Understanding Water Vapor 21PSTEM FOSS WW Content Study Jim Washburne Sept. 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 to water 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 Water & Air @ equilibrium Vapor Pressure (P V ) Variables: Vol. of air (V), Mass of water (m w ), Mass of dry air (m a ), Mass of water vapor (m v ) Mixing Ratio (m v / m a ) (g/kg) Specific Humidity (m v / (m v +m a )) (g/kg) Absolute Humidity (m v / V) (g/m 3 ) Relative Humidity (RH, a ratio) = actual/saturated RH = P V / P sat = water vapor content / water vapor capacity = observed / saturated (T a ) 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 vapor in 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.
Mixing ratio (g/kg) Types of Problems Tabular Problems Air Temp. ( o C) 2.8 1.1 2.8 32.2 Sat. Mixing ratio (g/kg) 11.1 13.2 22.3 36.5 Word Problems: If a room at 18.3 o C has a mixing ratio of 5.2 g/kg, a) What is the relative humidity? b) What is the dew point? Rel. Humidity (%) c) If the mixing ratio remains the same but the temp. of the room incr. to 26.7 o C, what is the new RH? o F o C Sat. Mix Ratio 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
Mixing ratio (g/kg) Types of Problems Tabular Problems Air Temp. ( o C) 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 Word Problems: If a room at 18.3 o C 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 o C c) If the mixing ratio remains the same but the temp. of the room incr. to 26.7 o C, what is the new RH? RH = 5.2/22.3 = 23% o F o C Sat. Mix Ratio 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
The dew point is the temperature to which the air must be cooled to become saturated and is an expression of water vapor content. In (a), the temperature exceeds 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 actual specific 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 temperature changes, even when the actual amount of water vapor in the air remains the same. Temperature Relative Humidity www.bom.gov.au/lam/humiditycalc.shtml
Two cases: Top: RH constant @ 50%; Bottom: T dew @ 10 o C Fairbank Toronto Pittsburgh Denver Phoenix Tair o C( o F) 10 (14) 0 (32) 10 (50) 20 (68) 30 (86) RH (%) 50 50 50 50 50 T o dew C 18 9 0 9 18 SH (g/kg) 1 2 4 7.5 14 RH (%) 100 100 100 53 29 T dew o C 10 0 10 10 10 SH (g/kg) 2 4 8 8 8 Constant RH emphasizes relative saturation Constant Tdew emphasizes actual amount of moisture in air
Tucson: Dew Points (used to) define Summer Monsoon
High Dew Points are uncomfortable Dew point C Dew point F Human perception Rel. humidity at 32 C (90 F) >Higher than 26 C > Higher than 80 F Severely high. Even deadly for asthma 65% and higher related illnesses 24 26 C 75 80 F Extremely uncomfortable, fairly 62% oppressive 21 24 C 70 74 F Very humid, quite uncomfortable 52% 60% Somewhat 18 21 C 65 69 F uncomfortable for 44% 52% most people at upper edge 16 18 C 60 64 F OK for most, but all perceive the humidity 37% 46% at upper edge 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%
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, the wet bulb depression, depends on the moisture content of the air and can be used to determine dew 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
TUS_201007(09 10) TEMP C DWPT C Series3 Series4 100 80 60 40 20 0 20 40 60 0 100 200 300 400 500 600 -- Cloud Top height -- region of cloud formation 700 800 900 1000
TUS_201007(29 30) 29TEMP 29DWPT 30_0TEMP 30_0DWPT DWPT 0.50 100 80 60 40 20 0 20 40 0 100 200 300 -- Cloud Top height -- 400 7/30, 0130z 7/30, 1900z 500 600 700 region of cloud formation 800 900 1000
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