NATS 101

Lecture 6 Part AAtmospheric Moisture

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Hydrological Cycle

85% of water vapor in atmosphere evaporates from oceans.Ahrens Fig. 4.1

85%85%15%15%

50%

50%

Ultimate source of all fresh waterUltimate source of all fresh water

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.

1 week

Colorado River Reservoir Levels

Lake Mead

http://www.arachnoid.com/NaturalResources/image.php?mead

Lake Powell

http://www.usbr.gov/uc/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, 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.

SVP depends on temperature. As water warms, more molecules have fast enough speeds to escape into the air.

Williams p62

dry E>C

saturatedE=C

warmed E>C

cooled E<C

Concept applies to an ice surface too. SVP over ice is lower because water molecules are bonded more tightly to ice.

For temperatures of interest, some water molecules are energetic enough to escape into atmosphere. So SVP>0.

SVP and TemperatureSVP almost 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

12

23

42

Relative Humidity

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.SVP and T Graph

RH= WaterVapor ContentWaterVapor Capicity

RH(%)= Vapor Pressure ×100SaturationVaporPressure

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 starts saturated at 0oC

Temp VP SVP RH

0oC 6 mb 6 mb 100%

10oC 6 mb 12 mb 50%

20oC 6 mb 22 mb 26%

30oC 6 mb 40 mb 14%

40oC 6 mb 70 mb 8%

Dew Point (DP)

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

• DP is a excellent 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 Dew Point

24 mb 20oC

12 mb 10oC

6 mb 0oC

3 mb -10oC

dry thermometer

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 Temperature

wet bulb

Ahrens, Fig 4.9

Wet Bulb Temperature(Ahrens, Appendix D)

Wet bulb temperature is about 30-40% of the way from the dew point to the temperature for surface conditions that characterize AZ

Application-Wet bulb temp gives maximum possible efficiency for a swamp cooler

Month MAX Dew P Wet 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

Summary: Moisture

• Air has a saturation level for water vapor, beyond no additional vapor can be added

• Saturation level depends on air temperature

• Actual water vapor content of air can be quantified by several measures

Vapor Pressure, Saturation Vapor Pressure, Relative Humidity, Dew Point, Wet-Bulb

Summary: Moisture Measures

• Saturation Vapor Pressure (SVP) - highest possible partial pressure for water vapor

• Vapor Pressure - actual partial pressure of water vapor

• Relative Humidity – is the ratio

actual content

saturation value

Summary: Moisture Measures

• Dew Point (DP) - temperature to which air must be cooled at constant pressure to become saturated. Plotted on WX maps.

• Wet Bulb Temperature - lowest temperature to which air can be cooled by evaporation of water. Higher than DP since water vapor is added to air.

Temp ≥ Wet Bulb Temp ≥ Dew Point

Next Class AssignmentFog and Clouds

• Ahrens - Reading3rd Pg: 87-105, 109-1184th Pgs: 89-108, 111-1215th Pgs: 91-108, 111-120

• Homework04 - D2L (Due Monday Feb. 22)

3rd-Pg 105: 4.13, 14, 15, 16

4th-Pg 108: 4.13, 14, 15, 16

5th-Pg 108: 4.13, 14, 15, 16

Do Not Hand In 4.16

NATS 101

Lecture 6 Part BCondensation

Cloud tracks caused by cloud condensation nuclei from ship exhausthttp://mm04.nasaimages.org

Cloud Condensation Nuclei

Small, airborne particles are necessary on which water vapor can condense to produce cloud droplets

Without such particles, RH>100% would be needed to produce clouds

Such surfaces are called Cloud Condensation Nuclei (CCN)

CCN are light and stay suspended for days

Cloud Condensation Nuclei

• Sources

Dust, volcanic ash, smoke, soot, salt, sulfate particles

• Concentrations

1,000-10,000 per cc Highest over cities Highest at surface

Ahrens, Meteorology Today, 5th Ed.

Cloud Condensation Nuclei

Water-SeekingSaltSulfuric acidNitric acid

Water-RepellingOilsGasolineWaxes

Condensation can occur on hygroscopic CCN for RH<100% Accounts for Haze when the RH approaches 60-80%

Ahrens, Meteorology Today, 5th Ed.

Haze over Melting Snow

Ahrens, Meteorology Today, 5th Ed.

Condensation

When air becomes supersaturated from either the temperature cooling or the addition of water vapor, water condenses onto CCN as small cloud droplets

Diameters of droplets are 2-20 microns, (10-100 times smaller than human hair)

Concentrations are 50-1,000 droplets per cc

Size of Cloud Droplets

Williams, The Weather Book, p73

100 times100 times

0.2

Summary: Condensation

Condensation

Can occur by cooling or moistening of air

CCN permit condensation at RH’s < 100%

Small (<0.2 to 1 microns) airborne particles

Responsible for Haze formation at RH < 100%

Next Class AssignmentFog and Clouds

• Ahrens - Reading3rd Pg: 87-105, 109-1184th Pgs: 89-108, 111-1215th Pgs: 91-108, 111-120

• Homework04 - D2L (Due Monday Feb. 22)

3rd-Pg 105: 4.13, 14, 15, 16

4th-Pg 108: 4.13, 14, 15, 16

5th-Pg 108: 4.13, 14, 15, 16

Do Not Hand In 4.16