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 (PV) Variables: Vol. of air (V), Mass of water (mw), Mass of dry air (ma), Mass of water vapor (mv) Mixing Ratio Specific Humidity Absolute Humidity (mv / (mv+ma)) (mv / ma) (mv / V) (g/kg) (g/kg) (g/m3) Relative Humidity (RH, a ratio) = actual/saturated 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 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. Sat. Mix oF oC Ratio 15 ‐9.4 1.9 Types of Problems 20 ‐6.7 2.2 25 ‐3.9 2.8 Tabular Problems 30 ‐1.1 3.5 Mixing ratio Air Temp. Sat. Mixing Rel. Humidity (g/kg) (oC) ratio (g/kg) (%) 35 1.7 4.3 2.8 ‐1.1 40 4.4 5.2 2.8 32.2 45 7.2 6.2 11.1 13.2 50 10.0 7.6 22.3 36.5 55 12.8 9.3 Word Problems: 60 15.6 11.1 65 18.3 13.2 If a room at 18.3 oC has a mixing ratio of 5.2 g/kg, 70 21.1 15.6 a) What is the relative humidity? 75 23.9 18.8 b) What is the dew point? 80 26.7 22.3 c) If the mixing ratio remains the same but the temp. 85 29.4 26.2 of the room incr. to 26.7 oC, what is the new RH? 90 32.2 30.7 95 35.0 36.5 Sat. Mix oF oC Ratio 15 ‐9.4 1.9 Types of Problems 20 ‐6.7 2.2 25 ‐3.9 2.8 Tabular Problems 30 ‐1.1 3.5 Mixing ratio Air Temp. Sat. Mixing Rel. Humidity (g/kg) (oC) ratio (g/kg) (%) 35 1.7 4.3 2.8 ‐1.1 3.5 2.8/3.5= 80 40 4.4 5.2 2.8 32.2 30.7 2.8/30.7= 9 45 7.2 6.2 11.1 18.3 13.2 11/13= 84 50 10.0 7.6 22.3 35.0 36.5 22/36= 61 55 12.8 9.3 Word Problems: 60 15.6 11.1 65 18.3 13.2 If a room at 18.3 oC has a mixing ratio of 5.2 g/kg, 70 21.1 15.6 a) What is the relative humidity? RH = 5.2/13.2 = 39% 75 23.9 18.8 b) What is the dew point? 80 26.7 22.3 Saturated at 4.4 oC c) If the mixing ratio remains the same but the temp. 85 29.4 26.2 of the room incr. to 26.7 oC, what is the new RH? 90 32.2 30.7 RH = 5.2/22.3 = 23% 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 o Two cases: Top: RH constant @ 50%; Bottom: Tdew @ 10 C Fairbank Toronto Pittsburgh Denver Phoenix Tair oC(oF) ‐10 (14) 0 (32) 10 (50) 20 (68) 30 (86) RH (%) 50 50 50 50 50 o Tdew C ‐18 ‐90918 SH (g/kg) 1 2 4 7.5 14 RH (%) 100 100 100 53 29 o Tdew 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 Rel. humidity Dew point °C Dew point °F Human perception at 32 °C (90 °F) Severely high. Even >Higher than 26 °C > Higher than 80 °F deadly for asthma 65% and higher related illnesses Extremely 24 –26°C 75 –80°F uncomfortable, fairly 62% oppressive Very humid, quite 21 –24°C 70 –74°F 52% – 60% uncomfortable Somewhat uncomfortable for 18 –21°C 65 –69°F 44% – 52% most people at upper edge OK for most, but all 16 –18°C 60 –64°F 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 -- Cloud Top height -- 500 region of cloud formation 600 700 800 900 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 -- 400 500 region of cloud formation 600 7/30, 0130z 7/30, 1900z 700 800 900 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.