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Old 01-03-2012, 07:41 PM
Status: "Enjoying the mountains" (set 3 days ago)
 
Location: Hot-Houston Texas
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Default Humidity and dew point

Can someone explain the difference between humidity and dew point?
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Old 01-03-2012, 08:52 PM
 
Location: Colorado Springs, CO
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from wikipedia

Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture, called the Absolute humidity.[1] In everyday usage, it commonly refers to relative humidity, expressed as a percent in weather forecasts and on household humidistats; it is so called because it measures the current absolute humidity relative to the maximum. Specific humidity is a ratio of the water vapor content of the mixture to the total air content (on a mass basis). The water vapor content of the mixture can be measured either as mass per volume or as a partial pressure, depending on the usage.

http://en.wikipedia.org/wiki/Humidity

dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature. The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease.

http://en.wikipedia.org/wiki/Dew_point
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Old 01-03-2012, 11:21 PM
 
Location: Western Massachusetts
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Perhaps these examples can help:

Let's it's 80% relative humidity and 55F (dew point is 49F). Sometime later it is 80% and 80F (dew point of 74F). So both days feel just as humid, right? Well, no. The 80F day feels much stickier. The 80% is the water vapor concentration in the air compared to the maximum water vapor concentration of air, which depends on the temperature. Since 80F air can contain much more water vapor, 80% humidity of 80F air is a lot more water vapor — so it feels more humid. The dew point of 74F means that 74F is the temperature that would make the relative humidity 100%. The higher the dew point, the more water vapor in the air.

Another reason the dew point can be useful is the increase in temperature during the day will make the relative humidity decrease by a lot even if the amount of water vapor stays the same or increases a little, because more water vapor can be in the air at hotter temperatures.

For example on July 23 last year in Western Mass, at 6 am it was 68F with a dew point of 63F. The relative humidity was 84%. At 4 pm more water vapor was in the air; the dew point was 68F. But the temperature was 96F and the air definitely didn't feel any drier. Wet clothes left outside would dry much faster, though.
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Old 01-04-2012, 02:39 AM
 
Location: Minnesota, USA
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Quote:
Originally Posted by I LOVE NORTH CAROLINA View Post
Can someone explain the difference between humidity and dew point?
Simple: the temperature and the dew point are the two variables used in determining relative humidity.

For a given temperature, the dew point is the temperature at which condensation would form if the absolute humidity (water vapor content of the air) was held steady.

Relative humidity is the "distance", expressed as percent, in between the condensation point and absolute temperature. For example, let's say both the temperature and dewpoint are 70. Condensation forms. The relative humidity, is therefore, 100%. Now let's take the temperature up to 75 and leave the dewpoint at 70. The relative humidity is now 85%. Not quite humid enough for condensation to form, but still sensibly very humid and muggy - maybe right before a thunderstorm. Let's say the temperature is 90, and the dewpoint 70 - you now have a relative humidity of 52% - just another hot and humid July day in Minneapolis.
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Old 01-04-2012, 09:16 AM
Status: "Enjoying the mountains" (set 3 days ago)
 
Location: Hot-Houston Texas
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Thanks everyone for your answers!
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