Lab 3

N | ] [ Materials Needed atures and pressures that commonly occur on * caleulator earth. The phase change of water is what makes auler the weather interesting, variable, and often unpredictable. When it changes phase, water Introduction either releases or consumes energy—a process What is humidity and how js itimportant to called latent heat transfer. atmospheric processes? This | Measure atmospheric moisture and uses two : Latent Heat : Water is a unique substance becausc it occurs in three phases (solid, liquid, and vapor) at temper- e

= Sublimation (680 cal) Heat Consumed Heat Released e " Evaporation (600 cal) o e Y y ‘ ] [ 3N ) —_— < o, 5% &, water vapog (.. L0 — - -— - L4 ) ° ®e o, Condensation (600 cal) ® e liquid water Deposition (680 cal) Figure 5-1. Energy transfer during phase changes. Ice - (liquid) water Water — water vapor Ice — water vapor Water vapor — water Water vapor — ice Water — ice Measures of Atmospheric Moisture The water vapor in the atmosphere comes from sources on the earth’s surface such as oceans, lakes, streams, soil, and plants. The amount of water vapor present depends on the amount of cnergy available to change this liquid surface water to gas. As the energy supply at the earth’s surface increases, liquid water molecules move faster and are more likely to leave the liquid sur- face and evaporate. For a given amount of ener- gy at the earth’s surface, there exists an upper consumed; energy used to melt the ice is now contained in the water Examples ice cubes melting in a glass; melting of falling snowflake wet puddie dries in +he Sun limit to the amount of liquid water that can be evaporated. There are several ways to measure atmoss pheric moisture at a given time and place. In one method, water vapor is considered to be like any other atmospheric gas, in that water vapor mole- cules exert a partial pressure proportional to their concentration in the atmosphere. This pressure is referred to as the vapor pressure. The “upper limit” of atmospheric vapor pressure is referred to as saturation vapor pressure, Since evap-

©ration rate depends o, available energy, this evaporating from a flat water surface equals the Saturation vapor Pressure increases with tem- number condensing on the surface. An air sam- Perature. Table 5-1 ang Figure 5-2 show the non- ple below the curve is unsaturated, indicating miede relationship between air temperature and that evaporation from a flat water surface would saturation vapor pressure. The Clausius-Clapeyron ~ exceed condensation onto the surface. The curve curve in Figure 5-2 depicts the equilibrium point shows how higher temperatures (indicating a between liquid waer and atmospheric water greater energy supply) increase the potential for vapor. At this point, the number of molecules evaporation. Table 5-1 Saturation Vapor Pressure (mb) at Sea-Level Pressure as a Function of Dry-Bulb Temperature (°C) T a0 o 80 35 0314 -30 0.519 -25 0.807 -20 1.254 70 -18 1.488 -16 1.760 -14 2.076 12 2.441 60 -10 2.863 -8 3.348 . -6 3.906 Qg 4 4545 E -2 5.275 o 0 6.108 = 2 7.055 @ 40 4 8.129 a 6 9.347 i o 13 11(2)3; ‘>° 30 Supersaturated 12 14.017 14 15.977 16 18.173 20 18 20.630 20 23.373 22 26.430 24 29.831 10 26 33.608 28 37.796 30 42430 = 47.551 I ;j 53.200 40 -30 -20 -10 DO 10 20 30 40 36 59.422 Temperature (°C) 38 66.264 | 40 73.777 Figure 5-2. Saturation'vvyapor pressure.

: ration mixing Dew point is another important measure of atmos- ~ how temperature relates to satu . : mixing ratio. Pheric moistyre, Consider the sample of air in Figure ratio and how dew point relates to g . is illustrates 3 Dew Point that you derived from Figure 5-4. This illustra 5-4 with a temperat : iXi io of 10g kg™ Its daf P::-l,: : t:iig{::;’:;:‘i:‘fi'; it 23. Now consider an air sam'pl'e with a otemperature of must be cooled in rder to reach saturation. We ~ 35.134 - 34°C and a relative humidity of 6-2635 134 0d = ,1-:—3 3 <100 may illustrate this idea o the graph by simply plot- What is its saturation mmn"g ;ano? STl 35,1 ting the value of temperature and mixing ratio of the What is its mixing rauo:"g‘_s_ Hnsaturated air parcel and drawing a horizontal line Plot the air sample on Fgguge 5"4- ’ that intersects the saturation curve. The temperature What is its dew point? 22~ © ; Exercise process when air near the earth’s surface is chilled to : ; : : its saturation point and dew forms on the ground. 24. Determine the dew point of one or two sites you measured in question 20. 22. What ig,}}& d‘j'w point of an air sample with a temper- ature of 30°C and a mixing ratio of 17 g kg'? 4.7 Site &) 10.8°C Remember that air is saturated at the dew point. Use Table 5-2 or Figure 5-3 to confirm that the mixing ratio from question 22 (17 g kg") is the saturation mixing ratio for the dew point Site 1) 19°C 60 (9/kg) g Ratio 8 -———-—-__-__. Mixin & ‘ o e T T S —" 0-40 -30 =20 -10 0 10 20 Temperature (°C) Figure 5-4. Dew point of an unsaturated air sample.

)T Table 5-4 Relative Humidity Table (°Q)

Relative Humidity Saturation V. oo ixi 0 Mixing Ratio Relative Sa"*tpor Pressure, mixing ratio, and the concept of Temp Ml?énfgg)‘l = (gfg--) Humidity uration help us to understand a much more : , 19.5) 2 commonly used . : o- 14°C __'LO_~.L___ 5 _'ji.fi_)__fi y measure of moisture—relative 0. | 88.70_% humidity. Remember th i 14°C : 9 : : er that a point on the satura- 19,4 2. 03 o tion vapor pressure curve represents the vapor 24°C - 2 Tl ¢ Pr(?ssure that saturated air would exert, and a 24°C 9. 2 2 : 4 point on the saturation mixing ratio curve 34°C 35.]) 7 19.92 % shows the mass of water vapor for a saturated sample. Since the atmosphere typically is not saturated, relative humidity (RH) measures how close an air sample is to the saturation point. Specifically, relative humidity is the ratio of the 15. In the winter, cold air is brought into homes and heated. How does this change the relative humidity actual amount of wat in th th ythear: er vapor in the air to the ~ : | : _ AL na heated up whout moisture, saturation level at a given temperature. In terms 2 be‘fl g r move Jaster of vapor pressure: So ain, paghcles sbeguiale : : and more water s taken From the Q€ RH = vapor pressure * 100% 16. Explain why the basement of a house often has high relative humidity in the summer. Int £ mixi : Dwma +the Summer +he aic is hotter and e humd. Thad air then goes into basements saturation vapor pressure vy, " : which raises hdws\df‘fg levels, RH = mixing ratio e e D e . 0, saturation mixing ratio 100% 17. Rank each of the following air samples from 1 : highest) to 5 (lowest) in order of water vapor con- Consider a sample of air with a temperature of (hghestiog { ) f P : Wi : tent. 20°C and a mixing ratio of 10 g kg". The satura- Saturation Actual t 28 ? ; é Mixing Mixin, on mxxxn'g ratio of the sample (derived from Temp. Ratio RH Ratif Rank Table 5-2) is 14.956 g kg". Thus we have: 10,410 2 A 14C 1014 g0 W3 | 14,450 SEEY B 20°C M.95C 6o% 8.91 _ 2 RH = mixing ratio R A0 14, 2 [ saturation mixing ratio - G Jj?(qu Al g i3 —i—— D 30°C <694 40% 11.08 4 10 g kg* B s 10.54 1 48 = g o 0 - 0, E 34°C d 30% 8 [——————1 195 g 1T 100/,] 66.8% —= 18. From the above example, can you suggest otie dis=SSH8 14. Determinc the saturation mixing ratio of the fol- advantage to using relative humidity? . lowing air samples, and calculate the relative T+ canmn o1 measure amourt of waoIShie .,\ humidity of each. the aic and oaly shows the aie SeATEEel : : ° 10D = 2100 = o 100 g q o F —— .| 00 = 100 o ’D' Iq O .359 '3" 5 o |0D.&