Transcript lecture 9 - The University of Arizona Department of

NATS 101
Lecture 9
Atmospheric Moisture
Hydrological Cycle
Ultimate source of all fresh water
15%
50%
85%
50%
Ahrens Fig. 4.1
85% of water vapor in atmosphere evaporates from oceans.
About 50% of precipitation that falls over land is runoff,
and the other 50% is transpired/evaporated.
Water vapor molecules reside in atmosphere for one week.
Humid Air
• Humid air is a mixture of molecules that
make up dry air (mostly N2 and O2) and
lighter water vapor (H2O) molecules.
• Each type of molecule contributes a fraction
of total air pressure, or a partial pressure,
proportional to the number molecules per
unit volume. The partial pressure of water
vapor is termed the vapor pressure.
Saturation Vapor Pressure
• The partial vapor pressure at which the rate of
evaporation equals the rate of condensation in a
closed system is called the saturation vapor
pressure or SVP. The SVP effectively denotes the
maximum water vapor that air the can “hold”.
• SVP depends strongly on temperature.
• Vapor pressure and SVP provide a measure of the
actual water vapor content and the air’s potential
capacity, respectively.
Williams p62
dry
E>C
saturated
E=C
warmed
E>C
cooled
E<C
SVP depends on temperature. As temperature increases,
more molecules are energetic enough to escape into the air.
Concept applies to an ice surface. SVP over ice is lower
because water molecules are bonded more tightly to ice.
For the temperatures of interest, some water molecules are
energetic enough to escape into atmosphere and SVP>0.
SVP and Temperature
SVP nearly doubles
with a 10oC warming
SVP and T Graph
Supercooled water
droplets can exist to
temps of -40oC
o
For temps below 0 C,
SVP runs 10%-30%
lower over ice
Ahrens Fig. 4.5
Relative Humidity
RH= Water Vapor Content
Water Vapor Capicity
RH(%)= Vapor Pressure ×100
Saturation Vapor Pressure
Air with a RH=100% is said to be saturated.
RH depends on air temperature (SVP).
RH changes by either changing air’s water
vapor content or the air’s temperature.
Relative Humidity
Ahrens Fig. 4.6
The RH for constant water vapor content
can fluctuate greatly during the course of
the day solely from the temperature changes
Relative Humidity
(Ahrens, Appendix B)
Consider air that is saturated at 0oC
Temp
VP
SVP
RH
0oC
6 mb
6 mb
100%
10oC
6 mb
12 mb
50%
20oC
6 mb
23 mb
28%
30oC
6 mb
42 mb
14%
40oC
6 mb
68 mb
9%
Other Measures of Humidity
SpecificHumidity= mass of water vapor in a parcel
total mass of all air in a parcel
Mixing Ratio= mass of water vapor in a parcel
mass of remaining air in a parcel
Values of specific humidityand mixing ratio
range from near 0% at poles to 3-4% in tropics.
Note that specific humidity  mixing ratio
Dew Point
• DP-temperature to which air must be cooled
at constant pressure to become saturated.
Higher DP  Higher water vapor content.
• DP is a good indicator of the actual water
vapor content since air pressures vary very
little along the earth’s surface.
• DP is plotted on surface weather maps.
• DP depression (Temp-DP) is plotted aloft.
Arizona Dew Point Ranges
Vapor Pressure
24 mb
12 mb
6 mb
3 mb
Dew Point
o
20 C
10oC
0oC
-10oC
Wet Bulb Temperature
Wet Bulb Temp -Lowest temp to which air can
be cooled by evaporation of water into it.
Warmer than dew point since moisture is being
added to air which raises dew point.
Measured with sling psychrometer.
wet bulb
dry thermometer
Ahrens, Fig 4.9
Wet Bulb Temperature
(Ahrens, Appendix D)
Wet bulb temperature lies about 30% of the
way from the dew point to the temperature
Application-Wet bulb temp gives maximum
possible efficiency for a swamp cooler
Month MAX
Dew P
Wet Bulb
June
100oF
37oF
65oF
o
o
o
July
100 F
63 F
75 F
Heat Index
Tucson
Record
MAX
July MAX
June MAX
Humidity reduces
the rate at which
sweat evaporates.
Thus, the cooling
rate is lowered.
Rocky Pt
Summer
Ahrens, Fig 4.8
Humid Air is Less Dense
Williams, p72
0.5%
lighter
Williams, p72
Summary: Moisture
• Water vapor comes from the evaporation of
sea water and resides in atmo. for ~1 week.
• Air has a saturation level for water vapor
• Saturation level depends on air temperature
• Humid air is less dense than dry air
• Water vapor content can be quantified by
RH, dew point temp, wet bulb temp
Assignment for Next Lecture
• Topic - Cloud and Fog Formation
• Reading - Ahrens pages 89-96
• Problems - 4.3, 4.13, 4.14, 4.15