Atmospheric Moisture

1. Atmospheric Moisture CH 5. pp. 122-152 Hydrologic Cycle

3. Boiling point is much higher than one would expect from molecular weight. Water is an Odd Molecule

4. Water is a Polar Molecule Net positive charge on one side and a negative side on the other.

5. Polar Molecule • Electrical negative will attract positive charge. • The electrical attraction of the polar molecules is quite strong. • The attraction makes water molecules join together, raising the temperature of the boiling and freezing point. • Polar attraction also makes water a good liquid solvent. Other molecules bind to water in solution. • Water one of the very few substances that is densest in its liquid state • Implications in the atmosphere?

6. Ice Crystal • 105 degree separation of hydrogen causes formation of a six sided crystal.

7. Water Vapor • As the number of water vapor molecules in the air increase in number, they will have more chance of being close to another molecule and being attracted to each other. • After there are a certain number of molecules in the air, any more molecules will cause the molecules to clump together forming a liquid. • This point is the saturation point.

8. Saturation Point • Saturation point dependent upon: • Temperature (the higher the temperature the more the molecules can escape from each other’s influence) • Pressure (the higher the pressure, the higher the number of molecules present)

9. Atmospheric Moisture Methods of achieving saturation • Adding water vapor to the air • Mixing cold air with warm, moist air • Lowering the temperature to the dew point Let’s talk about #3 Air Temperature can change by : Diabatic & Adiabatic Processes

10. Atmospheric Moisture Diabatic Processes Processes that involve the removal/input of heat Increase Heat; Increase Temp.; Increase volume Adiabatic Processes Processes that do not involve the removal/input of heat Expansion of air; Increase volume; Decreases Temp.

11. Evaporation/Condensation • Imagine a container of water covered with a lid • Some molecules go from liquid to gas (evaporation), and some go from gas to liquid (condensation). • The air above the liqud surface is at saturation when the number of molecules escaping equals the number of molecules reentering the water.

12. Saturation • Saturation occurs when evaporation=condensation (the air can’t hold any more water vapor) Increasing the temperature of the air increases the amount of water vapor it can hold (it takes more water vapor to reach saturation point)

13. Moisture Measurements • There are a number of measurements we can use to specify the amount of moisture (also referred to as humidity) in the air: • absolute humidity • specific humidity • vapor pressure • saturation vapor pressure • relative humidity • mixing ratio • saturation mixing ratio • wet-bulb temperature • dew-point temperature

14. Absolute Humidity • Absolute Humidity = mass of water vapor/volume of air • So, absolute humidity is like a water vapor density, commonly express in grams/m3 • Abs humidity is not a useful measurement for humidity--because it changes with volume and temperature changes that occur in the atmosphere. Why?

15. Expansion/Compression • What happens to the volume of a rising or sinking air parcel? • Consider a parcel of air at 1000mb • The parcel exerts 1000 mb of outward pressure to counteract the atmospheric pressure acting on the parcel • If no energy is added or taken away from the parcel, then the force of the molecules bumping into the side of the parcel will be constant

16. Expansion • Now, we raise the parcel to 500 mb • The outside pressure on the parcel decreases, so the volume of the parcel increases as the parcel expands

17. Specific Humidity • Specific humidity = mass of water vapor/total mass of air • For example, in a parcel, the mass of water vapor is 1g • The total mass of the parcel (N2, O2, Ar, H2O, other trace gasses) is 1 kg • Mass H20/Total mass of the parcel of air (including the water vapor) • Specific Humidity is 1g/kg • Specific humidity is not effected by expansion and compression changes in the air parcel • Much more useful in meteorology

18. Latitudinal Distribution of Specific Humidity Figure 5.9 • High at the equator • Low at the poles

19. Absolute & Specific Humidity Figure 5.7 For a given mass of water vapor in an air parcel, the absolute humidity changes as the parcel volume changes (lifts or descends). Figure 5.8 Specific humidity is concerned with the mass of vapor to mass of air, and is not affected by changes in parcel volume.

20. Mixing Ratio • Mixing ratio = mass of water vapor/mass of dry air • For example: In a parcel, the mass of water vapor is 1g; and the mass of the dry air in the parcel is 1.0 kg • The mixing ratio is 1g/kg

21. Vapor Pressure • Remember from Chap 1 that pressure is the force of collisions of molecules against a surface • The total pressure is the sum of the pressures of the different molecules If the total pressure in this parcel was 1000 mb-- Nitrogen would contribute 780 mb, Oxygen 210 mb, and Water vapor 10 mb.

22. Actual Vapor Pressure(Dalton’s Law) • The total pressure of the air parcel is due to the sum of partial pressures of each of the gasses comprising the parcel • The pressure due to water vapor is called the actual vapor pressure

23. Distribution of Actual Vapor Pressure January July Higher in moist regions Higher in warmer areas and warmer season Very low in cold air

24. Saturation Vapor Pressure • Actual vapor pressure tells us the total water vapor content of the air.. • Saturation vapor pressure indicates how much water vapor pressure is present when the air is saturated • Dependent on air temperature • It takes less moisture to saturate a cold parcel • Warm air can hold much more water vapor than cold air

25. Relative Humidity • How much water vapor is there divided by how much it can hold (X 100)=RH • Basically: Content/capacity • Relative Humidity can be calculated by: • Actual vapor pressure/saturation vapor pressure • Actual mixing ratio/saturation mixing ratio • We can change RH in two ways: • Change the amount of vapor in the air OR • Change the air temperature • Relative Humidity is affected by both temperature and pressure • Relative humidity is high at equator and poles

26. In most places, vapor content changes only slightly over a day, yet RH varies widely each day • This is due to daily change in air temperature. Lowest temperatures of early morning result in highest RH values, warm afternoon result in lowest RH values

27. Relative and Specific Humidity Relative humidity (RH) as an indicator of saturation reveals that desert air is far from saturated, and that cold polar air nears saturation. Graphs of RH contrast with specific humidity in the deserts and poles. Specific Humidity Relative Humidity Figure 5.14

28. Dew Point Temp (TDP) • The temperature that the air must be cooled to (assuming no changes in water vapor or pressure) in order for saturation to occur • If the dew point is < 32oF, it’s called Frost Point • Important measurement to predict formation of dew, frost, fog and minimum air temperatures…as well as a good indicator of severe weather, cumulus cloud ceiling heights • High TDP is a good indicator of the air’s actual vapor content.

29. Seasonal Dew Point Maps Figure 5.12A Figure 5.12B • January, July Average Dew Point Temperatures • High Dew Points=plentiful moisture • Low Dew Points=very dry air

30. Dew Point Reality • Can you feel dew point? • On a typical summer day the following is true: • Dew Point(F) Perception • 75+..........................Extremely uncomfortable • 70-74........................Very humid, quite uncomfortable • 65-69........................A bit uncomfortable for most people • 60-64........................Ok for most, but everyone begins to feel the humidity • 55-59........................Comfortable • 50-54........................Very comfortable • <=49........................”Feels like the west”, very pleasant, feels a bit dry to some

31. Wet Bulb Temperature • The lowest temperature that can be reached by evaporating water into the air • A good measure of how cool the skin can become by sweating • Note: the wet bulb temperature will always be less than or equal to the temperature • If you are a runner: • T = 90°F, RH = 90%...High wet-bulb temperature. • T = 90°F, RH = 10%...Low wet-bulb temperature. • You feel more comfortable when wet-bulb temperature is low • Wet-bulb temperature is related to the heat index

32. Measuring Moisture

33. Moisture MeasurementsHair Hygrometer • Human hair lengthens with increasing RH. (why you have a “bad hair day”) • Levers amplify the change in length. • Measures Relative Humidity directly. • Relatively cheap instrument.

34. Moisture MeasurementsSling Psychrometer • Two thermometers; one wet, one dry. Move through the air (either fan or swing around). • Wet thermometer measures wet bulb temperature

35. Moisture measurementsDew Point • Cool mirror until dew (or frost) forms on mirror. • Used by NWS at all automated surface observing stations.

36. Moisture MeasurementsElectrical Hygrometer • Uses a chemical film that absorbs moisture which changes the electrical resistance. • Used in radiosonde measurements.