1. Chapter 18

2. Water In The Atmosphere Water Vapor Source of all Condensation and Precipitation Most important gas in the atmosphere Only makes up about 4% of atmosphere

3. Water’s Change Of State Present in all states Solid, Liquid and Gas Changes states as it goes through water cycle Passes through atmosphere as gas (water vapor) Atmosphere holds enough water vapor to cover globe in 2mm deep layer

5. Solid To Liquid - Melting Energy must be transferred in form of heat Ice water stays at 0 degrees until all ice has melted Energy added is used to break apart crystals in ice Latent Heat Heat used to melt ice didn’t produce temperature change – it was hidden This hidden energy gets stored in liquid water Won’t get released until liquid returns to solid This is the energy source for storms

6. Liquid to Gas - Evaporation Energy absorbed during evaporation gives water molecules enough motion to escape surface of liquid and become gas Known as Latent Heat of Vaporization When you step out of shower and are freezing it is because the water on your skin is evaporating The energy needed to cause evaporation is coming from your skin Your skin then feels cooler

7. Gas To Liquid - Condensation Makes clouds and fog Water molecules have to release energy Latent Heat of Condensation Violent storms happen when a lot of this energy is released at once

8. Solid to Gas Sublimation Skips liquid phase Dry Ice is frozen carbon dioxide When exposed to room temperature it sublimates to gaseous state Deposition Water vapor goes directly to solid Frost

9. Humidity Amount of water vapor in the air Saturation Think of jar with lid on that is half full of water and the rest is dry air The water will evaporate causing movement in molecules of dry air When more molecules escape liquid water the pressure increases This forces some molecules to condense When the number of molecules evaporating are equal to the number of molecules condensing the air is said to be saturated Warm air holds more water vapor than cold air (pg 506)

10. Relative Humidity The ratio of the air’s actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressure Measures how near the air is to saturation Does not measure quantity of water vapor in air Can change in two ways Add or remove water vapor Change temperature

11. Relative Humidity

12. Dew Point Temperature that air has to cool to in order to reach its saturation point During summer, since warm air can hold a lot of water vapor, the warm days mean a lot of moisture in air When sun goes down the air cools Cool air can’t hold as much water vapor so it condenses and we wake up with dew covered surfaces High Dew Point = moist air Low Dew Point = dry air

13. Measuring Humidity Hygrometer Psychrometer A type of hygrometer Has two thermometers One wet-bulb One dry bulb

14. Measuring Humidity How A Psychrometer Works Air must continually pass over bulb Water evaporates from wet bulb The heat absorbed by evaporation causes wet-bulb temperature to drop (like your skin getting cold after shower) Amount of cooling is directly related to amount of water vapor in air Dry air will cause more evaporation and greater drop in temperature Use standard tables to determine relative humidity

15. Relative Humidity Table – pg 525

16. Why is Winter Air Dry? Water vapor content of air stays constant Warm air holds more water vapor than cold air In winter outside air is cold so it doesn’t hold a lot of moisture When you pump that air into your house and warm it up the relative humidity drops to less than 10% Causes static electricity, dry skin, sinus headaches, etc.

17. 18.1 Review What is the most important gas for understanding atmospheric processes? What happens to heat during a change of state? How does the temperature of air influence its ability to hold water? List two ways relative humidity can be changed? What does relative humidity describe about air? If the dew point is low, is the air moist or dry?

18. Cloud Formation Clouds form when air is cooled to its dew point Air must be saturated But these clouds form during warmest part of day – so how is that possible?

19. Air Compression and Expansion Think of pumping up bicycle tire When you put air in the hose gets warm Compressing air increases motion of gas molecules This causes temperature to rise Tire losing air When air is released it expands and cools Adiabatic Temperature Changes Changes that happen even though heat isn’t added or subtracted

20. Expansion and Cooling As you go higher in atmosphere pressure decreases With less pressure a parcel of air is able to expand as it moves higher Dry Adiabatic Rate Rate of cooling for unsaturated air 10 0 C/1000 meters Eventually parcel of air reaches its dew point Latent heat is released and clouds form Wet Adiabatic Rate Rate of cooling for saturated air 5 0 C/1000 meters

21. Adiabatic Cooling

22. Processes That Lift Air Air would prefer not to move around Four mechanisms cause air to rise Orographic Lifting Frontal Wedging Convergence Localized Convective Lifting

23. Orographic Lifting Elevated terrain acts as barrier Causes air to rise and reach dew point Clouds form and it rains on windward side Other side of barrier is dry because all of precipitation fell out Air descends and warms up making it even drier Rain Shadow Desert forms

24. Frontal Wedging Warm air meets cold air and a front forms Warm air is less dense so it rises over cold air

25. Convergence When two air masses run into each other the air has to go somewhere It can’t go down so it has to go up Leads to adiabatic cooling and possible cloud formation This is why it rains many afternoons in Florida

26. Localized Convective Lifting Unequal heating of Earth’s surface can cause pockets of air to be warmed more than others Rising parcels of air are called thermals Birds use this to soar high in sky People use it when they want to go hang gliding If parcel rises above condensation level then clouds form and may produce rain

27. Stability Stable air resists vertical movement Stable when air is cooler than surrounding environment This means it is more dense and doesn’t rise up to form clouds or cause precipitation Stable air tends to remain in original position Unstable air tends to rise – it is warmer and less dense than its surroundings

28. Stability Measurements We can tell if air is stable by knowing temperature of the atmosphere at various heights Measured using aircraft and radiosondes Environmental Lapse Rate Rate of change of air temperature with height

29. Degrees of Stability Stable air Temperature decreases gradually with increasing altitude Temperature inversion Most stable air Temperature increases with height Happens on clear nights Earth cools off quickly and air at surface is cooled while air above surface is still warm

30. Stability and Daily Weather Stable air You wouldn’t think clouds would form if air was stable But, remember, other processes cause air to lift When this happens clouds are usually widespread with little vertical thickness Precipitation, if any, is light to moderate Unstable air Towering clouds with powerful storms

31. Condensation Air must be saturated and there must be a surface for the water vapor to condense on Known as Condensation Nuclei Usually microscopic particles of dust, smoke or salt Water condenses and forms clouds, but they are so small they are still light enough to remain in the air So a cloud is made of millions and millions of tiny water droplets

32. 18.2 Review Describe what happens to air temperature when work is done on the air to compress it. What does stability mean in terms of air movement? What are the four mechanisms that cause air to rise? Describe conditions that cause condensation of liquid water in air. What is a temperature inversion? Hypothesize about other regions on Earth, other than the Florida peninsula, where convergence might cause cloud development and precipitation.

33. Clouds Visible mixtures of tiny droplets of water or tiny crystals of ice Show what is going on in the atmosphere Knowing the characteristics of cloud types helps you predict the weather. Types based on form and height

34. Cloud Form #1 - Cirrus High, white and thin Occur as patches or veil-like sheets Wispy fibers give feathery appearance

35. Cloud Form #2 - Cumulus Rounded individual cloud mass Usually have flat base and rise as domes or towers Fluffy tops described as cauliflower head

36. Cloud Form #3 - Stratus Sheets or layers that cover the sky No distinct units Usually bring precipitation

37. Cloud Height #1 – High Base above 6000 meters Cirrus, cirrostratus and cirrocumulus Thin and white Made of ice crystals Usually no precipitation If followed by cirrocumulus or cirrostratus may warn of approaching storm

38. Cloud Height #2 - Middle 2000 – 6000 meters Prefix “alto” as part of their name Altocumulus are larger and denser than cirrocumulus Altostratus are white or grayish sheet covering sky Often accompanied by light snow or drizzle

39. Cloud Height #3 - Low Form below 2000 meters Stratus, stratocumulus and nimbostratus Uniform, fog-like layer Covers most of sky Nimbostratus main precipitation makers Form in stable air when air is forced up

40. Clouds of Vertical Development These don’t fit in any of the other categories They may start in one layer and extend into another Form in unstable air Cumulus are usually fair weather but grow out of control under right circumstances – become cumulonimbus Produce rain or thunderstorms

41. Fog A cloud with its base at or very near the ground Physically same as cloud Difference is method and place of formation Clouds form when air rises Fog forms three ways Radiant cooling Movement of air over cold surface Water vapor added to reach saturation level

42. Fogs Caused by Cooling Warm moist air over Pacific moves over cold California Current then carried onshore by prevailing winds Cool, clear, calm nights air near surface of Earth cools when Earth loses heat gained during day and reaches its dew point As the air cools it becomes dense and sits in low areas

43. Fogs Caused By Evaporation Cool air moves over warm water Water evaporates from surface and air above reaches saturation Known as Steam Fog Most common in fall and early winter

44. How Precipitation Forms Cloud droplets <20micrometers in diameter This size would fall too slow Would also evaporate before it hit the ground Two Processes Bergeron Process Cold clouds Collision-Coalescence Process Warm clouds

45. Cold Cloud Precipitation The Bergeron Process Supercooling Cloud contains water vapor, ice crystals and water droplets Water droplets below freezing point Don’t freeze because there are too few freezing nuclei Water condenses on ice crystals Eventually crystals are heavy enough to fall If surface temp is above 4 O C the crystals melt on their way down and fall as rain

46. Warm Cloud Precipitation Collision-Coalescence Process Coalescence Cloud droplets fall Collide and combine with smaller droplets Drops become too large to stay suspended and fall as rain Usually in tropics

47. Liquid Forms of Precipitation Rain Size: 0.5-5mm diameter Drizzle Size: <0.5mm diameter Falls very slow

48. Solid Forms of Precipitation Snow Ice particles Usually falls as flakes small flakes at low temps larger flakes closer to freezing depends on moisture content in air

49. Solid Forms of Precipitation Sleet ice pellets rain falls through layer of freezing air

50. Solid Forms of Precipitation Freezing Rain Glaze Ice Rain doesn’t freeze until it strikes surface near ground

51. Solid Forms of Precipitation Hail Usually in cumulonimbus cloud Raindrops carried high in cloud by convection current – freezes Falls – gathers more precipitation Carried back up by updraft

52. 18.3 Review How are clouds classified? Compare and contrast clouds and fog. What must happen in order for precipitation to form? Describe how the temperature profile of air near Earth’s surface controls the type of precipitation that falls to the ground. Identify the following cloud types as producers of heavy, light, or generally no precipitation: Cirrocumulus Stratus Cumulonimbus nimbostratus