NATS 101
Lecture 9Atmospheric Moisture
Hydrological Cycle
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.
Ahrens Fig. 4.1
85%85%15%15%
50%
50%
Ultimate source of all fresh water
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,partial pressure, proportional to the number molecules per unit volume. The partial pressure of water vapor is termed the vapor pressure.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 saturation vapor pressurepressure 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.
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.
Williams p62
dry E>C
saturatedE=C
warmedE>C
cooled E<C
SVP and TemperatureSVP nearly doubles
with a 10oC warming
SVP and T Graph
Supercooled water droplets can exist to temps of -40oC
For temps below 0oC, SVP runs 10%-30% lower over ice
Ahrens Fig. 4.5
Relative Humidity
Air with a RH=100% is said to be saturatedsaturated.
RH depends on air temperature (SVP).
RH changes by either changing air’s water vapor content or the air’s temperature.
WaterVapor ContentRH=WaterVapor Capicity
Vapor Pressure ? 100RH(%)=Saturation Vapor Pressure
Relative Humidity
The RH for constant water vapor content can fluctuate greatly during the course of the day solely from the temperature changes
Ahrens Fig. 4.6
Relative Humidity(Ahrens, Appendix B)
Consider air that is saturated at 0oC
TempTemp VPVP SVPSVP RHRH
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%
mass of water vapor in a parcelSpecificHumidity=total mass of all air in a parcel
mass of water vapor in a parcelMixingRatio=mass of remaining air in a parcel
Values of specific humidityand mixing ratiorange from near 0% at poles to 3-4% in tropics.
Note that specific humidity mixing ratio<:
Other Measures of Humidity
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 indicatorgood 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 PressureVapor Pressure Dew PointDew Point
24 mb 20oC
12 mb 10oC
6 mb 0oC
3 mb -10oC
dry thermometer
Wet Bulb TempWet 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 Temperature
wet bulb
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
MonthMonth MAXMAX Dew PDew P Wet BulbWet Bulb June 100oF 37oF 65oF July
100oF 63oF 75oF
Heat Index
Humidity reduces the rate at which sweat evaporates. Thus, the cooling rate is lowered.
Tucson Tucson Record Record MAXMAX
Rocky Pt Rocky Pt SummerSummer
July MAX July MAX June MAX June MAX
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