ATMOSPHERIC MOISTURE

1
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.

2
Fig. 4-CO, p. 84
3
The Water Molecule

• Phase changes can be
• evaporation / condensation liquidlt--gtvapor
• sublimation / deposition vaporlt--gt solid
• melting / freezing
  solidlt--gtliquid
• 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

4
Fig. 4-1, p. 86
5
Fig. 4-2, p. 86
6
Fig. 4-3, p. 87
7
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.

8
Fig. 4-4, p. 87
9
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).

10
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

11
Fig. 4-5, p. 88
12
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

13
Fig. 4-6, p. 89
14
Fig. 4-7, p. 89
15
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)

16
Fig. 4-8, p. 89
17
Fig. 4-9, p. 90
18
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.

19
Fig. 4-10, p. 91
20
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.

21
Fig. 1, p. 92
22
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.

23
Fig. 4-11a, p. 93
24
Fig. 4-11b, p. 93
25
Fig. 4-12, p. 93
26
Fig. 4-13a, p. 94
27
Fig. 4-13b, p. 94
28
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.

29
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

30
Fig. 4-14a, p. 95
31

Fig. 4-14b, p. 95
32
Fig. 4-15, p. 96
33
Fig. 4-16, p. 97
34
Fig. 4-17, p. 97
35
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

36
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.

37
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.

38
Table 1, p. 98
39
Saturated Water Vapor Tables

• The text has a limited table of saturated water
  vapor over water. More extensive tables may be
  found by using Tetenss 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)

40
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.

41
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.

42
Table 4-1, p. 100
43
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.

44
Fig. 4-18, p. 100
45
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.

46
Fig. 4-19, p. 100
47
Fig. 4-20, p. 102
48
Table 2, p. 101
49
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 3.71A 75.5

50
Why Does Air Pressure Vary?

• Then the molecular weight of air is
• (75.5x2820.36x320.93x40 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.

51
Fig. 2, p. 101