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