Water in the Atmosphere

1. Water in the Atmosphere

2. Essential Points • Water is part of the atmosphere • Partial pressure, vapor pressure and humidity • Condensation and Clouds • Why it Rains • Hail, Snow and other forms of water

3. Water in the Atmosphere • Water vapor makes up 0-4% of the atmosphere • Most variable component • Residence time of a water molecule in the atmosphere = 10 days 1. Water is part of the atmosphere

4. Partial Pressure • The pressure that would be exerted by each gas in the atmosphere if all the other gases were removed • Proportional to the number of molecules present 2. Partial pressure, vapor pressure and humidity

5. Partial Pressure 2. Partial pressure, vapor pressure and humidity

6. Vapor Pressure 2. Partial pressure, vapor pressure and humidity

7. Vapor Pressure • The pressure of gas necessary to keep a liquid or solid from evaporating • The pressure of a substance in the atmosphere in equilibrium with a solid or liquid • If partial pressure > vapor pressure, material condenses • If partial pressure < vapor pressure, material evaporates 2. Partial pressure, vapor pressure and humidity

8. Vapor Pressure of Water 2. Partial pressure, vapor pressure and humidity

9. Vapor Pressure and Humidity • The higher the vapor pressure, the more moisture the air can hold • Vapor pressure increases with temperature • If partial pressure (water vapor actually in air) stays constant, then – • Humidity decreases as it gets hotter • Humidity increases as it gets cooler • Coolertemperatures can actually be less pleasant 2. Partial pressure, vapor pressure and humidity

10. Boiling Water • When the vapor pressure of water = atmospheric pressure, water boils • Since air pressure drops with altitude, so does the boiling point of water 2. Partial pressure, vapor pressure and humidity

11. Boiling Point of Water 2. Partial pressure, vapor pressure and humidity

12. Partial Pressure and Vapor Pressure • Partial pressure = Pressure water vapor actually exerts • Vapor pressure = Pressure water vapor could exert if the atmosphere were saturated • Most of the time the partial pressure is less than the vapor pressure 2. Partial pressure, vapor pressure and humidity

13. Adiabatic Expansion • Adiabatic = No heat transfer • Air warms or cools solely by expansion or compression • Why? Expansion does work, energy comes from internal heat • Dry air: 10 C/1000m (5.5 F/1000 feet) • Moist air: 6 C/1000m (3.3 F/1000 ft) 3. Condensation and Clouds

14. Lapse Rate and Stability • If air cools faster with altitude than 10C/km, it is unstable • If air cools 6-10 C/km with altitude, it is stable for dry air but not moist air • If air cools less than 6 C/km with altitude, it is stable • If air is warmer at high altitude than at the surface, it is inverted and extremely stable 3. Condensation and Clouds

15. “Inverted?” • If warm air rises, why is it an “inversion” to have warm air above cooler air? • Because normally, heating at the surface keeps air at the surface warmer • That’s why the troposphere is “tropo” • Warm air above is more stable, but also less common 1. Water is part of the atmosphere

16. Cloud Formation • Most cloud droplets require nuclei to form • Most nuclei are hygroscopic (attract water) • It is much easier to form water droplets than ice crystals 3. Condensation and Clouds

17. Why Don’t Clouds Fall Down? It’s a matter of Terminal Velocity – the faster things fall, the more air resistance they meet • Skydiver – 50 m/sec • Mouse – 5 - 10 m/sec • Cloud droplet – 1 cm/sec • The tiniest air motions keep such small particles suspended 3. Condensation and Clouds

18. Orographic Effects • Orographic Lifting • Clouds • Precipitation • Rain Shadows • Katabatic Winds • Chinook or Foehn Winds 3. Condensation and Clouds

19. Orographic Effects 3. Condensation and Clouds

20. Rain Forest, Washington 3. Condensation and Clouds

21. Rain Shadow, Nevada 3. Condensation and Clouds

22. Why Does it Rain? • Cloud droplets are about 1/100 mm in diameter • It takes over a million cloud droplets to make one raindrop • Coalescence of cloud droplets? Too Slow 4. Why it Rains

23. The Bergeron Process • Molecules are less tightly bound in liquids than in solids • Therefore vapor pressure of water is greater than that of ice • When ice and water droplets are present, water vapor can be saturated with respect to water droplets but supersaturated with respect to ice • Therefore water can evaporate from droplets and freeze onto ice. 4. Why it Rains

24. Freezing Rain 4. Why it Rains

25. Hail • Often associated with violent storms • Typically 1 cm or less • Can be 10 cm – capable of causing major damage • Stones typically show concentric structure • Probably form by repeated trips through supercooled regions 5. Hail, Snow and other forms of water

26. 5. Hail, Snow and other forms of water

27. 5. Hail, Snow and other forms of water

28. 5. Hail, Snow and other forms of water

29. 1. Water is part of the atmosphere

30. 1. Water is part of the atmosphere

31. 1. Water is part of the atmosphere

32. Record Hailstone, SD 2010 1. Water is part of the atmosphere

33. Fog • Radiation: Surface cools at night by radiation • Advection: Warm air carried over cool surface • Upslope: Small-scale orographic clouds • Steam: Warm water evaporates into cold air 5. Hail, Snow and other forms of water

34. Advection Fog, Labrador 5. Hail, Snow and other forms of water

35. Upslope Fog 1. Water is part of the atmosphere

36. Steam Fog 1. Water is part of the atmosphere

37. Frost 5. Hail, Snow and other forms of water

38. Ground Frost 5. Hail, Snow and other forms of water

39. Ice Shove 5. Hail, Snow and other forms of water

40. “Candle Ice” 5. Hail, Snow and other forms of water

41. Pancake Ice 5. Hail, Snow and other forms of water

42. Essential Points • Water is part of the atmosphere • Partial pressure, vapor pressure and humidity • Condensation and Clouds • Why it Rains • Hail, Snow and other forms of water