Humidity parameters. Saturation (equilibrium) vapor pressure Condensation balances evaporation

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Brittney Berry
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uidity paraeters Saturation (equilibriu) vapor

ratio & specific huidity Mass ratio of water vapor

Dew point & frost point eperature at which

out takes place Latent heat eat (energy) transfer

1 uidity paraeters Saturation (equilibriu) vapor pressure Condensation balances evaporation Miing ratio & specific huidity Mass ratio of water vapor and air and water content and wet air. Dew point & frost point eperature at which condensation occurs, teperature at which freeze out takes place Latent heat eat (energy) transfer associated with freezing and elting as well as condensation and evaporation processes. Condensation points robability of condensation to occur randoly of via condensation points

2 Mass iing ratio he hydrostatic equation for a iture of gases dz g R dz g R R d R with dz g dz g d d d ' ' effective olecular weight of gas with different coponents Mass iing ratio is an alternative way to epress the huidity level he densities of two different gases depend on the partial pressure of the two gas coponents.

3 For air (no ) N N g ole 0.95, Adopting the olecular weight of the gases g ole, Ar Ar 8.96 g ole g ole For huid air (with ) wet 8.96 g ole 8.05 g ole he ass iing ratio is deterined by the partial pressure of gas and water vapor ;

4 q q q q Specific huidity he specific huidity is closely correlated with the iing ratio and the two paraeters can be easily converted into each other. Value depends on latitude Average 0 g/kg

5 Assue an air pressure of 00 bar and a specific huidity of q=0 g/kg = 0.0, deterine the ass iing ratio and the water vapor pressure! q bar bar 00 bar Siilar value since air is the ain coponent air bar bar 5.8 bar 995 bar

6 uidity developents Assuing the average to be 0g/kg this would correspond to an increase in specific huidity of 3% over the last 40 years! Considering the latitude dependence there ight be a nuber of reasons, like a broadening of the distribution of the curve due to polar 940 waring. 950 But 960 statistical 970 fluctuations cannot 000 be ecluded 00 Year either

7 Relative huidity he relative huidity U is defined in ters of the saturated vapor pressure SV, which is the pressure above which ore water condensates out of the oist air than evaporates into the air. U SV ; U (90F) 5bar U ( ) SV 5bar U (60F) 8bar 0.66 U SV U SV he relative huidity indicates how oist (huid) the air is in our environent. Calculate the relative huidity U for a water vapor pressure of 5 b at a teperature of 60 F and 90 F! 5bar 48bar 83% 3% For the sae water pressure the air has an increasing relative huidity towards lower teperatures. his eans that condensation becoes easier towards lower teperatures. his point is defined as dew point.

Condensation and dew point he dew point is the teperature at which the water vapor in a saple of air at constant baroetric pressure condenses into liquid water at the sae rate at which it evaporates.

8 Condensation and dew point he dew point is the teperature at which the water vapor in a saple of air at constant baroetric pressure condenses into liquid water at the sae rate at which it evaporates. At teperatures below the dew point, water will leave the air and condense as liquid water drops. he dew point represents the saturation teperature for water in air. U 00% SV Window at 68 F roo teperature (0 o C) and water vapor pressure of 0 bar cools down at night due to insufficient insulation to 40 F (4 o C). his oves the SV fro bar to 9.5 bar, slightly below dew point, condensation occurs.

U SV For 33 o C (9F), SV= 50 bar For 4 o C (76F), SV= 30 bar For 6 o C (60F), SV= 7 bar Dew point teperature

9 he Dew oint is the teperature D to which a rising gas parcel (cloud) has to be cooled for condensation to take place, the saturated vapor pressure equalizes the partial pressure of water content. U SV For 33 o C (9F), SV= 50 bar For 4 o C (76F), SV= 30 bar For 6 o C (60F), SV= 7 bar Dew point teperature decreases by about o C for every 5% decrease of SV d d d 00% U % epirical linear relation 5% valid within o C 00% 60% o 30 C huid day 5% 00% 80% o 0 6 C stea bath 5%

10 Dew point in nature

n a huid, =0 o C, =00 bar day, the dew point can be deterined by cooling a etal plate until oisture fors at its surface. his happens when the teperature of the plate reaches D =5 o C.

11 n a huid, =0 o C, =00 bar day, the dew point can be deterined by cooling a etal plate until oisture fors at its surface. his happens when the teperature of the plate reaches D =5 o C. What is the relative huidity U? What is the ass iing ratio? Saturated SV SV SV SV U SV D D C C C.69ka.34ka 00bar bar 993.bar 6.9bar 3.4bar 7.% 993.bar.7 0

12 Mass iing ratio at dew point SV 0.66 SV SV SV 0.66 SV 0.66 SV Calculate the SV and the dew point for the observed ass iing ratios at 5 k and 5k altitude! Ae A 6.bar ln o C SV A 0 e z 000be bar bar SV 0.3bar ln 49 C 4K C 6. SV 0 e z C 000be bar ln 6. 8bar bar 0 44 C 9K Cloud foration is preferably only in altitudes up to a few k because of the teperature and huidity conditions. Above that water pressure is too low.

13 Soe ore eaples Assue a total air pressure of =000bar and specific huidity q=%! What is the iing ratio and the pressure of water vapor? q ; 0.66 ; ; b 000bar ; 30bar! ;

14 SV Calculate the saturated vapor pressure SV and deterine the dew point D at which SV=! eperature 0 C D 30bar ln 30 6, A e Check the etree case of q=0% specific huidity: D 74bar A e ln 44.8 D bar e o C SV A e A 6.bar D 4.9 ln D o C 6.bar e o C For high huidity conditions the dew point is reached at higher teperature! D

Latent heat Molecules are constantly evaporating fro the surface of a liquid. Molecules in the gas above the liquid are constantly hitting the surface and condensing.

his energy is called latent heat, which is the heat (energy) required or released for a syste for change of state (phase) without change of teperature.

15 Latent heat Molecules are constantly evaporating fro the surface of a liquid. Molecules in the gas above the liquid are constantly hitting the surface and condensing. he olecules that evaporate take energy fro the liquid (cooling); the olecules that condense add energy (heating). his energy is called latent heat, which is the heat (energy) required or released for a syste for change of state (phase) without change of teperature. Latent heat is typically associated with a phase transition of a substance fro one phase to another, such as the transition: ice water vapor. In the icroscopic picture latent heat is associated with the release of binding energy of a icroscopic syste. hase transition fro solid to liquid and fro liquid to gaseous, fro gaseous to plasa is endotheric and requires energy, inverse process releases energy! Energy of syste

16 Latent heat required for elting ice to water: L=80 cal/g=334 J/g Latent heat required for evaporating water: L=539 cal/g=57 J/g he latent heat of condensation of water in the teperature range fro 40 C to 40 C is approiated by the following epirical cubic function (with in C): L water () =

17 In cliate physics latent heat corresponds to an additional flu of energy which is required fro sun light for the evaporation of water into the atosphere and the release of heat with subsequent condensation at higher altitude.

18 Surface latent heat averaged over 50 years igh evaporation over ocean regions with war water as well as huid jungle regions. his provides cooling. Low evaporation over continents.

Ipact of latent heat on energy budget he release or absorption of latent heat during condensation or evaporation processes changes the energy flu and energy

F net F heat F latentheat F ground F latent heat F net F heat F latent heat F net F net F heat F net F heat F latent heat F latent heat F heat Daytie balance

19 Ipact of latent heat on energy budget he release or absorption of latent heat during condensation or evaporation processes changes the energy flu and energy budget of a cliate syste! his depends on the environental details and surface structure of the area. F net F heat F latentheat F ground F latent heat F net F heat F latent heat F net F net F heat F net F heat F latent heat F latent heat F heat Daytie balance over oist surface, waring the ground F ground F ground F ground F ground Nighttie balance over oist surface cooling the ground Daytie balance over surface waring the air Dry air over oist surface, causing evaporation

20 herodynaics of oist air For oist air condensation will occur and latent heat L is released with dq being the change of huidity. his changes the adiabatic conditions fro: Q 0 C d V d to: Q Ldq C d V d Ldq V d If air eleent is in hydrostatic balance d C d dz g s s d dz g z d C Lq d 0 g L q R L q cp dz Ldq s : saturated adiabatic lapse rate =0.067/ o C <

21 Release of latent heat increases the teperature in the volue eleent and eleent rises because of the increase in buoyancy d dz s d L q R L q cp Coparison, of very conditions, wet (huid) conditions, and noral atospheric conditions d dz s Destabilization requireent d K s 3 d 0 k K k

Molecular Speeds. Real Gasses. Ideal Gas Law. Reasonable. Why the breakdown? P-V Diagram. Using moles. Using molecules

Molecular Speeds. Real Gasses. Ideal Gas Law. Reasonable. Why the breakdown? P-V Diagram. Using moles. Using molecules Kinetic Theory of Gases Connect icroscopic properties (kinetic energy and oentu) of olecules to acroscopic state properties of a gas (teperature and pressure). P v v 3 3 3 But K v and P kt K v kt Teperature