ATMOSPHERIC MOISTURE

ATMOSPHERIC MOISTURE

• Water vapor -- an invisible gas which can easily form cloud droplets, ice particles.

• As a cloud droplet water vapor becomes visible.

• The different forms of precipitation can have water in either liquid or solid phases.

• When water changes phase much energy can be released or absorbed.

Fig. 4-CO, p. 84

The Water Molecule

• Phase changes can be:

• evaporation / condensation liquid<-->vapor

• sublimation / deposition vapor<--> solid

• melting / freezing solid<-->liquid

• Maximum amount of water vapor in the atmosphere is a called saturated vapor.

• Water vapor occurs naturally by evaporation liquid water and depends on T

Fig. 4-1, p. 86

Fig. 4-2, p. 86

Fig. 4-3, p. 87

The Water Molecule

• Warm air can hold more water vapor.

• Basically water vapor resides in the atmosphere between other gas molecules.

• It evaporates by having its velocity increased by a collision with a larger gas molecule of usually oxygen or nitrogen.

• Atomic mass of water molecule is 18,

• nitrogen 28, oxygen 32 and argon 40.

Fig. 4-4, p. 87

HYDROLOGIC CYCLE

• The process of evaporation increases the vapor pressure until water condenses and forms clouds.

• Clouds go through a process of further combination of water droplets to form ice or large drops which begin the process of precipitation.

• Evaporation also occurs over land from soil, lakes, streams and plants (transpiration).

HYDROLOGIC CYCLE

• 85% of water occurs from ocean evaporation, 15% from the land.

• The amount of water if all released at once would cover the earth 2.5 cm.

• Of the total water on the earth

• 97.5% is in the oceans

• 1.73% is in glaciers or snow

• 0.77% is fresh water,.0008% renewable/yr

Fig. 4-5, p. 88

HUMIDITY

• Humidity --specifying the amount of water vapor in the air.

• Lets consider a parcel of air a volume of air in an imaginary elastic container.

• ABSOLUTE HUMIDITY -- AH

• AH = mass of vapor/volume of air

• Unit is: g/m3

Fig. 4-6, p. 89

Fig. 4-7, p. 89

SPECIFIC HUMIDITY and MIXING RATIO

• Lets consider specifying humidity without using volume.

• Again consider a parcel of air.

• SPECIFIC HUMIDITY -- SH

• SH = Mass of vapor/total mass of air (g/kg)

• Consider a parcel of air. This time use

• another expression -- the MIXING RATIO

• MR = Mass of vapor/mass of dry air (g/kg)

Fig. 4-8, p. 89

Fig. 4-9, p. 90

Saturation Vapor Pressure

• There is a maximum amount of water vapor that air can hold. If that amount is exceeded the vapor will either condense as a liquid or freeze as a deposition.

• The amount of vapor held by water depends on the temperature because the amount of average volume that the air molecules occupy vary with their speed.

Fig. 4-10, p. 91

Boiling Point

• The boiling point is the maximum temperature of a liquid which is undergoing rapid evaporation.

• That point varies with pressure because the air pressure is involved with the process. This is because air molecules interact with the water molecules escaping the liquid.

Fig. 1, p. 92

Relative Humidity

• Relative humidity is expressed as two different ratios:

• RH = amount of water vapor (actual)

• amount of water vapor (for saturation)

• RH = actual vapor pressure x 100

• saturated vapor pressure

• One notices that as the daily temperature increases, the RH decreases.

Fig. 4-11a, p. 93

Fig. 4-11b, p. 93

Fig. 4-12, p. 93

Fig. 4-13a, p. 94

Fig. 4-13b, p. 94

Dew and Frost Points

• The temperature when saturated water vapor occurs or when the RH = 100 then one is at the temperature of the Dew Point if that T. The Dew Point comparison is made with respect to a flat surface of water. The Frost Point comparison is made with a flat surface of ice. If the Dew Point is below freezing, when saturated, the condensing vapor will become frost.

Polar and Desert Air

• The next two slides show:

• a) Polar Air T = 28oF (-2 oC) Saturated

• Dew Point = 28oF (-2 oC) Vapor Pres.

• Relative Humidity = 100 % 5.3 mb

• b) Desert Air T = 95oF (35 oC) Saturated

• Dew Point T= 50oF (10 oC) Vapor Pres.

• Relative Humidity = 21 % 56.2 mb

Fig. 4-14a, p. 95

Fig. 4-14b, p. 95

Fig. 4-15, p. 96

Fig. 4-16, p. 97

Fig. 4-17, p. 97

Relative Humidity in the Home

• If winter air is let into a home, lets say:

• T = -15 oC (5 oF) that has a RH = 100%

• The saturated vapor pressure is 1.9 mb

• When this air is heated to 68oF (20 oC) it becomes RH =8%. Here is how:

• At 20 oC the sat vap pressure is 23.4 mb RH = 1.9/(23.4) x 100 = 8.1%

Adding Water Vapor at Home

• With low humidity in the winter, one can put out pans of water near heaters to evaporate. Also many heating units allow the addition of water vapor to the heated air. One can purchase Humidifiers to perform this function. They add up to a gallon of moisture per room.

• Dry air can cause health problems, dry or cracked skin, irritate mucous membranes.

Relative Humidity in the Home

• During the summer, the air in the home has too much moisture. So the function of the air conditioner is first to remove the moisture and then secondly to lower the temperature.

• In dry climates Swamp Coolers are used.

• These blow air into a stream of falling water which cools the air by evaporation.

Table 1, p. 98

Saturated Water Vapor Tables

• The text has a limited table of saturated water vapor over water. More extensive tables may be found by using Tetens’s Equation both over water and ice.

• This are from the University of California at Davis Biometric website:

• http://biomet.ucdavis.edu/conversions/HumCon.htm

• (Tables appear at the end of document)

HOT DAYS -Human Discomfort

• We are naturally cooled by evaporation.

• If the air is too humid, the water vapor from the skin cannot evaporate instead it collects on the skin as perspiration.

• Less evaporation from the skin, we feel warmer.

• A measure of how cool the skin can become is the wet-bulb temperature.

HOT DAYS -Human Discomfort

• If the body gets too hot, the hypothalmus

• activates perspiration and a person can loose as much as 2 liters/hour.

• When this occurs a person must drink water to replace the liquid (salt). If not heat cramps. If the body temp continues to rise heat exhaustion and then heat stroke at 41oC. If further rise occurs T, death occurs.

Table 4-1, p. 100

HEAT INDEX (HI)

• The NWS developed the HI which incorporated the temperature and humidity to determine an apparent temperature.

• For example if the temperature is 95oF and the RH is 40% then the HI temp = 101oF.

• Since Heat Stroke can occur around 130oF, 95oF and RH =75 % can be deadly.

• This is equivalent to 110oF and RH = 35%.

Fig. 4-18, p. 100

HYGROMETERS

• Instruments to measure the relative humidity are hygrometers.

• Sling psychrometers

• contain dry and wet-bulb thermometers

• Aspirated pschrometers -- air is blown across both thermometers.

• Hair hygrometer - hair expands 2.5% from 0 to 100% RH.

Fig. 4-19, p. 100

Fig. 4-20, p. 102

Table 2, p. 101

Why Does Air Pressure Vary?

• Water vapor can occupy 4% of air.

• The molecular weight of water is 18

• of Nitrogen 28 of oxygen 32 of Argon 40.

• Since the Nitrogen/oxygen ratio is 78/21= 3.71 then one has to determine A the percentage of Oxygen in the atmosphere

• (3.71A + A + .0093 +.04) = 1.00

• A = 20.36, Nitrogen B= 3.71*A = 75.5

Why Does Air Pressure Vary?

• Then the molecular weight of air is:

• (75.5x28+20.36x32+0.93x40 +4x18)/100 =

• (2114 + 651.5 + 37.2 + 72)/100 =

• Molecular weight of wet air is = 28.75

• (7808 x 28 + 20.95x32 + 0.093x40)/100 =

• Molecular weight of dry air = 28.94

• So when water vapor enters, the molecular weight of air is reduced.

Fig. 2, p. 101