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Atmospheric moisture

Atmospheric moisture. Chapter 5 – GEOG 31062. Hydrologic Cycle. Evaporation / Condensation. Saturation = when evaporation equals condensation. How we measure moisture in the atmosphere. Absolute values Specific humidity Dew point Relative values Relative humidity.

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Atmospheric moisture

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  1. Atmospheric moisture Chapter 5 – GEOG 31062

  2. Hydrologic Cycle

  3. Evaporation / Condensation Saturation = when evaporation equals condensation

  4. How we measure moisture in the atmosphere • Absolute values • Specific humidity • Dew point • Relative values • Relative humidity

  5. Specific humidity is grams of water per kilogram of air Far more water vapor can exist in warmer air than colder air. Saturation specific humidity

  6. Relative humidity • Given as a percentage • Relates the total amount of water vapor present relative to the total amount possible • Depends on air temperature and total water vapor present • Why is this not an absolute measure of water in the atmosphere?

  7. Diurnal cycle of RH

  8. Dew point • Temperature to which the air must be cooled to reach saturation • (i.e. dew will form)

  9. Dew point/temperature relationships in unsaturated air (a) and saturated air (b & c)

  10. January Dew Points

  11. July Dew Points

  12. Precipitable water climatology (Pitt.)

  13. Addition of water vapor Mixing cold air with warm, moist air Cooling air to the dew point How do we reach saturation?

  14. Also… • Effect of Curvature • Small drops exhibit greater curvature: more evaporation takes place • Supersaturation may occur • Effect of nuclei • Droplets are not ‘clean’ • Haze can form at RH=80% • End result • Fairly complex saturation values in the atmosphere

  15. Ice nuclei • Ice Nuclei • Water only freezes at 0oC (32oF) if it has something to freeze onto • Supercooled droplets can exist down to -40°C • Spontaneous nucleation occurs at -40°C

  16. Measuring humidity

  17. How do we get air to dew point? • Diabatic processes • Adding or removing heat energy • Adiabatic Processes • No net exchange of energy (through the expansion of rising air)

  18. Lapse rates • Change of temperature with height • Dry adiabatic lapse rate • -1oC/100m (-5.5oF/1000ft) • Sinking parcels experience compression warming • Saturated adiabatic lapse rate • Occurs in saturated air • ~-0.5oC/100m (-3.3oF/1000ft) • Environmental lapse rate • What is actually happening in the atmosphere

  19. A comparison of adiabatic and environmental cooling rates

  20. Condensation • Dew or frost • Liquid or solid condensation on surface objects • Diabatic processes • Fog • Condensation near the ground of water vapor on nuclei in the air

  21. Advection fog / Radiation fog

  22. Upslope fog and Valley fog

  23. Precipitation or evaporation fog

  24. Heavy fog days

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