atm precip

8/11/2019 Atm Precip

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Precipitation and SoundingsPrecipitation comes in many differenttypes including snow, rain, drizzle, ice

pellets, freezing rain and hail. In thisPresentation we begin by looking at theforms and shapes of the particles thatmake the precipitation. We then look atthe different types of precipitation on theground. Finally, because the form of theparticles depends on the temperatureand humidity conditions that theyencountered in the air above, we learnabout soundings (the vertical profiles oftemperature, humidity and wind in theatmosphere). And since soundings aretaken during clear weather as well asstorms, we learn to relate each soundingto the type of weather it indicates.

A weather balloon about to lift a radiosonde (not the meteorologist) through theatmosphere to measure T, p, RH, and wind speed and direction.


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Crystal ShapesSnow crystals are built ona hexagonal plan but

assume many differentshapes including flatplates, long columns orneedles, clusters, bulletswith pyramidal endings,capped columns, and theclassic 6-sided stars (i. e.,dendrites). The next slideshows that crystal shapetells the temperature anddegree of supersaturationat which the crystals form.Stars form only at highhumidity at T -15 C.

At T 0 C, snow crystals get wet. They then stick togetherwhen they collide and can grow into giant clusters or colonies.


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A blizzard (heavy snowstorm with cold temperatures andstrong winds) with huge snowflakes, Upper Saddle River, NJ


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In the Good Old Days When Winter Was Really Winter

Great Lakes Effect Snowstorm, New York State, January 2007


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Raindrops

Small raindrops and cloud droplets are spheres, not tears my dears.(Surface tension pulls them into the most compact shape). Largeraindrops flatten on bottom (as they fall) and resemble hamburger buns.

Tear-shape drops only occur when the drops slide down surfaces that canget wet (like windows or skin on our faces).

Most raindrops (except those that form in clouds over tropical oceans)begin their lives as snow crystals or snowflakes several km above sea

level and melt when they fall into warmer air near the ground.


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Some winter storms produce trulyunBEARable weather. In addition to

snow, there can be freezing rain,which is rain that freezes on contactwith the ground. This is also calledblack ice, the most dangerous,paralyzing form of precipitation from

winter storms. Freezing rain createsa world without friction.

When T < 0 C at ground level, howcan we tell whether precipitation willcome in the form of snow, freezing

rain, or ice pellets?The answer is that we must look atthe SOUNDINGS, vertical profiles ofT, T d, and winds.

http://severInewx.atmos.uiuc.edu/06/online.6.1.html
http://severinewx.atmos.uiuc.edu/06/online.6.1.htmlhttp://severinewx.atmos.uiuc.edu/06/online.6.1.htmlhttp://severinewx.atmos.uiuc.edu/06/online.6.1.htmlhttp://severinewx.atmos.uiuc.edu/06/online.6.1.html


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Ice Pellets

Ice pellets (also calledsleet) are refrozensnowflakes. They occurwhen a layer of air aloftis warm enough (> 0 C)to melt snowflakes intoraindrops but the air

near the ground is coldenough to freeze thedrops. (The refrozencrystals never regaintheir virginal form.) Ice

pellets occur in winterstorms and sting whenthey hit your face.

Ice pellets are not hailstones, which are much larger accretions of frozenraindrops and cloud droplets. Hailstones only form in severe thunderstormsbecause they need large updrafts to suspend them long enough to grow large.


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FORMATION OF PRECIPITATION

To produce precipitation, cloud droplets and crystals must form and then growlarge enough to fall to the ground. There are three stages in the growth

process.1. Nucleation . Vapor molecules collide and stick to aerosols to form an embryodroplet or crystal. This begins to happen when RH < 100%. For example, saltparticles get wet and expand (deliquesce) when RH > 75%.

2. Diffusion . When air is slightly supersaturated with water vapor (RH > 100%),vapor molecules drift or diffuse toward the droplet or crystal and stick. Whenboth supercooled liquid droplets and ice crystals are present the crystals growat the expense of the droplets, which shrink and often evaporate.

3. Collision and Collection . (Coalescence for drops and Accretion for snow

and hail.) Large particles fall faster than small particles, so they collide and thelarge particles collect or engulf the smaller particles. At this stage, raindropsgrow like planets.

COALESCE


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Snowflake Growth by Accretion (Riming)

As snowflakes fall through acloud full of tiny supercooleddroplets, they collect thedroplets in their path, whichthen freeze on them. In thisaccretion or riming processthey grow and fall to theground. If air near the groundis warm they will melt to raindrops.

During the accretion processthe original pristine crystalform is covered anddestroyed. This specimenresembles an animal that hasbeen engulfed by parasites.

Planets form and grow by thesame accretion process (and

may melt for a time).


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Hailstones only form in thunderstorms. Large updrafts loft the stones, allowingtime to grow by accretion as raindrops and cloud droplets collide with them andfreeze. Hailstones often have alternating rings of cloudy and clear ice showingthat they have moved up and down within the storm. Cloudy ice has trapped airbubbles due to rapid freezing in cold air. Clear ice forms in air just below 0 C,so it freezes slowly enough for bubbles to escape.

Hailstones

Coffeyville, NB Record Hailstone


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The largest recorded hailstone in the United States by diameter (8 inches) andweight (1.93 pounds) fell in Vivian, South Dakota on July 23, 2010.


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PLANETARY ACCRETION

Clouds (Nebula) of gases and particlessurround newly forming stars. The particlescollide and often stick (if they have velcro-like

facets like snowflakes).Eventually they grow large enough so thatgravity holds them together. When this happens

they gather particles more efficiently and evencapture gases. In this way all the planets andmoons have formed.


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Accretion Although some

material may escape,

each meteor adds itsmass to the planet ormoon it crashes into.


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Meteor Crater, Arizona

The meteorite, about 1/40 th the width of the crater, slammed into Earth about

50,000 years ago. Although it disintegrated and produced what appears to be agreat hole, it added its mass to that of the Earth.


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The Cratered Moon

has no atmosphere,wind, or rain to washaway ancient craters

But how did the Moon form?The larger Earth should have

gathered most of the meteors.
http://upload.wikimedia.org/wikipedia/commons/0/03/Moon_formation.jpg


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http://en.wikipedia.org/wiki/Giant_impact_hypothesis

In the early days of the solar system, aplanetoid about the size of Mars, crashed into

Earth. Debris from the impact circled theEarth and accreted to form the Moon.
http://upload.wikimedia.org/wikipedia/commons/0/03/Moon_formation.jpghttp://upload.wikimedia.org/wikipedia/commons/0/03/Moon_formation.jpghttp://upload.wikimedia.org/wikipedia/commons/0/03/Moon_formation.jpghttp://upload.wikimedia.org/wikipedia/commons/0/03/Moon_formation.jpghttp://upload.wikimedia.org/wikipedia/commons/0/03/Moon_formation.jpghttp://upload.wikimedia.org/wikipedia/commons/0/03/Moon_formation.jpg


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ATMOSPHERIC SOUNDINGS AND WEATHER Introduction to Atmospheric Soundings

A sounding is a vertical profile of temperature, humidity and wind in the atmosphere. Ithelps to tell the weather in the air directly above, the vertical extent of clouds, the type ofprecipitation and the likelihood of thunderstorms or air pollution.

How Soundings are Obtained Every 12 hours at several hundred stations around the world, instrumented weatherballoons are released into the atmosphere. These balloons measure temperature,pressure, and humidity as they ascend through the atmosphere, and radio the readings

to the ground-based weather station. The wind above the ground is determined bytracking the balloon by a radar as it rises. The height of the balloon is determined fromthe pressure and the temperature mathematically by the hydrostatic equation, whichequates changes of pressure to the weight of a column of air.

Units for the Weather Variables

Temperature, T, for soundings is expressed in Centigrade.Dew Point Temperature, T d, is expressed in Centigrade and is a measure of Humidity.Pressure, p, is expressed in millibars (mb) or hectoPascals. 1 mb = 100 Pa (Pascals)Height, h, is expressed in meters.

Web sources : http://weather.uwyo.edu/upperair/sounding.html http://vortex.plymouth.edu/u-make.html

Th d i t t t i th t t t hi h th i b t t d ith t
http://weather.uwyo.edu/upperair/sounding.htmlhttp://vortex.plymouth.edu/u-make.htmlhttp://vortex.plymouth.edu/u-make.htmlhttp://vortex.plymouth.edu/u-make.htmlhttp://vortex.plymouth.edu/u-make.htmlhttp://weather.uwyo.edu/upperair/sounding.html


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The dew point temperature is the temperature at which the air becomes saturated with water vapor.Dew and clouds first appear when the air is cooled to the dew point. Further cooling can produceprecipitation. T d is an indirect measure of the amount of water vapor in the air. For example, whenTd = 0 C, each kilogram of air at a pressure of 1000 millibars (mb) is holding 3.84 grams of watervapor. In rough terms,

The amount of water vapor that the air can hold doubles for each 10 C increase of T.The amount of water vapor the air actually holds doubles for each 10 C increase of Td .

Thus, when T d = 10 C, 1 kg of air at 1000 mb is holding 7.76 g (nearly double 3.84), while when T d = 20 C, 1 kg of air at 1000 mb pressure is holding 14.95 g (nearly 4 times 3.84).

Plotting a Sounding Soundings are profiles of both T and T d. They are often

plotted on a graph known as an adiabatic chart. For mostadiabatic charts, the x-axis of this chart is proportional to T(or T d) while the y axis is related to pressure or height. Tand T d are plotted in the same manner as you would plotany graph. For each data point of T make a large dot andthen connect the dots with a solid line. For each data pointof T

d, make a small x and connect xs with a dashed line.

Data for Soundingz(m) T Td

0 10 31000 15 42000 10 10

4000 -9 -96000 -23 -40

Illustrative Sounding

Clouds and Precipitation


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Clouds and Precipitation Clouds and precipitation occur when the air is saturated, or when T T d. In the illustrativesounding, the cloud layer occurs from about 2000 to 4000 m. Many clouds are too thin or havebases that are too high to produce precipitation at the ground. In order to produce precipitation acloud base should be within about 3000 m of the ground and should in general be at least 2000 mthick. The thicker the cloud and the lower its base, the more likely it will produce precipitation.

Clouds often occur even where the sounding suggests that T is up to about 5 C higher than T d!This occurs in layers in which there are breaks in the clouds. As mentioned, this is common withthunderstorms. It also occurs when T is much below 0 C because of the difference betweensaturated mixing ratios of water and ice.

Temperature Inversions An inversion is a layer in which T increases with height. In the illustrative sounding, the inversionoccurs from 0 m to 1000 m. Inversions are most common just above the ground on clear nights andearly mornings. Pronounced inversions also occur in polar lands during winter. Inversions alsooccur at elevated surfaces of fronts.

Inversions are associated with pollution episodes because they act like lids, suppressing verticalmotions in the atmosphere. Ironically, weak inversions often occur several hours before severethunderstorms, where they help intensify the thunderstorms by delaying them until the atmospherebecomes extremely unstable, and by restricting the area over which they form. Instability will bediscussed below.

Thunderstorms Thunderstorms, which rise to heights of 8 17 km are exceptional because they pump warm,humid air to great heights and form in the midst of soundings that are colder and may be dry atmost altitudes. Therefore, thunderstorms do not match the sounding of the surroundingatmosphere. Few soundings are actually taken within thunderstorms, because they are so narrow,

so short lived and because their weather is so severe they shred the balloons But the


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so short-lived, and because their weather is so severe they shred the balloons. But thethunderstorm sounding can be estimated from the sounding of the surrounding atmosphere.

Soundings for Classical Weather Situations Very often it is possible to tell the type of weather to expect from the sounding. Soundings fordifferent types of weather conditions are described and graphed below.

1. Fair weather, with possible fog and pollution . Here the air near the ground is almost saturated(T - T d 0) so that fog is likely, particularly when the air cools at night. The air at 800 m is alsocooler (and denser) than the air at 1300 m. This inversion will trap any pollution near the ground ifthe winds are too light to blow it away. By contrast, the air aloft is very warm and dry, ( T - T d > 10)so that no clouds can form in it and no humid but cooler and denser air from the ground can riseinto it. Soundings like this are very common along the coast of California, where fog is frequent, andthey also resemble the soundings that occurred during some of the worst pollution episodes inLondon. In these cases, and on most clear nights, the coldest air is right at ground level and theinversion typically extends several hundred meters above ground.

2. Unstable air with a possible thunderstorm. Here air near the ground is humid and much warmerthan air above. Temperature decreases rapidly with height (i. e., the sounding line slopes sharply to

the left). For these soundings, thunderstorms are possible whenever the sounding temperaturedecreases more rapidly with height than the cooling rate for rising saturated air (discussed below).This situation is called unstable.

3. Warm front above with snow . Here, a mass of humid, tropical air ascends a dome of cold air atthe surface. Clouds and precipitation usually form in the warm air above the frontal surface (i. e. theinversion). The type of precipitation reaching the ground is determined by the temperature structureof the sounding. Because temperatures remain below 0 C at all heights in sounding #3, snow is

most likely If temperatures near the ground are well above 0 C rain will occur You should be able


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most likely. If temperatures near the ground are well above 0 C, rain will occur. You should be ableto draw a sounding with a frontal inversion that produces rain.

4. Warm front above with freezingprecipitation . This is the samegeneral situation as sounding #3,but now, T < 0 C near the ground,while T > 0 C in some layer aloft.Snow will form near the top of thissounding, melt or partly melt as itfalls through the zone with T > 0 Cand then, as it falls through the

cold air near the ground, eitherrefreeze to ice pellets in mid-air orfreeze on contact with the groundor any other object as freezingrain.

___________________________

The red dotted line in panel 2indicates T inside the thunderstormor the adiabatic cooling rate ofrising, saturated air. It shows that arising parcel (balloon) of saturatedair will be buoyant because it iswarmer than the surrounding air.

T of surroundingatmosphere

T in thunderstorm


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Topeka Sounding12Z 20 Jan 2010

From the afternoon of

19 Jan 2010 to themorning of 20 Janskies aloft were clearover Topeka andwinds blew from theEast. As the groundradiated heat to spaceit cooled and fogformed. The soundingfor the morning of 20Jan shows that aninversion has formedin the lowest 150 mand that the the air issaturated ( T = T d),while air aloft is dry.

0

200

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1400

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-15 -10 -5 0 5 10 15

T (o

C)

H e i g h t ( m ) T(

oC)

T(oC)

Inversion and Ground Fog


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Cross Sections


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Cross Sections Soundings provide a one-dimensional view of the vertical structure of the atmosphere above onepoint on earth. Cross sections provide a two-dimensional view of the atmosphere along a line onearth. Cross sections are "slices through the pie" that reveal the structure of fronts and theassociated forms of precipitation. This is done by contouring the cross section for temperature.

Cross sections show the sequence of precipitation form. With warm fronts there is a sequence ofrain, freezing rain, sleet (ice pellets), and snow as you move from the front to the cold air. However,freezing rain and sleet only occur near sharp fronts with large temperature contrasts (as in thediagram below to the left), because only then is there band where a refreezing zone lies below amelting zone. This does not occur with a weak front (as in the diagram below to the right). Even

when freezing rain and ice pellets do occur, they generally form in a narrower zone than either rainor snow.


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Global Precipitation

Precipitation averages just about 1 meter per year over Earth but, like wealth,varies widely from place to place and over time. Rainfall is generally greatestnear the Equator where the air rises, small in the subtropics where the air sinks,and tiny in the polar regions where the frigid air has a tiny vapor capacity. Thenext slide shows how precipitation varies over the year, as the main rain beltsfollow the Sun north and south with the seasons.


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http://en.wikipedia.org/wiki/File:MeanMonthlyP.gif

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