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NATS 101 Lecture 9 Atmospheric Moisture. Hydrological Cycle. Ultimate source of all fresh water. 15%. 85%. 50%. 50%. Ahrens Fig. 4.1. 85% of water vapor in atmosphere evaporates from oceans.

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Nats 101 lecture 9 atmospheric moisture

NATS 101Lecture 9Atmospheric Moisture

Hydrological cycle
Hydrological Cycle

Ultimate source of all fresh water





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

  • 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 apartial pressure,proportional to the number molecules per unit volume. The partial pressure of water vapor is termed thevapor pressure.

Saturation 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 thesaturation vapor pressureor 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.

Nats 101 lecture 9 atmospheric moisture

Williams p62



cooled E<C

dry 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 and Temperature

SVP 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
Relative Humidity

Air with a RH=100% is said to besaturated.

RH depends on air temperature (SVP).

RH changes by either changing air’s water vapor content or the air’s temperature.

Relative humidity1
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
Relative Humidity(Ahrens, Appendix B)

Consider air that is saturated at 0oC


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%

Dew point
Dew Point

  • DP-temperature to which air must be cooled at constant pressure to become saturated. Higher DP  Higher water vapor content.

  • DP is agood indicatorof 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
Arizona Dew Point Ranges

Vapor PressureDew Point

24 mb 20oC

12 mb 10oC

6 mb 0oC

3 mb -10oC

Wet bulb temperature
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 slingpsychrometer.

wet bulb

dry thermometer

Ahrens, Fig 4.9

Wet bulb temperature ahrens appendix d
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

MonthMAXDew PWet BulbJune 100oF 37oF 65oF July 100oF 63oF 75oF

Heat index
Heat Index

Humidity reduces the rate at which sweat evaporates. Thus, the cooling rate is lowered.

Tucson Record MAX

July MAX

June MAX

Rocky Pt Summer

Ahrens, Fig 4.8

Nats 101 lecture 9 atmospheric moisture

0.5% lighter

Williams, p72

Summary moisture
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
Assignment for Next Lecture

  • Topic -Cloud and Fog Formation

  • Reading -Ahrens pages 89-96

  • Problems -4.3, 4.13, 4.14, 4.15