WW2010
University of Illinois

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Meteorology
 
  introduction
 
  air masses, fronts
 
> clouds, precipitation
 
  el nino
 
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Clouds, Precipitation
 
  introduction
 
> development
 
  cloud types
 
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Development
 
  states of water
 
> relative humidity
 
  rising air
 
  convection
 
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  topography
 
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  rain or snow

User Interface
 
  graphics
> text

NOTE: We've guessed that you're not using a client that supports colored tables and have tried to compensate. Low graphics mode looks much better on clients that do... we recommend switching to Netscape 3.0 or Microsoft Internet Explorer.
.
Relative Humidity
indicates how moist the air is

Relative humidity may be defined as the ratio of the water vapor density (mass per unit volume) to the saturation water vapor density, usually expressed in percent:

Relative Humidity (RH) =
(Actual Vapor Density)
--------------------
(Saturation Vapor Density)
X 100%

Relative humidity is also approximately the ratio of the actual to the saturation vapor pressure.

RH =
(Actual Vapor Pressure)
--------------------------
(Saturation Vapor Pressure)
X 100%

Actual vapor pressure is a measurement of the amount of water vapor in a volume of air and increases as the amount of water vapor increases. Air that attains its saturation vapor pressure has established an equilibrium with a flat surface of water. That means, an equal number of water molecules are evaporating from the surface of the water into the air as are condensing from the air back into the water.

Saturation vapor pressure is a unique function of temperature as given in the table below. Each temperature in the table may be interpreted as a dew point temperature, because as the ground cools, dew will begin to form at the temperature corresponding to the vapor pressure in this table.

(C)Temp(F)|Sat Vapor Prs (mb) | (C)Temp(F)|Sat Vapor Prs (mb)

-18
-15
-12
-09
-07
-04
-01
02
04
07
10
13
16

00
05
10
15
20
25
30
35
40
45
50
55
60

1.5
1.9
2.4
3.0
3.7
4.6
5.6
6.9
8.4
10.2
12.3
14.8
17.7

|
|
|
|
|
|
|
|
|
|
|
|
|

18
21
24
27
29
32
35
38
41
43
46
49
52

65
70
75
80
85
90
95
100
105
110
115
120
125

21.0
25.0
29.6
35.0
41.0
48.1
56.2
65.6
76.2
87.8
101.4
116.8
134.2

Chart adapted from: Ahrens

For example, if the water vapor pressure in the air is 10.2 millibars (mb), dew will form when the ground reaches 45 degrees Fahrenheit (F). The relative humidity for air containing 10.2 mb of water vapor is simply 100% times 10.2 mb divided by the saturation vapor pressure at the actual temperature. For example, at 70 F the saturation vapor pressure is 25 mb, so the relative humidity would be

RH = 100% X (10.21 / 25.0) = 41%



states of water
Terms for using data resources. CD-ROM available.
Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.

rising air