Monday, Sept 22 (Week 5) Weather Review Weather Maps and Isopleths

1. Today • Monday, Sept 22 (Week 5) • Weather Review • Weather Maps and Isopleths • Humidity, Condensation, Fog (Chp 4) • Wednesday • Clouds, Precipitation (Chp 5) • Next Monday • Midterm Review

3. Homework #4 Solution

5. Weather Review • What happens today at 7:29 pm? • Autumnal Equinox • Astronomical event • Meteorological Fall (Sep-Oct-Nov)

6. Weather Review

7. Weather Review

8. Weather Review Mon am

9. Weather Review Mon pm

10. Weather Review Tue am

11. Weather Review Tue pm

12. Weather Review Wed am

13. Weather Review Wed pm

14. Weather Review Thu 5 am

15. Weather Review Thu 8 am

16. Weather Review Thu 11 am

17. Weather Review Thu 2 pm

18. Weather Review Thu 5 pm

20. HUMIDITY, CONDENSATION & CLOUDS

21. Earth’s Water Cycle

22. Review: PHASE TRANSITIONS Water is the only substance that can be found in the atmosphere in the gaseous, liquid and solid phases

23. SATURATION When Rate of Evaporation = Rate of Condensation

24. SATURATION • Saturation means that the rates of evaporation and condensation are in equilibrium and any additional water vapor that is added will throw this balance off. Rate of Evaporation = Rate of Condensation. • To compensate there will be condensation (i.e., cloud formation and/or precipitation). • At higher temperatures the rate of evaporation is higher, so more water vapor is needed to achieve equilibrium. Therefore saturation is temperature dependent

25. PARTIAL PRESSURES Total Pressure = PN2+ PO2 + PAr + PH20 + PCO2 + … Vapor Pressure: The pressure due to water molecules alone. If the total pressure were ~ 1000 mb, PN2 ~ 780 mb 78% PO2 ~ 210 mb 21% PH20 ~ 10 mb 1%

26. JANUARY WATER VAPOR (mb)

27. JULY WATER VAPOR (mb)

28. Summary of Humidity Definitons • Vapor Pressure Partial pressure of water vapor molecules • Saturation Vapor Pressure Partial pressure of water vapor in saturated air • Absolute Humidity Mass of water vapor in a fixed volume of air • Specific Humidity Mass of water vapor in a fixed total mass of air • Mixing Ratio Mass of water vapor in a fixed mass of dry air

29. RELATIVE HUMIDITY Relative Humidity = water vapor content water vapor capacity RH = 100 X actual vapor pressure saturation vapor pressure Measure of how close air is to becoming saturated, NOT how much water vapor is in the air. • If RH = 100%, then the air is saturated and condensation occurs.

30. RELATIVE HUMIDITY BASICS • When water vapor content or temperature changes, so does relative humidity. • With a constant water vapor content, cooling the air raises the RH and heating the air lowers it. • The Dew Point Temperature is a good measure of the water vapor content in the atmosphere.

31. TEMPERATURE vs. RH • Highest RH normally early morning • Lowest in the afternoon. • The change in temperature, changes the saturation vapor pressure.

32. TEMPERATURE vs. RH

33. POLAR AIR vs. DESERT AIR Polar Air Temp=28° DP=28° RH=100% Desert Air Temp=95° DP=41° RH=16%

34. Summertime Relative Humidity

35. SLING PSYCHROMETER Wick Wet Bulb Thermometer Dry Bulb Thermometer Gives Wet Bulb Temperature

36. HAIR HYGROMETER • As RH increase Hair length Increases • “Bad hair day”

37. ELECTRONIC MEASUREMENTS • Measuring Dew-Point Temperature • Cool a mirror until condensation appears on mirror • Use optical device to detect water and measure temperature • Measuring Humidity • Electric current through ceramic material • Change in Resistance = Change in Humidity

38. THE FORMATION OF DEW & FROST • Dew forms on objects when they cool below the dew point temperature. • Most likely on clear, calm nights due to increased radiative cooling • Frost forms when dew point is below 32°F • Frozen Dew when dew initially forms above 32° F and then the temperature drops to below freezing. Looks “pebbly”.

39. HEAT INDEX

40. Dew Frost Frozen Dew

41. CLOUD CONDENSATION NUCLEI • Good CCNS are hygroscopic (“like” water) • Natural CCNs • Sea salt particles • Natural sulfur emissions • Vegetation burning • CCNs from human activity • Pollutants from fossil fuels • Sulfur dioxide > particulate sulfuric acid and ammonium sulfate salts • Nitrogen oxides > gaseous nitric acid which can combine with ammonia to form ammonium nitrate particles

42. TYPICAL DROPLET SIZES

43. CLOUD DROPLET FORMATION • Below the Dew Point water vapor will tend to condense and form cloud/fog drops • Formation on cloud condensation nuclei (CCN) • Most effective CCN are water soluble. • Without particles clouds would not form in the atmosphere • RH of several hundred percent required for pure water drop formation

44. STEPS IN CLOUD FORMATION • Air cools causing RH to increase • Radiative cooling at surface or • Expansion in rising parcel • CCN take up water vapor as RH increases • Dependent on particle size and composition • If RH exceeds critical value, drops are activated and grow readily into cloud drops

45. FOG • Cloud in Contact with the Ground • Five Types • Radiation Fog • Advection Fog • Upslope Fog • Evaporation Fog • Precipitation Fog

46. RADIATION FOG • Surface cooling via radiation • Lowest air near the ground cools to dew point • Fog deepens from the ground up • Ideal Conditions • Moist ground • Clear Skies • Calm Wind • “Tule” Fog

47. RADIATION FOG Fig. 4.4, p. 98

48. RADIATION FOG

49. ADVECTION FOG • Warm air advects over cold surface • Cold surface cools air • Saturation = fog formation • Common on West Coast • Cold upwelled water

50. ADVECTION FOG “Fog Drip” • 20-40% of a Redwood’s water