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Chapter 4 Moisture and Atmospheric Stability This chapter covers: Water Water states of matter states of matter heat capacity and latent heat heat capacity.

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Presentation on theme: "Chapter 4 Moisture and Atmospheric Stability This chapter covers: Water Water states of matter states of matter heat capacity and latent heat heat capacity."— Presentation transcript:

1 Chapter 4 Moisture and Atmospheric Stability This chapter covers: Water Water states of matter states of matter heat capacity and latent heat heat capacity and latent heat Humidity and dew point Humidity and dew point Adiabatic temperature changes in the atmosphere Adiabatic temperature changes in the atmosphere Atmospheric stability Atmospheric stability

2 The Hydrologic Cycle

3 States of Matter: Solid, Liquid and Vapor

4 Gas (Vapor) widely spaced molecules widely spaced molecules no bonding between molecules no bonding between molecules molecules move at high speeds molecules move at high speeds very compressible very compressible

5 Liquid Closely spaced molecules Closely spaced molecules Moderate bonding between molecules Moderate bonding between molecules molecules move at medium speeds molecules move at medium speeds Slightly compressible Slightly compressible

6 Solid (i.e., ice) closely spaced molecules closely spaced molecules Strong, rigid bonding between molecules Strong, rigid bonding between molecules No molecule movement – only vibrations No molecule movement – only vibrations Fairly incompressible Fairly incompressible

7 Solid Water: Ice

8 Liquid Water

9 Water Vapor

10 Heat Capacity and Latent Heat of Water

11 Saturation Condition in which the air is holding the maximum amount of water vapor possible Condition in which the air is holding the maximum amount of water vapor possible Amount of water vapor present at saturation depends on Amount of water vapor present at saturation depends on Temperature; more vapor at higher temp. Very strong effect Temperature; more vapor at higher temp. Very strong effect Pressure; more vapor at higher pressure. Pressure; more vapor at higher pressure.

12 Absolute Humidity Amount of water vapor present in air Amount of water vapor present in air Given as grams water vapor per cubic meter of air Given as grams water vapor per cubic meter of air Value is affected by air pressure Value is affected by air pressure

13 Mixing Ratio Amount of water vapor present in air, but more useful than absolute humidity Amount of water vapor present in air, but more useful than absolute humidity Given as grams water vapor per kilogram of air Given as grams water vapor per kilogram of air Typically ranges from 0 to 4% Typically ranges from 0 to 4% Value is not affected by air pressure Value is not affected by air pressure

14 Saturation Air is limited in how much water vapor it can hold without water droplets forming Air is limited in how much water vapor it can hold without water droplets forming Saturation is the point at which air cant hold more water vapor Saturation is the point at which air cant hold more water vapor Mixing ratio at saturation depends on temperature, and somewhat on pressure Mixing ratio at saturation depends on temperature, and somewhat on pressure

15 Contrail: engine exhaust contains water vapor, exhaust cools, becomes saturated with water vapor and condensation occurs

16

17 Saturation Mixing-Ratio: How much water vapor can be present in air at different temperatures

18 Relative Humidity the humidity we feel Amount of water vapor in air relative to maximum possible amount (saturation mixing ratio) Example Temperature: 20 o C Saturation mixing ratio=14g vapor per 1 kg air Actual vapor content = 7 g per 1 kg air Relative humidity = 7 g / 14 g x 100% = 50%

19 Dew Point Temperature to which air must be cooled to become saturated Temperature to which air must be cooled to become saturated Assumes no change in mixing ratio Assumes no change in mixing ratio Relative humidity is 100% in air thats at its dew point Relative humidity is 100% in air thats at its dew point Stating airs dew point is essentially the same as stating its mixing ratio Stating airs dew point is essentially the same as stating its mixing ratio

20 Airs saturation mixing ratio and relative humidity change with temperature

21 Which has larger mixing ratio? Which has higher relative humidity? Antarctica Death Valley

22 Hotter: Higher mixing ratio, Lower relative humidity Colder: Lower mixing ratio Higher relative humidity

23 Relative Humidity, Mixing Ratio and Air Temperature Hotter air can hold much more water vapor than cold air Hotter air can hold much more water vapor than cold air Hotter air can have more vapor in it than cold air, yet have lower relative humidity Hotter air can have more vapor in it than cold air, yet have lower relative humidity

24 Relative Humidity Changes with Temperature Daily Relative Humidity Temperature

25 Air Temp, Dew Pt. & Relative Humidity in Heber

26 Dew Point Temperatures

27

28 Adiabatic Temperature Changes Air cools when it expands, warms when its compressedAir cools when it expands, warms when its compressed Rising air expands and coolsRising air expands and cools Sinking air is compressed and warmsSinking air is compressed and warms Adiabatic refers to temperature changes w/o heat transferAdiabatic refers to temperature changes w/o heat transfer Very important!

29 Adiabatic Temperature Changes

30 Dry & Wet Adiabatic Rates Saturated air cools less as it rises because condensation of water releases heat Dry adiabatic rate = 10 o C / 1000m = 5.5 o F / 1000 feetDry adiabatic rate = 10 o C / 1000m = 5.5 o F / 1000 feet Wet adiabatic rate = 5 to 9 o C / 1000m (2.75 to 5 o F/1000ft)Wet adiabatic rate = 5 to 9 o C / 1000m (2.75 to 5 o F/1000ft)

31 Dry & Wet Adiabatic Rates

32 Lifting Condensation Level As air rises, it expands and cools As air rises, it expands and cools Level (altitude) at which it is cooled to its dew point is the lifting condensation level Level (altitude) at which it is cooled to its dew point is the lifting condensation level – Clouds form above this level if air is rising

33 Causes of Lifting Orographic – wind blows over mountains Orographic – wind blows over mountains Frontal wedging – warm air forced over colder air Frontal wedging – warm air forced over colder air Convergence – winds blowing together Convergence – winds blowing together Convection – solar heating creates hot air that rises Convection – solar heating creates hot air that rises

34 Orographic Lifting Important along Wasatch Front, much of Western U.S.

35 Frontal Wedging Storm Fronts

36 Convergence

37 Convection

38 Cause of Rain Shadow Desert Rising air cools at wet adiabatic rate sinking air warms at dry adiabatic rate

39 Atmospheric Stability Stable Air = Air that tends to not rise Stable Air = Air that tends to not rise Unstable Air = Air that tends to keep rising (regardless of orographics, fronts, etc.) Unstable Air = Air that tends to keep rising (regardless of orographics, fronts, etc.) Importance – rising air cools, makes clouds, precipitation, even tornados Importance – rising air cools, makes clouds, precipitation, even tornados

40 What Controls Stability Depends on adiabatic cooling rate (dry and wet) vs. Environmental Lapse Rate Depends on adiabatic cooling rate (dry and wet) vs. Environmental Lapse Rate Environmental Lapse Rate = the actual, existing decrease in air temperature with altitude Environmental Lapse Rate = the actual, existing decrease in air temperature with altitude

41 Atmospheric Stability, cont. Three types of stability: Three types of stability: Absolute stability Absolute stability Absolute instability Absolute instability Conditional instability Conditional instability

42 Absolute Stability Environmental Lapse rate is less than wet adiabatic rate Environmental Lapse rate is less than wet adiabatic rate As air rises, it cools so much (even if its saturated) that it becomes cooler than surrounding air so it stops rising As air rises, it cools so much (even if its saturated) that it becomes cooler than surrounding air so it stops rising

43 Absolute stability

44 Absolute instability Environmental lapse rate is greater than dry adiabatic rate Environmental lapse rate is greater than dry adiabatic rate As air rises, despite cooling at dry adiabatic rate, it becomes progressively warmer than surrounding air and rises faster As air rises, despite cooling at dry adiabatic rate, it becomes progressively warmer than surrounding air and rises faster

45 Absolute Instability Absolute instability

46 Conditional Instability Environmental Lapse rate is greater than wet adiabatic rate, less than dry adiabatic rate Environmental Lapse rate is greater than wet adiabatic rate, less than dry adiabatic rate As air rises, if it is unsaturated it tends to not rise, but once its saturated it keeps rising As air rises, if it is unsaturated it tends to not rise, but once its saturated it keeps rising

47 Conditional Stability Conditional Instability

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49 NWS Storm Prediction Center Focuses on dangerous thunderstorms Focuses on dangerous thunderstorms Produces estimates of convective stability for locations across the country twice daily Produces estimates of convective stability for locations across the country twice daily Main website: http://www.spc.noaa.gov Main website: http://www.spc.noaa.govhttp://www.spc.noaa.gov Soundings (weather balloon data which provide information on environmental lapse rate and more) with stability analysis (somewhat advanced scientifically): http://www.spc.noaa.gov/exper/soundings/ Soundings (weather balloon data which provide information on environmental lapse rate and more) with stability analysis (somewhat advanced scientifically): http://www.spc.noaa.gov/exper/soundings/ http://www.spc.noaa.gov/exper/soundings/

50 Chapter 4 END


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