ES 106 Clouds Precipitation Condensation and cloud formation A Condensation nuclei necessary for clouds to form 1 dust pollen salt smoke 2 at sub freezing temperature form by deposition of water vapor onto ice B cloud types all clouds are three basic shapes or combinations modifications 1 three predominant shapes a cirrus curl of hair b cumulus puffy masses c stratus layer or sheet 2 three predominant levels a high clouds above 6000 m often ice crystals b middle clouds from 2000 to 6000 m c low clouds less than 2000 m above ground level 3 clouds of vertical development extend from low level to towering heights II Fog cloud at the ground Created by cooling other than lifting A Advection fog from movement of warm moist air over cooler surface CA current B Radiation fog when ground cools on clear night air cooled by conduction C Upslope fog forms when humid air moves up topographic high ground D Evaporation fogs 1 steam fog cool air moves over warm water a warm water can evaporate because molecules have high energy b fog formed because cool air causes immediate condensation of vapor 2 frontal fog or precipitation fog a warm air lifted by frontal lifting results in rain b rain falls through cold air below condensation of vapor into fog in cold air I III Precipitation A droplets formed on condensation nuclei or ice crystals too small to fall 1 average size about 20 microns 0 02 mm 2 needs to be about 2 mm to fall 100 times as large 3 growth of droplets by Bergeron Process or collision coalescence B Bergeron Process mid latitudes and high latitudes 1 ice forms in supercooled cloud upon impact with objects at 10 O C 2 ice crystal preferentially attracts water vapor over attraction of liquid droplets 3 becomes too large to be supported falls melts into raindrops or not C Collision coalescence low latitudes Tropics 1 some large droplets begin to fall 2 run into 20 micron droplets etc and become larger fall faster 3 coalescence may be function of electrical charge of droplets too D forms of precipitation 1 rain liquid droplets at least mm in diameter a from nimbostratus or cumulonimbus clouds b drops larger than 4 mm break up into smaller drops c less than mm is drizzle or mist 2 snow aggregates of ice crystals fluffy or clumps depends on temperature 3 freezing rain sleet or glaze created when rain falls into colder air or onto subfreezing surfaces and freeze upon contact 4 hail ice balls from cm to 5 cm usually a created by updrafts in cumulonimbus clouds lifting ice balls b successive trips allow numerous layer of ice to form 5 rime hoarfrost pogonip deposition from vapor onto surface E measuring precipitation 1 standard rain gauge funnels water into narrow tube a calibrated to area differences to magnify the water height b errors usually from high wind not allowing water to enter funnel 2 snowfall measurements complicated by drifting 3 annual precipitation maps show colored bands for annual rainfall a drawn from data of many stations b ALL classes between an area of low rainfall and high rainfall are shown without leaving out those in between c 40 60 is NOT next to 140 to 180 the 60 to 80 80 to 100 and 100 to 140 are shown in between d Be sure you do this in lab next week 4 weather radar shows water density of clouds but inaccurate for snow Air Pressure and Wind I Pressure A 14 7 lab in2 exerted in all directions up down sideways B Measuring air pressure with barometer 1 millibars standard sea level pressure 1013 2 mb 2 inches of mercury a rises in evacuated tube from pressure on open dish b standard sea level pressure 29 92 inches 3 aneroid barometer uses partly evacuated metal chamber a high fair b low storm c overgeneralization 4 barograph records pressure continuously II Wind A Horizontal movement of air advection 1 flows due to pressure differences Pressure Gradient Force a from high to low b created by unequal heating of Earth s surface 2 affected by surface friction 3 affected by Coriolis Effect B pressure gradient force 1 maps drawn of pressure shown with isobars equal pressure lines 2 spacing of isobars shows the pressure gradient 3 wind blows more strongly with larger pressure gradients 4 initial direction from high pressure toward low pressure but C Coriolis Effect begins to affect direction 1 general mechanism a deflected to right of their path in Northern Hemisphere b deflected to left of their path in Southern Hemisphere c regardless of direction of travel d not affected at equator 2 affect on wind flow a changes direction at 90O angle to wind flow b does not affect wind speed c wind speeds affect amount of Coriolis Effect 1 greater speeds more deflection 2 slower speeds less deflection D friction of Earth s surface affects wind flow 1 upper levels of atmosphere not affected by friction a wind flow follows isobars b geostrophic winds 2 slows wind speeds at lower levels of atmosphere a reduces amount of Coriolis Effect pressure gradient prevails b surface winds directed toward low pressure at angle across isobars c surface roughness affects amount of surface friction III High pressure and low pressure A Low pressure called a cyclone 1 northern hemisphere cyclones turn counterclockwise as winds blow inward toward low pressure and are deflected to right by Coriolis effect 2 southern hemisphere cyclones turn clockwise by same effect 3 flow inward results in surface convergence a creating uplift and storminess due to expansion and cooling b consequent divergence aloft may become stronger than surface convergence and intensify cyclone B High pressure called anticyclone the opposite of cyclone 1 winds flow outward 2 surface divergence at center a convergence aloft where air drawn into area of divergence b subsiding air precludes rainfall because it is compressed and warms C these effects are the basis for fair and stormy indications on barometer D isobar maps show high pressure ridges and low pressure troughs IV General circulation of the atmosphere A Greatest heating in tropics creates uplift of rising air 1 flow from poles to equator would occur without Coriolis Effect or friction 2 these break single circulation into smaller cells with surface directions B Idealized global circulation 1 equatorial low created by Sun heating a abundant precipitation b 20 30O N and S of equator c Cooling aloft and poleward flow at tropopause 2 descending air about 30O N and S of equator a subtropical high pressure b descending air does not rain desert belts across Earth 3 wind flow between
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