WW2010 University of Illinois

 WW2010

 welcome

 > online guides

 archives

 educational cd-rom

 current weather

 index

 Online Guides

 introduction

 > meteorology

 remote sensing

 projects, activities

 Meteorology

 introduction

 air masses, fronts

 > clouds, precipitation

 el nino

 forces, winds

 hurricanes

 hydrologic cycle

 light, optics

 midlatitude cyclones

 severe storms

 weather forecasting

 Clouds, Precipitation

 introduction

 > development

 cloud types

 precipitation

 Development

 states of water

 > relative humidity

 rising air

 convection

 convergence

 topography

 fronts

 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