Water in the Atmosphere

1. Water in the Atmosphere Lab 5 October 5, 2009

3. Properties of Water Physical States Gas (Water Vapor)? Molecules move freely and mix well with other molecules Liquid Molecules are close together and constantly bump one another Solid In ice, molecules are arranged in a hexagonal crystal Only natural substance that occurs naturally in all three states on Earth�s surface

5. Phases of Water Condensation Evaporation Melting Freezing Sublimation Molecules have enough energy to escape from the surface of ice into air above and directly into the vapor phase Deposition Water vapor molecule attaches itself to an ice crystal and changes to ice

6. Evaporation Water has a very high surface tension Takes energy to break the hydrogen bonds on a water surface in order to evaporate What can enhance evaporation from the surface of water? When temperatures are increases, molecules move faster (gain energy) and can break the surface tension more easily Wind also enhances evaporation Wind enhances: by blowing the vapor molecules already in the air away. Therefore, this prevents saturation from occurring which would allow for a greater amount of evaporation.Wind enhances: by blowing the vapor molecules already in the air away. Therefore, this prevents saturation from occurring which would allow for a greater amount of evaporation.

7. What happens when we heat the planet up?What happens when we heat the planet up?

9. Measuring Water Vapor Water vapor is clearly important in the atmosphere Greenhouse effect, latent heat How do we measure water vapor Absolute humidity, relative humidity, mixing ratio, vapor pressure

14. Zonally Averaged Specific Humidity The specific humidity is highest in warm, muggy tropics and as we move away from the tropics, it decreases. Major deserts of the world are at around 30 degrees. This figure shows that at that latitude, the average air contains nearly twice the water vapor than the air at 50 degrees. Therefore, the air of the desert is not �dry� and the water vapor content is not very low. The specific humidity is highest in warm, muggy tropics and as we move away from the tropics, it decreases. Major deserts of the world are at around 30 degrees. This figure shows that at that latitude, the average air contains nearly twice the water vapor than the air at 50 degrees. Therefore, the air of the desert is not �dry� and the water vapor content is not very low.

16. Vapor Pressure (e)? The air�s moisture content may also be described by measuring the pressure exerted by the water vapor in the air. Dalton�s Law The total pressure exerted by the gases in a mixture is equal to the sum of the partial pressures of each individual component in a gas mixture. For 1000 mb of air: 78% N2 = 780 mb 21% O2 = 210 mb 1% H2O(v) = 10 mb ---> actual vapor pressure More air = more pressure Higher vapor pressure = Larger # of water vapor molecules Last bullet: more water vapor molecules = more vapor pressure just like when you have a balloon and put more AIR molecules into it, it�s total pressure increases.Last bullet: more water vapor molecules = more vapor pressure just like when you have a balloon and put more AIR molecules into it, it�s total pressure increases.

17. Saturation Vapor Pressure (es)? Recall: when evaporation and condensation are in equilibrium, the air is saturated with water vapor. Saturation vapor pressure describes how much water vapor is necessary to make the air saturated at any given temperature. It is the pressure that that amount of vapor would exert.

18. Saturation vapor pressure depends primarily on the air temperature. Exponential relationship When water and ice both exist below freezing at the same temperature, the saturation vapor pressure just above water is greater than the saturation vapor pressure over ice. The statement that water has a higher saturation vapor pressure than ice means that at any temperature below freezing, it takes more vapor molecules to saturate air directly above water than it does to saturate air above ice. This occurs because it is harder for molecules to escape from ice than water. (more air = more pressure?)?The statement that water has a higher saturation vapor pressure than ice means that at any temperature below freezing, it takes more vapor molecules to saturate air directly above water than it does to saturate air above ice. This occurs because it is harder for molecules to escape from ice than water. (more air = more pressure?)?

23. Representing Atmospheric Conditions As with station plots and contouring, it's favorable to be able to represent vertical atmospheric conditions in a simple manner Skew-t diagrams Let's first discuss

24. Skew T Diagrams Why are skew T diagrams useful? Forecasting applications: Temperature and dew point profile of atmosphere Daily maximum temperature Level of cloud formation Stable vs. unstable air Precipitation type (icing forecasting)? Level of tropopause CAPE (Convective Available Potential Energy)? Microburst forecasting And many more�

26. Isobars (pressure)?

27. Isotherms (temperature)?

28. Dry Adiabats

29. Saturation Adiabats

30. Saturation Mixing Ratio

33. @ 950 mbT=15?CTd=0?C

36. Skew-T Uses Locations and magnitudes of inversions Stable/unstable layers Cloud base heights Precipitation types Severe weather potential First, we must understand how an air parcel travels vertically in the atmosphere