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Understanding Water Vapor Concentration in the Atmosphere

Learn about quantifying water vapor with vapor pressure, mixing ratio, specific humidity, and more. Discover key concepts like saturation vapor pressure and humidity variables. Dive into technical definitions, equations, and saturation states.

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Understanding Water Vapor Concentration in the Atmosphere

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  1. Lecture 7 Water Vapor

  2. Concentration of water vapor can be quantified by: Vapor pressure Mixing ratio Specific humidity Absolute humidity Relative humidity Dew point depression Wet-bulb temperature Water Vapor amount in the air is variable.

  3. Warmer air can hold more water vapor at equilibrium than colder air. Air that holds this equilibrium amount is saturated. If air is cooled below the saturation temperature, some of the water vapor condenses into liquid, which releases latent heat and warms the air. Thus, temperature and water vapor interact in a way that cannot be neglected.

  4. Saturation Vapor Pressure Vapor pressure: Air is a mixture of gases. All of the gases contribute to the total pressure. The pressure associated with any one gas in a mixture is called the partial pressure. Water vapor is a gas, and its partial pressure in air is called the vapor pressure. Symbol e is used for vapor pressure. Units are pressure units: kPa.

  5. Saturation Air can hole any proportion of water vapor. For humidities greater than a threshold called the saturation humidity, water vapor tends to condense into liquid faster than it re-evaporates. This condensation process lowers the humidity toward the equilibrium (saturation) value. The process is so fast that humidities rarely exceed the equilibrium value.

  6. Saturation Thus, while air can hold any portion of water vapor, the threshold is rarely exceeded by more than 1% in the real atmosphere. Air that contains this threshold amount of water vapor is saturated. Air that holds less than that amount is unsaturated.

  7. Saturation The equilibrium (saturation) value of vapor pressure over a flat surface of pure water is given the symbol: es For unsaturated air,e < es Air can be slightly supersaturated (e > es). When there are no surfaces upon which water vapor can condense.

  8. Saturation – Technical Definition Sealed Container Water Vapor Liquid Water

  9. Water Vapor Fluxes Flux of water molecules from vapor to liquid Flux of water molecules from liquid to vapor

  10. Saturation Saturation exists when these two fluxes of water vapor are equal Flux of water molecules from vapor to liquid Flux of water molecules from liquid to vapor

  11. Saturation Vapor Pressure • Formula for es(T) called the Clausius-Clapeyron Equation • Approximation: Where e0 = 0.611 kPa, T = 273 K, Rv = 461 J K-1 Kg-1 is the gas constant for water vapor. Absolute temperature in Kelvins must be used for T.

  12. Clausius-Clapeyron Equation This equation describes the relationship between temperature and saturation vapor pressure. Because clouds can consist of liquid droplets and ice crystals suspended in air, we must consider saturations with respect to water and ice.

  13. Teten’s Formula Is an empirical expression for saturation vapor pressure with respect to liquid water that includes the variation of latent heat with temperature. B = 17.2694, T1 = 276.16 K, T2 = 35.86 K

  14. Exercise • Calculate es(T) for T = 0C, 10C, 20C, 30C, 40C

  15. Graph of Clausius-Clapeyron Equation

  16. System of Dry Air + Water Vapor • Assume system is closed • i.e., no exchange of mass with environment Dry air + water vapor

  17. Saturation, Sub-Saturation, Super-Saturation Super-saturated air Saturated air Sub-saturated air

  18. Super-Saturation and Condensation Suppose air becomes super-saturated “Excess” water vapor will condense

  19. Supersaturation • Supersaturation occurs when e > es • Supersaturation is a temporary state • Water vapor condenses until state of supersaturation is relieved

  20. Humidity Variables • Mixing Ratio the ratio of mass of water vapor to mass of dry air is called the mixing ratio, r or w. It is given by: Where ε = rd/rv = 0.622 g vapor/g dry air is the ratio of gas constants for dry air to that for water vapor. r is proportional to the ratio of partial pressure of water vapor (e) to partial pressure of the remaining gases in the air (P-e).

  21. Humidity Variables The saturated mixing ratio, rs, is where es is used in place of e. Units are g/g but is usually presented as g/kg: = grams of water vapor per kilogram of dry air.

  22. Humidity Variables • Specific Humidity The ratio of mass of water vapor to mass of total (moist) air, q, to a good approximation is given by:

  23. Humidity Variables • Absolute Humidity The concentration of water vapor in air is called the absolute humidity, and has units of grams of water vapor per cubic meter (g/ m3). Because absolute humidity is essentially a partial density, it can be found from the partial pressure using the ideal gas law for water vapor:

  24. Humidity Variables • Relative Humidity The ratio of actual amount of water vapor in the air compared to the equilibrium (saturation) amount at that temperature is called the relative humidity.

  25. Cooling a Parcel -- Constant Pressure

  26. Reminder • Recall where nv = number of moles of water vapor and n = total number of moles i.e., e is proportional to p.

  27. Cool the system at constant pressure • Closed system  nv and n remain constant  e remains constant

  28. Start with Sub-Saturated Air Cool air at constant pressure e

  29. Cool at Constant Pressure e

  30. Cool at Constant Pressure e

  31. Cool at Constant Pressure e

  32. Cool at Constant Pressure e

  33. Cool at Constant Pressure e

  34. Saturation Achieved Continue to cool air e

  35. Super-Saturation! e

  36. Dew • Dew forms when super-saturation occurs near a surface, e.g., a blade of grass

  37. DEW

  38. Dew Point (Td) • Definition: The temperature at which saturation would first be achieved if the air were cooled at constant pressure

  39. Temperature and Dew Point So, Td is the temperature that satisfies es(Td) = e. e Td T

  40. Note • If the Td < 0C and super-saturation occurs, frost forms • Water vapor turns directly to ice • Note: Frost is not frozen dew!

  41. Frost

  42. Relative Humidity (RH) where w is the mixing ratio and wsis the saturation mixing ratio

  43. Relative HumidityApproximation Simpler, as es is a function of T only.

  44. Exercise • Let T = 20.0C and e = 12.0 hPa • Calculate RH using the approximate form • First, calculate es(T)

  45. Increased Accuracy • For greatest accuracy, use the exact form of RH and use tabulated values of ws • Best source: Smithsonian Meteorological Tables(SMT)

  46. Supersaturation • When condensation is occurring on a surface, a thin layer of air next to the surface is supersaturated • i.e., RH > 100% • Technically, Td > T where condensation is occurring • However, Td – T is quite small and cannot be measured by standard instruments • So, for practical purposes, Td T

  47. Adiabatic Cooling (Adiabatic expansion due to falling pressure)

  48. Closed system  nv/nis constant But, p is decreasing Therefore, e is decreasing

  49. RH of Expanding Parcel e is decreasing due to expansion But, parcel is cooling  es is also decreasing It turns out that es decreases faster than e

  50. e and es for a rising parcel z esdecreases faster than e e decreases as parcel rises e es

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