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Chapter 5: Forms of Condensation and Precipitation Clouds – visible aggregate of minute droplets of water or tiny crystals of ice or a mixture of both Cloud formation: major factor is adiabatic cooling reaching saturation level Lifting condensation level – height at which a rising parcel cools to the dew point Condensation has two requirements Air must be saturated (or nearly so) There must be a surface on which condensation can occur On the ground, blades of grass serve as the surface for dew formation In the air there must be small particles of dust Cloud condensate nuclei: The name for small surfaces suspended in the air on which condensation can form Especially near oceans, salt forms the major hygroscopic nuclei particles Suspended salt occurs from evaporation of sea spray droplets Dust storms, volcanic eruptions, and pollen are major sources of particles on land Hygroscopic nuclei – water seeking particles that allow condensation to begin Hygroscopic nuclei can begin to form water droplets at a relative humidity below 100% Water droplets on hygroscopic particles tend to grow quickly and become large Hydrophopic nuclei – water repelling particles that allow condensation to begin Hydrophobic nuclei tend to form droplets only when the relative humidity is 100% Water droplets on hygrophobic particles tend to grow slowly and are generally smaller Clouds are classified on the basis of two criteria Form Cirrus – high, white, and thin (cirrus is from the Latin word meaning curl or filament) They are separated or detached and form delicate veil-like patches or extended wispy fibers that often have a feathery appearance Cumulus – globular, individual cloud masses Normally they have a flat base and appear as rising domes or towers and are frequently described as having a cauliflower-like structure Stratus – sheets or layers that cover much or all of the sky While there may be minor breaks, these clouds have no distinct individual units Height High – above 6000 meters (20 000 feet) usually ice Cirrus – wispy horsetails Cirrostratus – transparent, fibrous or smooth, forming halos around the sun or moon Cirrocumulus – white patches composed of very small cells or ripples (mackerel sky) Middle – 2000 to 6000 meters usually water droplets Altocumulus – large patches of rounded masses or rolls that may or may not merge cells usually have a more distinct outline because they are formed of water droplets instead of ice Altostratus – formless layer of grayish clouds covering all or most of the sky the sun is usually visible through the cloud layer with indistinct limbs Low – below 2000 meters (6500 feet) Stratus – a uniform layer that frequently covers much of the sky and occasionally produces light precipitation Stratocumulus – stratus clouds that develop a scalloped bottom or long parallel rolls or broken globular patches Nimbostratus – from the Latin nimbus (rain cloud) and stratus (to cover with a layer) as the name implies, these clouds are one of the major precipitation producers and start as middle clouds (from thickening altostratus) but lowers into low category as precipitation continues Clouds of vertical development – have bases in the low height range but extend upward into the middle or high altitudes Cumulus – individual masses that develop domes or towers Usually form on clear days when unequal surface heating causes parcels of air to rise convectively above the lifting condensation level and are obvious because of their flat cloud bottom Often called ‘fair weather clouds’ Cumulonimbus – dark, dense, billowy clouds of considerable vertical extent forming huge towers and often exhibit an anvil head The tops of these clouds turn to ice and appear fibrous Cloud varieties extend the 10 basic cloud types with adjectives (See Cloudwise from NOAA at http://www.srh.noaa.gov/srh/jetstream/clouds/cloudwise/types.html or Jetstream Online School for Weather http://www.srh.noaa.gov/jetstream/clouds/m2.htm for better pictures) Uncinus – hook-shaped Cirrus uncinus appear like commas resting on their sides and are bad weather precursors Fractus – broken into smaller pieces Mammatus – having rounded protuberances on the under side Associated with stormy weather in cumulonimbus clouds Lenticular – lens shaped Most frequently, lenticular altocumulus clouds form on the lee side of mountains as air forms waves that lift crests above the lifting condensation level Fog – a cloud with its base at or very near the ground Fog formed by cooling: occurs when the temperature of the air close to the ground falls below the dew point Radiation fog – results from radiative cooling of the ground and adjacent air Calm air results in patchy fog less than a meter deep Light breezes up to 3 to 5 km/hr create enough turbulence to carry the fog 10 to 30 meters upward without dispersing the fog Air containing radiation fog is cool and dense and tends to flow downhill or into valleys These fogs usually dissipate 1 to 3 hours after sunrise Fog lifts: actually, radiation fog does not lift, rather the sun heats the ground and air just above it resulting in temperatures above the dew point – the warm level of the air increases in depth until the fog is gone Advection fog – results from air moving horizontally over a cold surface below A certain amount of turbulence is, again, necessary for development of advection fog Winds between 10 and 30 km/hr because turbulence facilitates cooling through a thicker layer of air and also lofts the fog to greater heights Upslope fog – results from relatively humid air moving up a gradual sloping plain or, in some cases, up a steep mountainside A certain amount of turbulence is, again, necessary for development of Fog formed by evaporation Steam fog – if cool air moves over warm water, enough moisture may evaporate to saturate the air immediately above the water and when rising meeting cold air above condenses to form fog that looks like steam An extreme example called arctic sea smoke occurs when very cold arctic air moves over quite warm ocean water (sometimes as much as a 30°C temperature difference) Frontal fog – (sometimes called precipitation fog) if warm air rising over fairly humid cool air drops rain into the cooler air, enough rain can evaporate to saturate the cooler air and form fog Dew and frost Dew – the condensation of water vapor on objects that have cooled enough to drop below the dew point of the surrounding air White frost – (sometimes called hoar frost) forms when the dew point of the air is below freezing so that deposition occurs How precipitation forms Cloud droplets are very tiny – typically 0.02 mm (20 μm: compare to human hair at 75 μm) This occurs because condensation nuclei are abundant and available water is distributed among numerous droplets instead of concentrating into fewer large droplets Cloud droplets are so tiny they fall incredibly slowly It would take several hours to fall 1000 m But droplets this size would evaporate in just a few meters Typical raindrops are 2 mm in diameter – in order to fall, droplets must increase volume by a factor of 1 000 000 times Two processes account for droplet accumulation to form precipitation The Bergeron Process (precipitation from cold clouds) typical for middle latitudes Even on sweltering days, the temperature at cloud tops can be –50°C The Bergeron process depends upon the co-existence of water vapor, liquid cloud-droplets, and ice crystals Cloud droplets do not freeze at 0°C as might be expected Pure water suspended in the air typically supercools and will not freeze until –40°C Alternately, supercooled water will freeze it if impacts an object This explains why planes flying through liquid cloud droplets ice up It also explains why falling liquid water drops instantly freeze when they hit tree branches or the ground (ice storms) Supercooled water will also freeze on contact with solid particles that have a crystal structure similar to that of ice (AgI, for example) Such materials are called freezing nuclei (which are a requirement just as condensation nuclei are required for condensation during fog formation) In contrast to condensation nuclei, freezing nuclei are rare and do not become active until the temperature reaches –10°C with 100% saturation levels of humidity 100% relative humidity with respect to liquid water at –10°C is the same as 110% relative humidity with respect to ice Given these facts, a cloud at –10°C where each ice crystal is surrounded by many thousands of liquid droplets and with air at 100% saturation wrt water is supersaturated, 110% wrt ice Under these conditions the ice crystals will collect more water from surrounding droplets than they lose to sublimation This situation will generate ice crystals that become large enough to fall During the descent of such ice crystals, these crystals will enlarge as they intercept cloud drops that freeze on them Air movement breaks up these delicate crystals which then act as freezing nuclei for other liquid droplets – a chain reaction that produces many snow crystals Accretion of 10 to 30 snow crystals forms a large mass called a snowflake If the surface temperature is above 4°C, these snowflakes melt and continue to fall as rain The Collision–Coalescence Process (precipitation from warm clouds) typical for the tropics Clouds made entirely of liquid droplets must contain some droplets larger than the typical 20 μm if precipitation is to form Large droplets form only when ‘giant’ condensation nuclei are present or when hygroscopic particles (such as sea salt) exist (which begin to form droplets below 100% relative humidity) Because rate of fall is size dependent, these ‘giant’ droplets fall most rapidly As ‘giant’ droplets fall through a cloud they coalesce with the smaller, slower droplets and become even larger causing them to fall faster and grow even larger If such large droplets can collect the equivalent of a million normal droplets they will become large enough to fall to the surface without evaporating This large number of required collisions requires clouds that have great vertical thickness and updrafts will help add to this process Remember, larger drops fall faster but higher speeds increase friction with the air At a typical 2 mm size, raindrops begin to flatten on the bottom At 4 mm, raindrops form depressions At 5 mm, raindrops will form a toroid (donut), and break apart into many smaller drops These smaller drops will updraft and build the chain reaction to form a rainstorm Forms of precipitation Rain Rain – drops of water that fall from a cloud and have a diameter of at least 0.5 mm (This size excludes drizzle and mist which both have smaller droplets) Much of the world’s rainfall begins as snow, graupel, or hail which melts before reaching the surface Warm cloud rain (especially over the ocean) has fewer condensation nuclei thus forming a range of droplet size where raindrops quickly form by collision-coalescence Drizzle – fine, uniform drops of water having a diameter less than 0.5 mm Often formed by stratus or nimbostratus clouds where precipitation may occur for hours Mist – precipitation containing the finest droplets able to reach the ground These tiny droplet appear to float and their impact is almost imperceptible Virga – rain that enters unsaturated air and evaporates before reaching the ground Snow Snow – precipitation in the form of snowflakes or, more often, aggregates of ice crystals The size, shape, and concentration of snowflakes depend to a great extent on the temperature at which they form At very low temperatures moisture content is very low resulting in small, individual, sixsided ice crystals called ‘powder’ At temperatures warmer than about –5°C, ice crystals join together into clumps of composite snowflakes These flakes tend to be heavy and high in moisture content (ideal for snowmen and snowballs) Sleet and Glaze Sleet – the fall of small particles of ice that are clear to transparent Sleet requires an above-freezing layer of air overlying a subfreezing layer of air near the ground Glaze – or freezing rain occurs when an above-freezing layer of air overlies a thin subfreezing layer of air near the surface so that the raindrops do not have time to freeze Hail – precipitation in the form of hard, rounded pellets of irregular lumps of ice Large hailstones, when cut in half, reveal nearly concentric shells of ice of differing densities and degrees of opaqueness The layers of ice accumulate as the hailstone travels up and down in a strong convective cloud Clear layers form in warmer parts of the cloud where liquid water accumulates and freezes slowly while rapid freezing of supercooled droplets in cold parts of a cloud traps air bubbles giving that layer a milky appearance Most hail is 1 – 5 cm in diameter (pea to golf ball size) The heaviest authenticated hailstone was 14 cm 766 g (1.67 lb) Hail is only produced in large cumulonimbus clouds where updrafts can reach speeds of 160 km/hr Rime – a deposit of ice crystals formed by the freezing of supercooled fog or cloud droplets on objects whose surface temperature is below freezing Rime has a unique, feathery appearance Wind may cause rime to form only on the windward side of objects Precipitation measurement Standard instruments Standard rain gauge – a 20 cm funnel opening narrows to a tube with a cross section onetenth that size thereby magnifying the depth of the rain to increase accuracy and allowing the measurement of rain depth of 0.01 inches Trace of precipitation: report made for rain depths less than 0.01 inches Tipping-bucket rain gauge – similar to a standard gauge for collection but contains see-saw dual bucket that will tip each time 0.01 inches of rain is collected the data being recorded on a drum chart (or electronically) so that both amount and rate of rainfall can be recorded Weighing gauge – another rate recording gauge that works by collecting rain in cylinder that rests on a spring balance the data being recorded in real time on a chart Measuring snowfall Two measurements are required to record snowfall accurately Depth Usually measured with a calibrated stick but best done with several averaged values taken in an area free of obstructions to prevent errors caused by drifting Water equivalent Usually measured by melting snow and weighing the resulting water mass Measurement errors Rain gauges These can measure low for multiple reasons Splashing: some rain splashes out of the gauge Wetting: some water doesn’t run down the funnel Evaporation: in dry climates, evaporation can be a factor Proper exposure is also critical Buildings, trees, other tall objects may block rain falling at an angle – gauges should be twice as far from such objects as the objects are high Wind causes turbulence – windscreens are often placed around the gauge In the US, errors are thought to range from 7 to 20% – in higher latitudes errors are thought to be as much as 80% Weather radar Radar between 3 and 10 cm will penetrate cloud droplets but are reflected by raindrops, ice crystals, and hailstones Such radar is not only useful to detect where precipitation occurs, but by the strength of the reflected image the amount of precipitation can also be accurately assessed Intentional weather modification Weather modification strategies fall into three broad categories 1. Employment of energy to forcefully alter weather Example: use of helicopters to mechanically mix air to disperse fog at an airport 2. Modifying land and water surfaces to alter natural interaction with lower atmosphere Example: spread dark substance on land to increase absorption/radiation (untried) 3. Triggering, intensifying, or redirecting atmospheric processes Example: cloud seeding with dry ice or silver iodide (cheap and has been done) Cloud seeding was first accomplished in 1946 by Vincent J. Schaefer who spurred growth of ice crystals by seeding a cloud with dry ice It was later discovered that AgI could do the same thing Certain conditions must be met for successful cloud seeding a. Clouds must be present b. A portion of the cloud must contain supercooled droplets Orographic cloud seeding has been tried at Vail and Beaver Creek, Colorado A paper mill in South Africa was putting Na and K chlorides into the air and seeding clouds Fog and cloud dispersal Dry ice seeding of supercooled fog can open clear areas Seeding will not work with warm fog Warm fog has been dispersed using mechanical mixing (helicopters) with warmer and dryer air above to fog Turboclair (underground jet engines) installed at Orly Airport (Paris, France) heat the air and can clear 900 meters of the airstrip Hail suppression Hail cannons in Europe fired large smoke rings into clouds to prevent hail (late 1800s early 1900s) Russia reported great success by seeding hail clouds (increased nucleation should reduce size of hailstones) US made similar seeding tests in Colorado Studies showed all of these methods to be ineffective Frost prevention: reduce losses in agriculture Large daytime air masses where temperature drops below 0°C (difficult to combat) Or small low lying areas in night time radiation cooling (easier to combat) Methods used Covering plants with insulating materials Warming methods Sprinklers: latent heat of freezing can prevent crop damage Mixers: large windmills can mix warmer air higher up with cooler air below Heaters: probably the most successful but also the most expensive method