Understanding Water Vapor -...
Transcript of Understanding Water Vapor -...
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