F) to be oppressive.

Here is a formula to calculate the dew point in degrees Celsius to within ±0.4 °C. It is valid for

0 °C < T < 100 °C
0.01 < RH < 1.0
0 °C < Td < 50 °C
where

T = temperature in degrees Celsius
RH = is the relative humidity as a fraction (not percent)
Td = the dew point temperature to be calculated
The formula is:

where

and

a = 17.27
b = 237.7 °C
ln is the natural logarithm.
For a derivation of the above formula as well as an error estimation of it, see e.g. [1].

There is also a very simple approximation which allows you to convert in your head between the dew point, the dry bulb temperature and the relative humidity, which is accurate to within about ±1 °C as long as the relative humidity is above 50%. This can be expressed as a simple rule of thumb: 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. In equations this is:
or

RH = 100 − 5(T − Td)

where in this case RH is in percent, and T and Td are in degrees Celsius. The derivation of this, a discussion of its accuracy, comparisons to other approximations, and more information on the history and applications of the dew point are given in the Bulletin of the American Meteorological Society .

In Fahrenheit
For example, a relative humidity of 100% means dew point is same as air temp. For 90% RH dew point is 3 degrees Fahrenheit lower than air temp. For every 10 percent lower, dew point drops 3 deg.

Tf_d is in Fahrenheit RH same as above.

Answer 10

noooo..its the spot where the family dog leaves a load on the rug!!!