Understanding Water Vapor
21PSTEM – FOSS WW Content Study
Jim Washburne
Sept. 2010
Graphics from:
Understanding Weather & Climate: wps.prenhall.com/esm_aguado_uwac_3
a) Consider a hypothetical jar containing pure water and an overlying
volume that initially contains no water vapor.
b) As evaporation begins, water vapor starts to accumulate above the
surface of the liquid. With increasing water vapor content, the
condensation rate likewise increases
c) Eventually, the amount of water vapor above the surface is enough
for the rates of condensation and evaporation to become equal. The
resulting equilibrium state is called saturation (c).
Humidity refers to the amount of water vapor in the air.
The part of the total atmospheric pressure due to
water vapor is referred to as the vapor pressure.
The vapor pressure of a volume of air depends on both the
temperature and the density of water vapor molecules.
The saturation vapor pressure is an expression of the maximum
water vapor that can exist. The saturation vapor pressure depends
only on temperature.
Terminology
Water & Air
@ equilibrium
 Vapor Pressure (PV)
Variables: Vol. of air (V), Mass of water (mw),
Mass of dry air (ma), Mass of water vapor (mv)
Mixing Ratio
(mv / ma)
(g/kg)
Specific Humidity
(mv / (mv+ma))
(g/kg)
Absolute Humidity
(mv / V)
(g/m3)
Relative Humidity (RH, a ratio) = actual/saturated
RH = PV / Psat = water vapor content / water vapor capacity
= observed / saturated (Ta)
RH = MR / Sat MR
RH = SH / Sat SH
RH = AH / Sat AH
Dew Point = Temperature when it reaches 100% saturation
Relative humidity, RH, relates the amount of water vapor
in the air to the maximum possible at the current temperature.
RH = (specific humidity/saturation specific humidity) X 100%
More water vapor can exist in
warm air than in cold air, so
relative humidity depends on
both the actual moisture
content and the air
temperature.
If the air temperature
increases, more water vapor
can exist, and the ratio of the
amount of water vapor in the
air relative to saturation
decreases.
Types of Problems
Tabular Problems
Sat. Mixing ratio (g/kg)
Rel. Humidity
(%)
oF
oC
Sat. Mix
Ratio
15
‐9.4
1.9
20
‐6.7
2.2
25
‐3.9
2.8
30
‐1.1
3.5
35
1.7
4.3
Mixing ratio
(g/kg)
Air Temp.
(oC)
2.8
‐1.1
40
4.4
5.2
2.8
32.2
45
7.2
6.2
11.1
13.2
50
10.0
7.6
22.3
36.5
55
12.8
9.3
60
15.6
11.1
65
18.3
13.2
70
21.1
15.6
75
23.9
18.8
80
26.7
22.3
85
29.4
26.2
90
32.2
30.7
95
35.0
36.5
Word Problems:
oC
If a room at 18.3
has a mixing ratio of 5.2 g/kg,
a) What is the relative humidity?
b) What is the dew point?
c) If the mixing ratio remains the same but the temp.
of the room incr. to 26.7 oC, what is the new RH?
Types of Problems
Tabular Problems
oF
oC
Sat. Mix
Ratio
15
‐9.4
1.9
20
‐6.7
2.2
25
‐3.9
2.8
30
‐1.1
3.5
35
1.7
4.3
Mixing ratio
(g/kg)
Air Temp.
(oC)
Sat. Mixing ratio (g/kg)
Rel. Humidity
(%)
2.8
‐1.1
3.5
2.8/3.5= 80
40
4.4
5.2
2.8
32.2
30.7
2.8/30.7= 9
45
7.2
6.2
11.1
18.3
13.2
11/13= 84
50
10.0
7.6
22.3
35.0
36.5
22/36= 61
55
12.8
9.3
60
15.6
11.1
65
18.3
13.2
70
21.1
15.6
75
23.9
18.8
80
26.7
22.3
85
29.4
26.2
90
32.2
30.7
95
35.0
36.5
Word Problems:
oC
If a room at 18.3
has a mixing ratio of 5.2 g/kg,
a) What is the relative humidity?
RH = 5.2/13.2 = 39%
b) What is the dew point?
Saturated at 4.4 oC
c) If the mixing ratio remains the same but the temp.
of the room incr. to 26.7 oC, what is the new RH?
RH = 5.2/22.3 = 23%
The dew point is the temperature to which the air must be cooled to become
saturated and is an expression of water vapor content. In (a), the temperature
exceeds the dew point and the air is unsaturated. When the air temperature is
lowered so that the saturation specific humidity is the same as the actual
specific humidity (b), the air temperature and dew point are equal. Further
cooling (c) leads to an equal reduction in the air temperature and dew point
so that they remain equal to each other. When the temperature at which
saturation would occur is below 0 °C, we use the term frost point.
Change in amount of water vapor in saturated air with temperature.
The air's capacity for water vapor increases as air temperature
increases. Air with a temperature of 30°C can hold more than three
times as much water vapor as air at 10°C.
Relative humidity varies significantly when the temperature
changes, even when the actual amount of water vapor in
the air remains the same.
Temperature ________
Relative Humidity _ _ _ _
www.bom.gov.au/lam/humiditycalc.shtml
Two cases: Top: RH constant @ 50%; Bottom: Tdew @ 10 oC
Fairbank
Toronto
Pittsburgh
Denver
Phoenix
Tair oC(oF)
‐10 (14)
0 (32)
10 (50)
20 (68)
30 (86)
RH (%)
50
50
50
50
50
Tdew oC
‐18
‐9
0
9
18
SH (g/kg)
1
2
4
7.5
14
RH (%)
100
100
100
53
29
Tdew oC
‐10
0
10
10
10
SH (g/kg)
2
4
8
8
8
Constant RH emphasizes relative saturation
Constant Tdew emphasizes actual amount of moisture in air
Tucson: Dew Points (used to) define Summer Monsoon
High Dew Points are uncomfortable
Dew point °C
Dew point °F
> Higher than 26 °C
> Higher than 80 °F
24 – 26 °C
75 – 80 °F
21 – 24 °C
70 – 74 °F
18 – 21 °C
65 – 69 °F
16 – 18 °C
60 – 64 °F
13 – 16 °C
10 – 12 °C
< 10 °C
55 – 59 °F
50 – 54 °F
< 49 °F
Human perception
Rel. humidity at 32 °C (90 °F)
Severely high. Even 65% and higher
deadly for asthma
related illnesses
Extremely uncomfortable, fairly 62%
oppressive
Very humid, quite 52% – 60%
uncomfortable
Somewhat uncomfortable for 44% – 52%
most people at upper edge
OK for most, but all perceive the humidity 37% – 46%
at upper edge
Comfortable
38% – 41%
31% – 37%
Very comfortable
A bit dry for some
30%
The simplest and most widely used instrument for
measuring humidity is the sling psychrometer, which
has two thermometers called the wet bulb and dry bulb.
The difference between the two temperatures, the
wet bulb depression, depends on the moisture content
of the air and can be used to determine
dew point and relative humidity.
Dew Point from dry & wet bulb
RH from dry & wet bulb
Now consider Lab notebook p.31
Calculate RH – Example p.31,#2
Calculate RH – Example p.31,#3
Orographic Effect ‐ 1
Orographic Effect ‐ 2
Orographic Effect ‐ 3
Orographic Effect ‐ 4
Orographic Effect ‐ 5
Orographic Effect ‐ 6
Orographic Effect ‐ 7
Dragon’s Breath
wps.prenhall.com/esm_aguado_uwac_3/11/2880/737513.cw/index.html
Using Radiosonde Data From a Weather Balloon Launch
http://mynasadata.larc.nasa.gov/preview_lesson_nostds.php
?&passid=34
TUS_201007(09‐10)
TEMP C
‐100
‐80
‐60
‐40
DWPT C
Series3
Series4
‐20
0
20
40
0
100
200
300
400
-- Cloud Top height -500
region of cloud formation
600
700
800
900
1000
60
TUS_201007(29‐30)
29TEMP
‐100
‐80
‐60
29DWPT
30_0TEMP
‐40
30_0DWPT
DWPT 0.50
‐20
0
20
40
0
100
200
300
-- Cloud Top height --
400
500
region of cloud formation
600
7/30, 0130z
7/30, 1900z
700
800
900
1000
EXTRA SLIDES
Another View of Orographic effect
Unlike the DALR, the SALR is not a
constant value. If saturated air cools
from 30 °C to 25 °C (a 5° decrease),
the specific humidity decreases from
27.7 grams of water vapor per
kilogram of air to 20.4. A 5 °C drop in
temperature from 5 °C to 0 °C lowers
the specific humidity only 1.7 grams
for each kilogram of air. This brings
about less warming to offset the
cooling by expansion, as well as a
greater saturated adiabatic lapse rate.
Dew Point is measure of how much moisture there is in the air
Change water vapor to Log axis
Find best fit line – slopes are equal