Chapter 5 CONDENSATION DEW FOG CLOUDS Dew Frost Dew forms on nights with ideal radiational cooling conditions Air near the surface cools to its dewpoint Water droplets form on grass and other low lying vegetation Air becomes saturated dewpoint depression 0 relative humidity 100 Condensation occurs Water droplets don t really form on trees and brush because they are taller and as you move up in the atmosphere at surface level it gets warmer When the dewpoint is below freezing we refer to it as the frost point This is the same process as dew formation but in this instance frost forms instead Haze Fog salt dust volcanic ash etc are particles in the air onto which water vapor can condense Condensation nuclei they are very light so they may be suspended in the air for several days most abundant in deserts sand oceans seas salt and cities pollution most scarce in forests mountains and polar regions rain droplet 2000 m cloud droplet 20 m condensation nuclei 0 2 m condensation nuclei are water seeking some particles act as better condensation nuclei than others Hygroscopic Most active in promoting condensation Hydrophobic Allow condensation to occur at relative humidity s of less than 100 Ex salt condensation nuclei are water repelling These condensation nuclei resist condensation even with relative humidity s of greater than 100 Ex oil There are generally enough hygroscopic particles to allow for condensation at relative humidity s of only 75 or more Haze a layer of dust or salt particles Dry haze occurs in the afternoon when the relative humidity is at a minimum low enough that condensation does not occur even for the most hygroscopic condensation nuclei and floating particles remain small and selectively scatter some rays of sunlight Visibility may be reduced but not severely Wet haze occurs in the morning when the relative humidity is at a maximum as water collects on the condensation nuclei the particles increase and scatter sunlight much more efficiently Because the floating particles are larger they cause the visibility to be reduced to less than half of dry haze visibility a cloud on the ground Fog When the air approaches saturation condensation begins forming on condensation nuclei when millions or more floating water droplets form we get fog Formation of fog occurs in one of two ways 1 Cooling radiation 2 Evaporation and mixing Radiation ground fog fog produced by radiational cooling shallow moist layer near the surface with drier air above radiational cooling has caused only the air near the surface to reach saturation leaving upper layers dry condensation occurs in the surface layer and fog forms Favorable conditions for radiation fog Clear skies long nights Light wind less than 5 kts to bring more of the moist air in direct contact with the cool ground A strong breeze is unfavorable as it would cause drier air above to mix down to the surface Clear skies and lights winds are typically associated with areas of high pressure General characteristics of radiation fog Forms upward from the ground during the night During the day the sun warms the ground Warm air rises and mixes with the foggy air above increased temperatures cause some evaporation and fog dissipation This allows more sunlight to reach the ground which further accelerates fog dissipation The fog typically dissipates around the edges first where the fog layer is the thinnest If the fog is thick and the sun angle is low the fog may not completely dissipate a type of radiation fog that forms when cold air drains into a valley floor Valley fog Advection fog Advection Warm moist air has a relatively high dewpoint so when it moves over a cold surface the air fog that occurs when warm moist air moves over a cold surface the transfer of given air properties of air by horizontally moving it near the surface cools to dewpoint and fog forms Ex fog over the pacific coast of California the result of warm moist air from the Gulf of Mexico being advected the fog that forms as moist air moves up along an elevated hill plain or mountain Advection Radiation fog over the cooler landmasses of the Gulf coast the cooler ground is the result of radiational cooling causing fog to form Upslope fog Evaporation mixing fog saturation Cold air moves over a warm lake in early fall Warm water evaporates from the lake into the air which increases the water vapor content Evaporation fog is seen when exhaling on a cold day or as steam fog Occurs in winter and spring on the east side of the Rocky Mountains fog produced when two unsaturated air masses mix and create of the air and thus raises the dewpoint Advection fog tends to occur near the coast Clouds Cloud Types low clouds 0 to 6500 feet 1 2 middle clouds 6500 to 26000 feet 3 high clouds 20000 to 60000 feet 4 clouds with vertical development within each group clouds are identified by appearance Stratus St layer sheet like Cumulus Cu heap puffy Cirrus Ci curl of hair wispy poorly defined edges Nimbus violent rain rain clouds Clou ds high clouds seen less frequently than cirrus mackerel sky fill gill appearance high clouds that form a thin sheet like layer can form halo around sun or moon high clouds composed mainly of ice crystals that move generally west to east Cirrus Ci Cirrocumulus Cc Cirrostratus Cs due to light refraction via ice crystals Altocumulus Ac Altostratus As entire sky with the sun faintly visible through the layer gloomy day clouds Nimbostratus Ns middle cloud semi puffy composed mainly of water droplets thin low cloud steady rainfall water droplet composed dark grey layer middle clouds composed of water droplets and ice crystals that often cover the Scud ragged irregular shred of nimbostratus cloud low solid uniform layer of grey cloud cover no precipitation low flat and lumpy clouds that often occur near sunset low clouds limited vertical development fair weather clouds cumulus clouds with vertical development head of a cauliflower thunderstorm clouds flat top caused by the tropopause anvil caused by Stratus St Stratocumulus Sc Cumulus humilus Cu Cumulus congestus Tcu Cumulonimbus Cb upper level winds Unusual clouds Remember that clouds are the result of air that s saturated and has started condensing Many different ways to get condensation many different uncommon peculiar clouds found at crest of mountain wave formed by air forced over mountain to Lenticular saturation Pileus condensed Mammatus Nacreous cold water mother of pearl clouds cap cloud moist air deflected over
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