Condensation in the Atmosphere The atmosphere contains a mixture of dry air and a variable amount of water vapor (0-4% or 0-30 g/kg) An air parcel is said to be saturated with liquid water when sufficient numbers of molecules exist so that in the presence of a plane surface of pure water at the air temperature, equal fluxes of molecules are escaping from the liquid surface as are entering the liquid surface from the air borne vapor. Generally we ignore the short period when air borne water
droplets are too small to be considered a plane surface of pure water and assume that if the air exceeds saturation, there is a net flux of droplets into the liquid phase, maintaining the vapor content at saturation or 100% relative humidity. Variation of Saturation with Temperature Tetens formula for computing saturation vapor pressure (over
plane surface of pure water) is: esl (T ) a(T 273.16) es =6.1078exp T b water
ice a 17.2693882 21.8745584 b 35.86 7.66 rsl =0.611 esl , p esl
rsl =0.611 esl p esl esi (T ) curved function
How does atmosphere form cloud? e > esl (T ) I. Adiabatic A. Increase e
a) Evaporation - can only increase RH to 100%! It CANNOT form a cloud by itself! dqliq dt B. (e es (T ))
Decrease T a) Expansional cooling b) Conduction with cold surface in parcel, ie ice hydrometeor
How does atmosphere form cloud? e > esl (T ) II. Diabatic A. Increase e
a) B. Evaporation - still cannot moisten a parcel because an adjacent parcel must have temperature only infintesimally different Decrease T a)
Radiational Cooling i. b) Tends to occur only at interface between: moist and dry layer between cloud and clear air
Conduction with cold surface i. c) Molecular diffusion of heat very inefficient, especially when diffusing a cool layer upward Parcel Mixing
i. ii. Due to curvature of saturation variation with temperature Mix two subsaturated parcels to achieve super saturation and the formation of cloud droplets Cloud formed by breath on cold day Mixing over a relatively warm lake
on a cold day Fogs Fog is a cloud in contact with the ground The reasons for fog formation mirror all the ways that saturation can be achiweved, i.e. Radiation (radiation fog, ground fog) Conduction (sea, advection fogs) Mixing is still involved
Mixing (steam fog, frontal fog, advection fog) Expansional cooling (upslope fog) Fog Types Advection Fogs Sea Fog advection of warm moist air over a cold sea surface leads to mixing of warm moist and conduction cooled air producing saturation and fog Advection of warm moist air over cold land surface leads to mixing of warm moist and conduction cooled air producing
saturation and fog (e.g. warm air advection over a snow cover) Land and sea breeze fog Tropical air fog Ice fog Snow fog Role of Dew Cooling of the surface causes moisture of the air in contact with the surface to be deposited as dew This causes a net downward transport of moisture
into the ground and the formation of a dew point inversion The dew point inversion may inhibit fog formation However, once the sun rises and the surface warms, the dew acts as a reservoir of water to allow fog to persist for several hours. Role of Droplet Settling Small liquid droplets settle very slowly Settling depletes liquid water content at top
of fog and increases it below This weakens radiative divergence at the top Hence low CCN contents produce more settling (larger droplets) and lower water contents Radiation Fog 1. Radiation cools the surface, surface air cools by conduction
2. Radiation divergence across top of moist layer cools the air above, destabilizing air above 3. Static instability at layer top causes turbulence to overturn air, mixing cold air from below, forming saturation 4. Once cloud layer forms, radiational cooling at top of fog layer is greatly enhanced, further increasing overturning and increasing fog water content Valley Fog
1. 2. 3. 4. 5. 6. 7. Nocturnal radiation cooling along side walls produces sinking motion along sidewalls
Dew deposition at the surface creates a dew point inversion at the surface Converging cold and somewhat dry air flows over the valley force upward motion and deepen the inversion About 3 h before fog formation, mountain wind forms, providing continuity for the downslope flow, but restricting upward motion in valley center. Cooling is then capped to low and mid levels of the valley by the strengthening inversion Radiation cooling at the top of the inversion layer leads
to the formation of a thin cloud layer The thin cloud layer enhances radiation divergence and deepens to the surface Marine Fog Differs from Radiation fog:
Radiation does not rapidly affect surface temperature Less CCN- more drizzle (Giant salt nuclei) Moisture flux up Heat flux down Results of model experiments show: Case 1: upward moisture flux, downward heat flux , ie cold water/warm air promotes fog Case 2: upward heat and moisture flux, ie fog if air
above is cold and moist Fog Produced By Marine Stratus Lowering Radiational cooling lowers base of stratus cloud: Fog Streets Marine Stratocumulus
Exist over Large spans of the eastern Pacific, eastern Atlantic and western Indian Oceans These are upwelling regions of cool water so air naturally near saturation in marine PBL These are also regions of large scale subsidence aloft Dynamics of Marine
Stratocumulus Subsidence Drying Aloft Moistening from the cool ocean surface Radiation divergence at top of marine PBL Entrainment of low theta-e but high theta
air from above PBL inversion Other Factors: Drizzle: Weakens radiation divergence Shear: enhances entrainment
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