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Air parcels

Air parcels. A parcel is a “blob” of air Small enough to have only one value of T, p, ρ, etc. Large enough to contain a significant number of molecules. (Are there enough particles to talk about temperature as average kinetic energy, for example?). Lapse Rates.

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Air parcels

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  1. Air parcels • A parcel is a “blob” of air • Small enough to have only one value of T, p, ρ, etc. • Large enough to contain a significant number of molecules. (Are there enough particles to talk about temperature as average kinetic energy, for example?)

  2. Lapse Rates • Parcel lapse rate – the rate at which temperature changes as the parcel is lifted to a higher altitude • Environmental lapse rate – the rate at which the air surrounding the parcel changes as altitude increases

  3. The Adiabatic Lapse Rate • An adiabatic process is one during which no heat is exchanged between the substance in question and its surroundings • Many atmospheric motions occur rapidly enough that parcels do not exchange a significant amount of heat with the environment • Examples: • rising air in a thunderstorm • Air rising over a topographic barrier (like a mountain)

  4. The Adiabatic Lapse Rate • The adiabatic lapse rate for DRY air on Earth is • Γd = g/cp • Γd = 9.81 m s-2 / 1004 J kg -1 C-1 • Γd = 0.00977 C m-1 • Γd = 9.77 C km-1

  5. The Adiabatic Lapse Rate • This means that a rising(sinking) air parcel will cool(warm) at a rate of about 10 oC per km of ascent(descent) unless: • It exhanges significant mass or heat with the environment • It becomes saturated with respect to water vapor • It rises(sinks) so slowly that radiation heat transfer is possible

  6. Humidity, Condensation and Clouds • Circulation of water in the atmosphere • Evaporation, condensation and saturation • Humidity • Dew and frost • Fog • Clouds

  7. Circulation of Water in the Atmosphere • evaporation • condensation • precipitation • hydrologic cycle • The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of precipitation for the planet.

  8. Stepped Art Fig. 4-1, p. 80

  9. Evaporation, Condensation and Saturation • saturation • condensation nuclei • In very clean air, about 10,000 condensation nucleiare typically found in one cubic centimeter of air,a volume approximately the size of your fingertip.

  10. HumidityMixing Ratio (w) • The ratio of the mass of water vapor in air to the mass of dry air: • w = mv / md • Usually expressed in g kg-1 • Some typical values: • Tropical marine boundary layer air: w ≈ 18 g kg-1 • Polar air: w ≈ 1 g kg-1 • Stratospheric air: w ≈ 0.1 g kg-1

  11. Specific Humidity The ratio of the mass of water vapor in air to the total mass of the air (dry air plus water vapor): SH = mv / (md + mv) w = SH / (1 – SH) SH = w / (1 + w)

  12. Vapor Pressure • actual vapor pressure • saturation vapor pressure • “Saturation” describes a condition of equilibrium: liquid water is evaporating at exactly the same rate that water vapor is condensing.

  13. Vapor Pressure Saturation vapor pressure depends only on temperature… Formula: Saturation vapor pressure Saturation vapor pressure at 273 K = 6.11 mb Latent heat of vaporization = 2.5x106 J kg-1 Gas constant for water vapor = 461 J kg-1 K-1 273 K Temperature

  14. Vapor Pressure Saturation vapor pressure depends only on temperature… Formula:

  15. Vapor Pressure Saturation vapor pressure depends only on temperature… Graph:

  16. Relative Humidity • definition of relative humidity • saturation and supersaturation • condensation • relative humidity and temperature • When the general public uses the term “humidity”, they mean “relative humidity.”

  17. Relative Humidity The ratio of the actual vapor pressure to the saturation vapor pressure. f = e / es Since es depends on temperature, the relative humidity measures closeness to saturation, not actual water vapor content.

  18. Fig. 4-5, p. 83

  19. Fig. 4-7, p. 85

  20. Relative Humidity and Dew Point • dew point temperature: the temperature to which air must be lowered to reach 100% relative humidity • dew point depression and relative humidity • The dew point temperature is useful for forecasting heatindex, precipitation probabilities, and the chance of frost.

  21. Measuring Humidity • psychrometers • hygrometers

  22. Topography and Clouds • orographic uplift • rain shadow • The rain shadow works for snow too. Due to frequentwesterly winds, the western slope of the Rocky Mountainsreceives much more precipitation than the eastern slope.

  23. Collision and Coalescence Process • terminal velocity • coalescence • warm clouds • A typical cloud droplet falls at a rate of 1 centimeter per second.At this rate it would take 46 hours to fall one mile.

  24. Instruments • standard rain gauge • tipping bucket rain gauge • It is difficult to capture rain in a bucket when thewind is blowing strongly.

  25. Doppler Radar and Precipitation • radar • Doppler radar

  26. Stepped Art Fig. 5-39, p. 135

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