Water in the Atmosphere All 3 phases solid liquid gas under commonly observed temperatures and pressures Vapor 04 by volume surface Daltons Law of Partial Pressures total air pressure ID: 1025044
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1. ATMOSPHERICMOISTURE(Chapter 4)
2. Water in the AtmosphereAll 3 phases (solid, liquid, gas) under commonly-observed temperatures and pressuresVapor: 0–4% by volume (@ surface)Dalton’s Law of Partial Pressures: total air pressureis the sum of the partial pressures of the individual constituents … + vaporpressure (e)[units = millibars]
3. Average January Vapor Pressure (mb)
4. Average July Vapor Pressure (mb)
5. e = vapor pressurees = saturationvapor pressureVapor Pressure, Saturation Vapor Pressure, and TemperatureGauge recordssaturation vaporpressure for 20°Ces depends onlyon the temperature!Pressuregauge0
6. SATURATION VAPORPRESSURE AND TEMPERATUREThe vapor pressure of saturatedair (the saturation vapor pressure)depends ONLY on TEMPERATURE!
7. Cloud Condensation Nucleii (CCN)For cloud droplets to form, liquid water needs to condense onto something.CCN include salt, soil particles, ash, dust,various pollutants, aerosols, etc.In completely pristine air, cloud formation wouldbe very difficult.
8. CONDENSATION AND TEMPERATUREAs air cools, condensation becomes more likely.
9. SATURATION VAPORPRESSURE AND TEMPERATUREThe vapor pressure of saturatedair (the saturation vapor pressure)depends ONLY on TEMPERATURE!Cooling the air lowers the saturation vapor pressure,making condensation morelikely
10.
11. ABSOLUTE HUMIDITY () Absolute humidity varies as the air’s volume (or density)changes. Note that water vapor remains constant.upMass of water vapor per volume of air
12. SPECIFIC HUMIDITY (q)Mass of water vapor per mass of airSpecific humidity is conservative for pressure (temperature) changes.MIXING RATIO (w): Mass of water vapor per mass of dry air
13. Relative Humidity (RH)Relative humidity is the amount of water vapor in the air divided by the maximum possible amount for a given temperature. x 100 (%) moisture in airmax possible amountBut es only depends on temperature! RH is NOT conservative for pressure and temperature changes.RH measures proximity to saturation
14. RELATIVE HUMIDITY (RH)Assume unsaturated airAir is closer to being saturated, so RH increases
15. RELATIVE HUMIDITYDespite same water content (vapor pressure), RH lowers because of higher temperatureand thus higher saturation vapor pressure.
16. SATURATION VAPORPRESSURE AND TEMPERATUREIt takes a lot morewater vapor to saturate warm airthan cold air.
17. Constanttemperature,increasingvapor pressureRH increasesIncreasingtemperature,constantvapor pressureRH decreases
18. DIURNAL VARIATION OF RELATIVE HUMIDITYRH tends to be highest when it is coldest.
19. LATENT HEATPhase changes occur at constant temperature.(pp. 34–35)
20. Phases of Water
21. ICELIQUID WATER(note higher density)Phases of Water
22. Dew Point Temperature (Td)Temperature below which any further cooling produces condensation.Td (°F)T(°F)RH (%)midnight5060703 a.m.5055836 a.m.50501009 a.m.4949100saturationcondensationLatent heat is released and dew point drops very slowly(or not at all) as vapor is converted to liquid. Dew forms(or cloud droplets form in the atmosphere).If Td ≤ 32°F (0°C), it is called the frost point (Tf).T ≥ Td
23. DEW POINT TEMPERATURE AND SOURCE REGION
24. JANUARY DEW POINT TEMPERATURE
25. JULY DEW POINT TEMPERATURE
26. DEW POINT TEMPERATUREJanuaryJuly
27. Psychrometerwet fabricTTwdry bulbwet bulbwindWet bulb temperature (Tw):lowest temperature attainedby evaporating water into the airEvaporation removes heatFrom the air and the temperature drops(evaporation is a coolingprocess that removes latent heat from the air)T minus Tw = wet bulb depressionT minus Td = dew point depressionlarge value = dry airT ≥ Tw
28. Summary of Humidity VariablesVapor pressure (partial pressure of water vapor) [units = mb]Saturation vapor pressure (vapor pressure of saturated air) [mb]Absolute humidity (mass of vapor per volume of air [g/m3]Specific humidity (mass of vapor per total mass of air) [g/kg]Mixing ratio (mass of vapor per mass of dry air) [g/kg]Relative humidity (proximity to saturation) [%]Dew point temperature (temperature at which condensation occurs via cooling) [degrees]Frost point temperature (dew point below freezing) [degrees]Wet bulb temperature (temperature at which condensation occurs via evaporation) [degrees]Dew point depression (temperature minus dew point temp.) [degrees]Wet bulb depression (temperature minus wet bulb temp.) [degrees]
29. HYDROLOGIC CYCLE
30. Quick Summary—Humidity (Chapter 4):Water exists in all three phases under normal conditions on Earth.Phase changes occur at a constant temperature, with latent heat being taken in from the air or released to the air.The vapor pressure of saturation depends only on air temperature.The easiest way to saturate air is to cool it, thus lowering the amount of vapor needed to saturate it.The numerous variables used to characterize humidity levels are all designed to suit different purposes.