Understanding Water Vapor

21PSTEM – FOSS WW Content StudyJim WashburneSept. 2010

Graphics from: Understanding Weather & Climate: wps.prenhall.com/esm_aguado_uwac_3

a) Consider a hypothetical jar containing pure water and an overlying volume that initially contains no water vapor.

b) As evaporation begins, water vapor starts to accumulate above the surface of the liquid. With increasing water vapor content, the condensation rate likewise increases

c) Eventually, the amount of water vapor above the surface is enough for the rates of condensation and evaporation to become equal. The resulting equilibrium state is called saturation (c).

Humidity refers to the amount of water vapor in the air.The part of the total atmospheric pressure due towater vapor is referred to as the vapor pressure.

The vapor pressure of a volume of air depends on both the temperature and the density of water vapor molecules.

The saturation vapor pressure is an expression of the maximum water vapor that can exist. The saturation vapor pressure depends

only on temperature.

Terminology

Vapor Pressure (PV)

Relative Humidity (RH, a ratio) = actual/saturated

Absolute Humidity(mv / V)(g/m3)

Variables: Vol. of air (V), Mass of water (mw), Mass of dry air (ma), Mass of water vapor (mv)

Specific Humidity(mv / (mv+ma))

(g/kg)

Water & Air@ equilibrium

Mixing Ratio(mv / ma)

(g/kg)

RH = PV / Psat = water vapor content / water vapor capacity= observed / saturated (Ta)

RH = MR / Sat MR RH = SH / Sat SH RH = AH / Sat AH

Dew Point = Temperature when it reaches 100% saturation

Relative humidity, RH, relates the amount of water vaporin the air to the maximum possible at the current temperature.

RH = (specific humidity/saturation specific humidity) X 100%

More water vapor can exist in warm air than in cold air, so

relative humidity depends on both the actual moisture

content and the air temperature.

If the air temperature increases, more water vapor can exist, and the ratio of the amount of water vapor in the

air relative to saturation decreases.

Types of ProblemsTabular Problems

Mixing ratio(g/kg)

Air Temp.(oC)

Sat. Mixing ratio (g/kg)

Rel. Humidity(%)

2.8 ‐1.1

2.8 32.2

11.1 13.2

22.3 36.5

oF oC Sat. MixRatio

15 ‐9.4 1.9

20 ‐6.7 2.2

25 ‐3.9 2.8

30 ‐1.1 3.5

35 1.7 4.3

40 4.4 5.2

45 7.2 6.2

50 10.0 7.6

55 12.8 9.3

60 15.6 11.1

65 18.3 13.2

70 21.1 15.6

75 23.9 18.8

80 26.7 22.3

85 29.4 26.2

90 32.2 30.7

95 35.0 36.5

Word Problems:

If a room at 18.3 oC has a mixing ratio of 5.2 g/kg,a) What is the relative humidity?

b) What is the dew point?

c) If the mixing ratio remains the same but the temp.of the room incr. to 26.7 oC, what is the new RH?

Types of ProblemsTabular Problems

Mixing ratio(g/kg)

Air Temp.(oC)

Sat. Mixing ratio (g/kg)

Rel. Humidity(%)

2.8 ‐1.1 3.5 2.8/3.5= 80

2.8 32.2 30.7 2.8/30.7= 9

11.1 18.3 13.2 11/13= 84

22.3 35.0 36.5 22/36= 61

oF oC Sat. MixRatio

15 ‐9.4 1.9

20 ‐6.7 2.2

25 ‐3.9 2.8

30 ‐1.1 3.5

35 1.7 4.3

40 4.4 5.2

45 7.2 6.2

50 10.0 7.6

55 12.8 9.3

60 15.6 11.1

65 18.3 13.2

70 21.1 15.6

75 23.9 18.8

80 26.7 22.3

85 29.4 26.2

90 32.2 30.7

95 35.0 36.5

Word Problems:

If a room at 18.3 oC has a mixing ratio of 5.2 g/kg,a) What is the relative humidity?

RH = 5.2/13.2 = 39%b) What is the dew point?

Saturated at 4.4 oCc) If the mixing ratio remains the same but the temp.

of the room incr. to 26.7 oC, what is the new RH?RH = 5.2/22.3 = 23%

The dew point is the temperature to which the air must be cooled to becomesaturated and is an expression of water vapor content. In (a), the temperatureexceeds the dew point and the air is unsaturated. When the air temperature is

lowered so that the saturation specific humidity is the same as the actualspecific humidity (b), the air temperature and dew point are equal. Further cooling (c) leads to an equal reduction in the air temperature and dew point

so that they remain equal to each other. When the temperature at which saturation would occur is below 0 °C, we use the term frost point.

Change in amount of water vapor in saturated air with temperature.The air's capacity for water vapor increases as air temperature

increases. Air with a temperature of 30°C can hold more than three times as much water vapor as air at 10°C.

Relative humidity varies significantly when the temperaturechanges, even when the actual amount of water vapor in

the air remains the same.

Temperature ________Relative Humidity _ _ _ _

www.bom.gov.au/lam/humiditycalc.shtml

Fairbank Toronto Pittsburgh Denver Phoenix

Tair oC(oF) ‐10 (14) 0 (32) 10 (50) 20 (68) 30 (86)

RH (%) 50 50 50 50 50

Tdew oC ‐18 ‐9 0 9 18

SH (g/kg) 1 2 4 7.5 14

RH (%) 100 100 100 53 29

Tdew oC ‐10 0 10 10 10

SH (g/kg) 2 4 8 8 8

Two cases: Top: RH constant @ 50%; Bottom: Tdew @ 10 oC

Constant RH emphasizes relative saturation

Constant Tdew emphasizes actual amount of moisture in air

Tucson: Dew Points (used to) define Summer Monsoon

Dew point °C Dew point °F Human perception Rel. humidity at 32 °C (90 °F)

> Higher than 26 °C > Higher than 80 °FSeverely high. Even deadly for asthmarelated illnesses

65% and higher

24 – 26 °C 75 – 80 °FExtremely uncomfortable, fairly oppressive

62%

21 – 24 °C 70 – 74 °F Very humid, quite uncomfortable 52% – 60%

18 – 21 °C 65 – 69 °F

Somewhat uncomfortable for most people at upper edge

44% – 52%

16 – 18 °C 60 – 64 °FOK for most, but all perceive the humidity at upper edge

37% – 46%

13 – 16 °C 55 – 59 °F Comfortable 38% – 41%10 – 12 °C 50 – 54 °F Very comfortable 31% – 37%< 10 °C < 49 °F A bit dry for some 30%

High Dew Points are uncomfortable

The simplest and most widely used instrument for measuring humidity is the sling psychrometer, which

has two thermometers called the wet bulb and dry bulb.

The difference between the two temperatures, thewet bulb depression, depends on the moisture content

of the air and can be used to determinedew point and relative humidity.

Dew Point from dry & wet bulb

RH from dry & wet bulb

Now consider Lab notebook p.31

Calculate RH – Example p.31,#2

Calculate RH – Example p.31,#3

Orographic Effect ‐ 1

Orographic Effect ‐ 2

Orographic Effect ‐ 3

Orographic Effect ‐ 4

Orographic Effect ‐ 5

Orographic Effect ‐ 6

Orographic Effect ‐ 7

Dragon’s Breath

wps.prenhall.com/esm_aguado_uwac_3/11/2880/737513.cw/index.html

Using Radiosonde Data From a Weather Balloon Launch

http://mynasadata.larc.nasa.gov/preview_lesson_nostds.php?&passid=34

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TUS_201007(09‐10)TEMP C DWPT C Series3 Series4

-- Cloud Top height --

region of cloud formation

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TUS_201007(29‐30)29TEMP 29DWPT 30_0TEMP 30_0DWPT DWPT 0.50

-- Cloud Top height --

region of cloud formation

7/30, 0130z 7/30, 1900z

EXTRA SLIDES

Another View of Orographic effect

Unlike the DALR, the SALR is not a constant value. If saturated air cools from 30 °C to 25 °C (a 5° decrease), the specific humidity decreases from

27.7 grams of water vapor per kilogram of air to 20.4. A 5 °C drop in temperature from 5 °C to 0 °C lowers the specific humidity only 1.7 grams for each kilogram of air. This brings

about less warming to offset the cooling by expansion, as well as a

greater saturated adiabatic lapse rate.

Dew Point is measure of how much moisture there is in the air

Change water vapor to Log axis

Find best fit line – slopes are equal