Chapter 12 Lecture Outline. Moisture, Clouds, and Precipitation

Transcription

1 Chapter 12 Lecture Outline Moisture, Clouds, and Precipitation

2 Focus Question 12.1 Why is latent heat important to our weather?

3 Water s Changes of State Heat energy One calorie is the heat necessary to raise the temperature of one gram of water one degree Celsius Latent heat Stored or hidden heat Not derived from temperature change Heat exchanged between water and surroundings during phase change Important in atmospheric processes

4 Water s Changes of State Three states of matter 1. Solid ice 2. Liquid water 3. Gas water vapor To change state, heat must be either absorbed or released

5 Water s Changes of State Processes Evaporation Liquid is changed to gas calories per gram added Latent heat of vaporization Condensation Water vapor (gas) is changed to a liquid c of heat energy/gram is released Latent heat of condensation

6 Water s Changes of State

7 Water s Changes of State Processes Melting Solid is changed to a liquid 80 calories per gram added Latent heat of melting Freezing Liquid is changed to a solid 80c of heat energy/gram is released Latent heat of fusion

8 Water s Changes of State Processes Sublimation Solid is changed directly to a gas 680 calories per gram of water are added Deposition Water vapor (gas) changed to a solid 680c of heat energy is released

11 Water Phase Changes

12 Focus Question 12.2 If the temperature remains unchanged and the amount of water vapor in the air decreases, how does relative humidity change?

13 Humidity: Water Vapor in the Air Amount of water vapor in the air Saturated air Air filled to capacity with water vapor Capacity is temperature dependent Warm air has a much greater capacity Water vapor adds pressure Vapor pressure

14 Humidity: Water Vapor in the Air Measuring humidity Mixing ratio Mass of water vapor in a unit of air compared to the remaining mass of dry air Measured in g/kg Relative humidity Ratio of the air s actual water vapor content compared with the amount of water vapor required for saturation (at that temperature and pressure)

15 Humidity: Water Vapor in the Air

16 Humidity: Water Vapor in the Air Measuring humidity Relative humidity Expressed as a percent Saturated air Content equals capacity Has 100% relative humidity Dew point temperature Temperature to which a parcel of air would need to be cooled to reach saturation

17 -Relative humidity can be changed in two ways: Relative humidity can be 1. Adding moisture raises the changed relative humidity in two ways Removing moisture lowers the relative humidity 2. Changing the air temperature Lowering the temperature raises the relative humidity Raising the temperature lowers the relative humidity

19 Humidity: Water Vapor in the Air Relative Humidity change with Temperature Change - It is related to T!

20 Dew point temperature Cooling the air below the dew point causes condensation e.g., dew, fog, or cloud formation Water vapor requires a surface to condense on Dew Point Temp does not change when the air temp changes

21 Humidity: Water Vapor in the Air Two types of hygrometers are used to measure humidity: 1. Psychrometer Compares temperatures of wet-bulb thermometer and dry-bulb thermometer Greater difference = lower relative humidity If air is saturated, both thermometers read the same temperature 2. Hair hygrometer Reads the humidity directly

23 Humidity: H 2 O Vapor in the Air Hair hygrometer Reads the humidity directly

24 Focus Question 12.3 Describe the importance of adiabatic cooling in cloud formation.

25 The Basis of Cloud Formation: Adiabatic Cooling Adiabatic temperature changes Air is compressed Motion of air molecules increases Air warms Descending air is compressed Air expands Air parcel does work on the surrounding air Air cools Rising air expands

26 The Basis of Cloud Formation: Adiabatic Cooling Adiabatic rates Dry adiabatic rate Unsaturated air Rising air expands & cools at : 10 C/1000 m (5.5F/1000 ft.) Descending air compresses and warms at: 10 C/1000 m (5.5F/1000 ft.) Wet adiabatic rate Begins at condensation level Air has reached the dew point Condensation is occurring and latent heat is being liberated Heat released by condensing water reduces cooling rate Rate varies from 5 C to 9 C/1000 m

27 The Basis of Cloud Formation: Adiabatic Cooling

28 Focus Question 12.4 How do orographic lifting and frontal wedging force air to rise?

29 Processes That Lift Air Orographic lifting Elevated terrains act as barriers Result can be a rainshadow desert

30 Processes That Lift Air Frontal wedging Cool air acts as a barrier to warm air Fronts are part of middle-latitude cyclones

31 Processes That Lift Air Convergence Air flows together and rises

32 Processes That Lift Air Localized convective lifting Unequal surface heating causes pockets of air to rise because of their buoyancy

33 Focus Question 12.5 How does atmospheric stability affect our weather?

34 The Weathermaker: Atmospheric Stability Stability of air determines: Type of clouds that develop Intensity of the precipitation

35 The Weathermaker: Atmospheric Stability Types of stability Stable air Resists vertical displacement Cooler and denser than surrounding air Wants to sink No adiabatic cooling Widespread clouds with little vertical thickness Precipitation is light to moderate Absolute stability Environmental lapse rate less than wet adiabatic rate

36 The Weathermaker: Atmospheric Stability

37 The Weathermaker: Atmospheric Stability Absolute instability Acts like a hot air balloon Rising air Warmer and less dense than surrounding air Rises until it reaches altitude with same temperature Adiabatic cooling Environmental lapse rate greater than dry adiabatic rate Clouds are often towering Conditional instability Atmosphere is stable for an unsaturated parcel of air but unstable for a saturated parcel

40 Focus Question 12.6 What is the function of condensation nuclei in cloud formation?

41 Condensation and Cloud Formation Condensation Water vapor changes to a liquid and forms dew, fog, or clouds Water vapor requires a condensation surface On the ground Grass, a car window, etc. In the air are tiny bits of particulate matter called condensation nuclei Dust, smoke, ocean salt crystals, etc.

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43 Condensation and Cloud Formation Clouds Made of millions and millions of Minute water droplets, or Tiny crystals of ice Classification based on form Cirrus High, white, thin Cumulus Globular, puffy, cloud masses Vertically developed Stratus Sheets or layers that cover much of the sky Horizontally developed

44 Condensation and Cloud Formation

46 Condensation and Cloud Formation Clouds classified based on height High clouds Above 6000 m Cirrus, cirrostratus, cirrocumulus Middle clouds 2000 to 6000 m Altostratus and altocumulus Low clouds Below 2000 m Stratus, stratocumulus, and nimbostratus (nimbus means rainy )

48 Condensation and Cloud Formation Clouds of vertical development From low to high altitudes Called cumulonimbus Often produce rain showers and thunderstorms

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50 Focus Question 12.7 Define fog.

51 Fog Fog is a cloud with its base at or near the ground Considered an atmospheric hazard Most fogs form because of Radiation cooling, or Movement of air over a cold surface

52 Fog Fogs caused by cooling Advection fog Warm, moist air moves over a cool surface Radiation fog Earth s surface cools rapidly Forms during cool, clear, calm nights Upslope fog Humid air moves up a slope Adiabatic cooling occurs

53 Fog

54 Fog

55 Fog Evaporation fogs Steam fog Cool air moves over warm water Water has a steaming appearance Frontal fog, or precipitation fog Forms during frontal wedging when warm air lifted over colder air Rain evaporates to form fog

56 Fog

57 Fog

58 Focus Question 12.8 Describe the 2 mechanisms that produce precipitation.

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60 How Precipitation Forms Cloud droplets < 20 micrometers (0.02 millimeter) in diameter Fall incredibly slowly Formation of precipitation 2 ways Bergeron process Temperature in the cloud is supercooled Ice crystals collect water vapor Large snowflakes form and fall to the ground or melt and turn to rain

61 How Precipitation Forms

62 How Precipitation Forms Collision-coalescence process Warm clouds Large hygroscopic condensation nuclei Large droplets form Droplets collide with other droplets during their descent

63 How Precipitation Forms

64 Focus Question 12.9 Explain why snow can sometimes reach the ground as rain, but the reverse does not occur.

65 Forms of Precipitation Rain and drizzle Rain Droplets have at least a 0.5 mm diameter Drizzle Droplets have less than a 0.5 mm diameter Snow Ice crystals, or aggregates of ice crystals

66 Forms of Precipitation Sleet and glaze Sleet Small particles of ice in winter Occurs when warmer air overlies colder air Rain freezes as it falls Glaze, or freezing rain Impact with a solid causes freezing

67 Forms of Precipitation

68 Forms of Precipitation Hail Hard rounded pellets Concentric shells Most diameters range from 1 to 5 cm Formation In large cumulonimbus clouds Layers of freezing rain are caught in violent up- and down-drafts Pellets fall when they become too heavy

69 Forms of Precipitation

70 Forms of Precipitation Rime Forms on cold surfaces Freezing of supercooled fog Freezing of cloud droplets

71 Focus Question How is precipitation measured?

72 Measuring Precipitation Rain Easiest form to measure Measuring instruments Standard rain gauge Uses a funnel to collect rain Cylindrical tube measures in cm or inches Radar is also used to measure the rate of rainfall

73 Measuring Precipitation

74 Measuring Precipitation

75 Measuring Precipitation Snow has two measurements: 1. Depth 2. Water equivalent General ratio is 10 snow units to 1 water unit Varies widely