Fog and Cloud Development. Bows and Flows of Angel Hair

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Claire Simon
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1 Fog and Cloud Development Bows and Flows of Angel Hair 1

2 Ch. 5: Condensation Achieving Saturation Evaporation Cooling of Air Adiabatic and Diabatic Processes Lapse Rates Condensation Condensation Nuclei Dew, Frost, Fog Cloud Droplets Dry and Moist Adiabatic Processes and Lapse Rates 2

3 Ch. 6: Cloud Development Cloud Lifting Mechanisms Equilibrium States and Atmospheric Stability Stability vs. Environmental Lapse Rate Clouds vs. Stability Factors Affecting Parcel Buoyancy and Environmental Lapse Rate 3

4 Achieving Saturation The purpose of saturating or supersaturating air is to produce condensation of water vapor into cloud or fog droplets or ice crystals Add water vapor to air Reduce the air temperature to the dew point temperature Cool the air by contact with a cold surface Mix warm moist air with cooler air 4

5 Cooling of Air to Get Condensation Diabatic Adiabatic Cold ground cools air by conduction Heat transfer involved Fog No heat transfer between air parcel and environment 5

6 Adiabatic Process Adiabatic: no heat exchange between a system and its surroundings Adiabatic expansion: P, T Adiabatic compression: P, T 6

7 Rising Air Parcels Parcel expands, loses pressure Environmental pressure decreases as altitude increases Parcel starts with same pressure as environment Rising parcel adiabatic expansion temperature decreases 7

8 The Basics: Clouds and Fog Condensed water droplets Water vapor + Air Get condensation to occur by reducing air temperature Saturate air by lowering the air temperature until it is less than or equal to the dew point temperature 8

9 Condensation Nuclei Cloud Condensation Nuclei (CCN) and Ice-forming Nuclei (IN) These are aerosol particles that provide the surfaces on which condensation of a water droplet or ice crystal takes place CCN IN (salt water-soluble) (dust non-soluble) 9

10 Dew and Frost Water condenses onto cold surfaces Temperature of air in contact with a cold surface is reduced to the dew point temperature or below Favorable conditions for dew formation: coldest air temperatures Clear night sky Calm night winds 10

11 Types of Dew/Frost (Liquid) Dew T min and DP > 0 C Frost T min and DP < 0 C Frozen Dew T min < 0 C DP > 0 C 11

12 Hazes and Hygroscopic Nuclei Hygroscopic: water seeking Vapor molecules Salt particle (CCN) 12

13 Hazes and Hygroscopic Nuclei Hygroscopic: water seeking Vapor molecules Vapor molecules stick to hygroscopic nuclei 13

14 Hazes and Hygroscopic Nuclei Salty haze droplet resists evaporation down to 70 75% RH Vapor molecules Deliquescence 14

15 Radiation Fog Air near ground cools to the dew pt. Radiative cooling of the ground 15

16 Warm, moist air Advection Fog Cool surface Air cools to dew point as it blows over cold surface 16

17 Evaporation Fog Warm, humid air mixes with cool air aloft Temperature reduced to dew pt.; vapor condenses into fog Warm, humid air rises Warm, moist surface Moisture from surface evaporates into warmed air above 17

18 18

19 Condensation in Rising Air z When parcel s T drops below the DP, condensation occurs Lifting Condensation Level (LCL) Parcel rises; loses temperature DP T Unsaturated air parcel 19

20 Lapse Rates z Lapse Rate: rate at which temperature decreases as altitude increases z 2 γ = T 2 T 1 z 2 z 1 = ΔT Δz z 1 T 2 T 1 T 20

21 z (km) Dry and Moist Adiabatic Processes T ( C) Condensation releases latent heat into the air parcel Warms it up; parcel loses temperature less rapidly T ( C) 21

22 Dry Adiabatic Lapse Rate Γ d = 10 C/km Moist Adiabatic Lapse Rate: varies with temperature and moisture content Range: 4 to 10 C/km Average: Γ m = 6 C/km Environmental Lapse Rate (γ): varies with weather conditions, such as solar heating, cloud cover, winds,... 22

23 Lifting Mechanisms Convection Topographic Lifting Surface Convergence Frontal Lifting Cold air 23

24 Cloud Formation by Simple Convection Warm air is less dense and more buoyant than cold air z γ Γ d T env T par T 24

25 Equilibrium States Equilibrium: steady-state; balance Disturb a system at equilibrium; subsequent behavior characterizes stability of system Stable Neutral Unstable 25

26 Equilibrium States Equilibrium: steady-state; balance Disturb a system at equilibrium; subsequent behavior characterizes stability of system Stable 26

27 Equilibrium States Equilibrium: steady-state; balance Disturb a system at equilibrium; subsequent behavior characterizes stability of system Unstable 27

28 Equilibrium States Equilibrium: steady-state; balance Disturb a system at equilibrium; subsequent behavior characterizes stability of system Neutral 28

29 z γ < Γ Stable Γ γ T 29

30 z γ > Γ Unstable Γ γ T 30

31 z γ = Γ Neutral Γ γ T 31

32 Absolute vs. Conditional Stability γ < Γ m < Γ d Absolutely Stable γ < Γ γ > Γ γ = Γ Stable Unstable Neutral 32

33 Absolute vs. Conditional Stability Γ m < Γ d < γ Absolutely Unstable γ < Γ γ > Γ γ = Γ Stable Unstable Neutral 33

34 Absolute vs. Conditional Stability Γ m < γ < Γ d Conditionally Unstable γ < Γ γ > Γ γ = Γ Stable Unstable Neutral 34

35 Clouds vs. Stability Stable Convection suppressed; horizontal layer formed Unstable Convection encouraged; vertical, puffy development 35

36 Temperature Inversions z Γ d γ Temperature increases as altitude increases; γ < 0 C/km T 36

37 Typical Situations Parcels are superheated by the ground z Γ m γ Γ d LCL T (Ex.: absolutely stable atmosphere) 37

38 z Ground warms up during the daytime atmosphere destabilizes z T Ground cools off during the nighttime atmosphere stabilizes T 38

Stability and Cloud Development. Stability in the atmosphere AT350. Why did this cloud form, whereas the sky was clear 4 hours ago?

Stability and Cloud Development AT350 Why did this cloud form, whereas the sky was clear 4 hours ago? Stability in the atmosphere An Initial Perturbation Stable Unstable Neutral If an air parcel is displaced