Growth of Cloud Droplets and Overview of Precipitation Growth Of

MTO 412E Physics of Growth of Cloud Droplets and Cloud and Precipitation Precipitation Too little : Drought • Condensation • Cool and Cold Clouds Growth of – Bergeron Process Cloud Droplets – Riming and Aggregation and Overview of • Warm Clouds and Overview of Too much : – Collision and Coalescenc Precipitation Flood • Precipitation and Precipitation types • Precipitation Measurement

Condensation Condensation

• Can lead to quick growth for small water • Condensation occurs when water vapor droplets. changes to a liquid. • Once radii of 20 microns is passed, the • For condensation to take place, the air must be saturated and there must be a surface on efficiency of condensation is reduced. which the vapor can condense. • By itself condensation would produce few • In the air above the ground, tiny hygroscopic raindrops. (water -absorbent) particles known as condensation nuclei serve as the surfaces on which water vapor can condense.

Condensation onto water • When an air parcel is lifted and cooled, the vapour pressure eventually exceeds the SVP. • This condition is called supersaturated • Droplets cannot form from pure water vapour until very high supersaturations (> 5 times SVP) are reached • Particles are essential to provide surfaces on which condensation can start • These particles are called cloud condensation nuclei (CCN) ****

1 The Bergeron Process Bergeron Process

• Ice crystals will gain water vapor from – Also called the ice crystal process neighboring water droplets and grow to a suitable size for precipitation. • Need: • Bergeron process most effective when ice to – Water Vapor droplet ratio is 1:100,000 – Ice crystals • Non -convective precipitating clouds most – Supercooled water drops in equilibrium likely use this process since the liquid water with the vapor content is lower than convective clouds.

Saturation over ice Bergeron Process Saturation • There is a saturated vapour pressure over ice in the Vapor Pressure with respect to same way as over water Liquid Water • At any temperature below 0 oC, the SVP over ice is slightly less than the SVP over water Vapor Saturation • The difference is not constant - it is greatest at -12 Pressure Vapor Pressure o C with respect to • Ice crystals grow very fast if there are water Ice droplets present as well, because a low supersaturation over water is a high one over ice. **** 0
C Temperature

Bergeron Process Bergeron Process Below 0
C, the saturation vapor pressure with respect to ice is less than that with respect to a plane surface of liquid wate r Since the ice is now in a supersaturated environment, vapor at the same temperature. So, f or equilibrium there is a lower molecules will go to the ice crystal and the ice crystal will vapor pressure for ice than for liquid water. grow. Consider a supercooled liquid water drop next to an ice crystal:

Key Point: If the liquid drop is in equilibrium with the environment, then the ice crystal will be in a supersaturated environment.

2 Bergeron Process Bergeron process Since the vapor molecules are going toward the ice crystal, the drop now finds itself in an unsaturated environment. The drop will begin to evaporate, thereby supplying more water • Freezing nuclei start the growth of ice vapor molecules for the ice crystal to acquire. crystals in supersaturated conditions and other water droplets are then attracted • Common in mid -latitudes (at higher altitudes) • Cloud seeding utilizes this process using silver iodide or dry ice (which are good freezing nuclei)

Bergeron (ice crystal) process Bergeron Process

• is operative in cold clouds • Depends on the co -existence of supercooled water and ice in the same cloud. • air that is saturated with respect to water is • Saturation vapor pressure over ice is less than that supersaturated with respect to ice, promoting over water at the same temperature. deposition (saturation vapor pressure lower • Leads to a continuous transfer, in which supercooled droplets surrender water vapor which over ice) is subsequently deposited onto the ice crystals. • Occurs in either mixed clouds or glaciated clouds

Bergeron-Findesin -Wegner teorisi Bergeron Process

3 Water droplets and ice crystal are in equilibrium; water vapor molecules > liquid is saturation vapor pressure over water is Riming greater than it is over ice

• Analogous to droplet collision • Riming is the collision of ice crystals and supercooled water droplets. • Causes rapid grow of ice crystals and acceleration of their fall speeds.

Ice crystal growth

accretion ice crystals collide with supercooled water droplets

aggregation ice crystals collide with other ice crystals

4 Aggregation

• The joining together of two ice crystals to Ice nuclei form a single larger crystal.

• Deposition nuclei • Freezing nuclei • Contact nuclei

“hygroscopic”

Accretion Aggragation

• Accretion (Riming ) - Ice crystals collide • Aggragation - Ice crystals collide and stick with supercooled droplets and freeze on together. contact to form graupel . • This process forms snow flakes. • Graupel may fracture forming many ice particles. • This is the process involved in the growth of hail.

Droplet growth Droplet Growth

• Many CCN are hygroscopic (they absorb water • Cloud droplets are 100X smaller than vapour) and deliquescent (they dissolve into the typical rain drops. water they have absorbed) – Cloud droplet - 20 µm • eg salt from evaporation of sea spray – Rain drop - 2000 µm • This lowers the saturated vapour pressure over the surface and enables condensation to occur at low • Droplets will stay the same size if they are supersaturations (typically 0.5%) at their equilibrium vapor pressure. ****

5 Buharlaşmada eğrilik etkisi... Curvature Effect

• Curvature Effect - Droplets with a smaller radius have a higher equilibrium vapor pressure. • Air that is saturated w.r.t. a flat surface is unsaturated w.r.t. a curved surface. • For droplets to exist, the surrounding air must be supersaturated (RH >100%)

Eriyik etkisi, küçük damlacıklar... Solute Effect

• Solute effect - Equilibrium vapor pressure is less for a droplet with salt in solution. – Salt is hygroscopic and makes it harder for water to evaporate.

• Water droplets are competing among themselves for available water vapor. – Increasing RH results in larger droplets.

Terminal Velocity Growth of droplets

• Smaller droplets can collide to form larger • Growth of droplets by condensation is rapid for droplets. small droplets (< 10 µm) but slows down • Terminal velocity - Speed at which the • Smallest drizzle drops are ~ 200 µm diameter - cannot grow these by water condensation alone force of air resistance balances the force of gravity for a falling droplet. • Larger droplets must grow by collision and combination – Larger droplets have a higher terminal velocity. • Collision growth is slow for small droplets, • Typical rain drop falls 600X faster than a increases with diameter typical cloud droplet. ****

6 Damlanın zamanla büyümesi

condensation

collision

Collision and coalescence

• is operative in warm clouds and the lower portions of cool clouds

• larger droplets have a higher terminal The most important factor in the production velocity (drag =gravity) than smaller of raindrops is the cloud’s liquid water droplets, increasing the probability of content. collisions and coalescence

Collision & Coalescence – contd. How surface area depends on the size

• In clouds warmer than -15 °C(5 °F), collision between droplets play a significant role • Larger drops may form on larger condensation nuclei (salt particles or through random collision droplets; turbulent mixing between cloud and drier environment) • Amount of air resistance depends on the size of the drop and its rate of fall --- speed of falls increases until the air resistance = gravity – Terminal velocity – Larger drops means less evaporation also • Coalescence: Merging of droplets by collision • Forces that hold together tiny droplet together are so strong th at if the droplets collide with another droplet, they would not stick together

7 Collision & coalescence – contd. Cloud droplet rising & then falling through a warm cumulus cloud by growth and coalescence • Rising air currents slow the rate at which drops fall --- thick cloud with strong updrafts will maximize the time droplets spend in a cloud --- the bigger size droplets • When the fall velocity of the drop > updraft velocity, droplet slowly descends; when it reaches the bottom of the cloud, size ~ 5 mm --- typically occur in a rain shower originating in the warm, convective cumulus clouds • Factors in the production of raindrops – Cloud’s liquid water content (most important) – Range of droplet sizes, cloud thickness, updrafts of the cloud, electric charge of the droplets and the electric field in the cl oud

Formation of Precipitation Formation of Precipitation

How does a cloud cloud condensation nuclei (CCN) required droplet become a rain droplet (0.5 to several hygroscopic material that cloud droplets (~0.02 mm) mm)? form on Collision/coalescence - droplets supercooling occurs if no CCN are available collide with each other and enlargen , larger droplets less susceptible to evaporation due to the curvature effect (Figure 8.4)

• Warm cloud all portions > 0 oC

o Contributing factors • Cool cloud lower portions > 0 C upper portions < 0 oC

o • Range of droplet sizes • Cold cloud all portions < 0 C • Cloud thickness • Mixed cloud -40oC < T < –20oC • Updrafts in cloud • Glaciated cloud T < -40oC • Electric charge of raindrops and cloud droplets “supercooled ” water T < 0 oC

8 Cb’nin Anatomisi Collision-coalescence process

• Drops collide as larger drops overtake smaller drops as they fall, with.... • … further attraction (coalescence) from the attraction of particles of opposite electrical charge

Çarpışma - Collision Birleşme

• Collisions between drops – Dependent on absolute size of the collector drop and relative size of the droplets below. – Most efficient when droplets are slightly smaller than the collector drop.

Coalescence Coalescence

• When droplets stick together after colliding. • Coalescence - Growth of large droplets as • Coalescence efficiencies are usually they overtake smaller droplets and merge assumed to be near 100% with them. • The longer a droplet is in a cloud the more time it has to grow in size. • A large droplet (200 µm) will take 12 minutes to fall through a 500 m cloud.

9 Ice Crystal Growth Ice Crystal Growth

The ice crystals grow by deposition at the expense of • Methods of Ice Crystal Growth the liquid drops shrinking by evaporation. – Aggregation • Methods of Ice Crystal Growth • Formation of snowflakes – Diffusional growth • Ice crystals collide and stick together. • Water vapor molecules diffuse toward and deposit on the ice crystal. – Riming • A crystal falling through supercooled droplets will collect many of these drops. The drops will freeze on contact.

Ice crystal Process Distribution of ice and water in a cumulonimbus cloud

• Bergeron process of rain formation: A process that produces precipitation; involves tiny ice crystals in a supercooled cloud growing larger at the expense of the surrounding liquid droplets

• Ice crystals and liquid cloud droplets must coexist in clouds at below freezing

• Accretion or riming of ice crystals: Ice crystals grow larger by colliding with the supercooled liquid droplets; the droplets freeze into ice and stick to the ice crystal

Freezing Water or Ice

• Water droplets at temperatures below • If both ice crystals and water droplets are freezing are supercooled . present in a cloud, which will grow faster? • A cloud is glaciated (only ice particles) at temperatures below -40°C. • Saturation vapor pressure over ice is less than that over water. • If enough water molecules organize to form a nucleus of ice, the rest of the droplet will freeze (homogeneous freezing) • Ice crystals will grow at the expense of water droplets.

10 Collision & Coalescence: a) warm cloud composed only of small cloud droplets of uniform size; b) different size droplets The rôle of ice

• The freezing point of pure water is -40 oC • At temperatures between 0 and -40, there will usually be a mixture of water droplets and ice crystals ****

PRECIPITATION Formation of Precipitation is any form of water that falls from the

Ice crystal process (Bergeron) - atmosphere and reaches the earth’s surface ice crystals due to their lower saturation vapour pressure grow (rain, hail, sleet, freezing rain, etc.) at the expense of ambient supercooled water droplets with a higher saturation vapour Why is fog not a type of precipitation? pressure (Figure 8.7, 8.9) approximately one million cloud droplets are required to equal the volume of one raindrop

thus condensation is not the mechanism by which precipitation occurs

Precipitation Processes Damla büyüklük karşılaştırılması • Average diameter cloud droplets ~ 0.02 mm • Typical raindrop size ~ 2 mm • Growth of cloud droplets by condensation is slow to produce rain; clouds can develop and begin to rain in less than an hour • 1 million average size cloud droplets will make a average size raindrop – Other processes?? • Two important processes on how rain is produced – Collision-Coalescence Process – Ice -crystal (or Bergeron) process

11 Relative sizes of raindrops, cloud droplets, & relative sizes condensation nuclei

• condensation nuclei 0.2 microns • cloud droplets 20 microns • Raindrops 2000 microns

What are condensation nuclei?

Why Cloud Droplets Don’t Fall How Large is Large Enough?

• Gravity vs. Updraft – Terminal Velocity – The final speed obtained by an object falling through the atmosphere. A balance between gravity and the frictional forces (drag) on the object.

Precipitation Mechanisms Types of Precipitation

• For precipitation to form, millions of cloud • Snow droplets must somehow coalesce into drops large enough to sustain themselves during • Rain their descent. • Graupel • The two mechanisms that have been proposed to explain this phenomenon are: • Hail  the Bergeron process (ice crystal) , which • Sleet produces precipitation from cold clouds (or cold cloud tops) primarily in the middle latitudes, and • Freezing Rain  the warm cloud process most associated with the tropics called the collision -coalescence process .

12 Precipitation Types • Rain (Meteorology definition!): falling drop diameter ≥ 0.5 mm • Drizzle: Water drop diameter < 0.5 mm • Most drizzle falls from stratus clouds; also, rain passing through undersaturated zone and undergo evaporation leading to smaller -sized droplets – drizzle • Virga : Precipitation that falls from a cloud but evaporates before reaching the ground • Raindrops that reach the earth’s surface are seldom larger than ~6mm as collision between raindrops tend to break them apart into many smaller drops

Precipitation Types – Contd. Precipitation Types – contd. • Snow: Much of the precipitation reaching the ground • When snowflakes fall through very cold air with a low begins as snow moisture content, they do not readily stick together & • During summer, freezing level is usually high & powdery flakes of ‘dry’ snow accumulates on ground snowflakes falling from a cloud melt before reaching the • Flurries: Light snow showers that fall intermittently for surface short duration; often from developing cumulus clouds • During winter, freezing level is much lower, and falling • Snow Squall: A more intense snow showers snowflakes have a better chance of survival (comparable to summer rain showers); usually form • Snowflakes can fall ~300 m below the freezing level from cumuliform clouds before completely melting • Ground Blizzard: Drifting + Blowing snow after snow • Fallstreaks : Falling ice crystals that evaporate before fall ended reaching the ground • Blizzard: Weather with low temp & >30 knot winds • Ice crystals have been observed falling at temp ~-47°C bearing large amounts of fine, dry, powdery snow

Sleet & Freezing Rain • Sleet: Partially snowflake (or cold raindrop) passing Precipitation through warmer air undergoes partial melting; when it again goes through subfreezing surface layer of air, partially melted snowflake or cold raindrop turns back Types? into a tiny transparent ice pellet, called, sleet rain - liquid precipitation > 0.5 mm droplet size • Freezing Rain: Supercooled liquid drops upon striking a drizzle - liquid < 0.5 mm droplet size cold surface, form a thin veneer of ice – this form of snow - solid - flakes or aggregates precipitation is called freezing rain ice pellets (sleet) - originates as rain but freezes • Freezing drizzle: If the water droplets are small, then, it freezing rain or drizzle - supercooled water - freezes upon is called freezing drizzle impact to ground • Rime: White/Milky granular deposit of ice formed by the hail - small spheres or chunks of ice, 5 - 50 mm - successive rapid freezing of supercooled water drops when they accretion of water droplets around ice core come in contact with an object in below-freezing air

13 Sıcaklık ve yağış tipi...

Figure 8.22 (8.19 of 6th)

Sleet – partially snowflake (cold droplet) freezes into Yağış Tipleri a pellet of ice before reaching the ground

Accumulation of rime on tree branches Ice storm caused tree limbs to break & Power lines to sag

14 Precipitation Types Types of precipitation • The two most common and familiar forms of precipitation are: • Rain  drizzle, shower, cloudburst, virga  rain (drops of water that fall from a cloud and • Snow  flurries, squall, water equivalent have a diameter of at least 0.5 millimeter) and • Sleet  snow (precipitation in the form of ice crystals or, more often, aggregates of ice crystals). • Freezing rain • Hail

see Figure 8.22

Precipitation Types (cont.) Forms of precipitation

• Other forms include: • Rain – may begin as snow  sleet (falling small particles of ice that are clear to translucent), – Bergeron Process – summer freezing level at mid -  glaze (formed when supercooled raindrops turn to ice on latitudes is approximately 12,000 ft . colliding with solid objects),  hail (hard, rounded pellets or irregular lumps of ice • Freezing rain produced in large cumulonimbus clouds), and – Falls as rain from warm air mass that lies above  rime (a deposit of ice crystals formed by the freezing of cold air supercooled fog or cloud droplets on objects whose surface temperature is below freezing). – An inversion situation – climb to find warm air • drizzle (smaller droplets of rain, yet larger than mist) • Sleet – raindrops freeze as they fall • mist (smallest water droplets visible) • gaupel (watery hail)

More forms of precipitation

• Glaze – raindrops freeze upon ground impact – Occurs when surface temperature is near freezing – An " icestorm " • Hail – Irregular lumps of ice that form in layers in CB’s – Alternate layers of clear ice and rime ice (opaque ice) that are associated with severe updrafts • Virga – Streamers of precipitation trailing beneath clouds but evaporating before reaching the ground

15 Rain

• Rain - Consists of water droplets – D ≥ 0.5mm • Drizzle - Consists of uniform sized droplets – D < 0.5 mm • Virga - Streaks of precipitation that evaporate before they reach the ground.

Raindrops < 2mm nearly spherical; Drizzle or Rain >2mm, elliptical

• Why does a warm stratus cloud produce smaller rain droplets than a warm cumulus cloud? • Stratus clouds are less than 500 m thick and droplets may only grow to 200 µm (Drizzle) • Cumulus clouds have updrafts which keep the droplets in the cloud longer allowing them to grow to 5000 µm (Large Raindrop)

US Precipitation Raindrop Shape

• Not tear drop shaped!

16 Snow

• Snow Shower : A form of precipitation Snow from a convective cloud that is short in duration, starts and stops suddenly, and • Results from the growth of ice crystals often has a rapid change in intensity. through deposition, riming and aggregation. • Flurries : A very light and usually brief • A snowflake’s structure depends on the show shower. temperature and moisture conditions that exist when the crystal is formed. • Snow Squall : A sudden wind associated with snowfall that occurs – Warmer conditions provide for riming and a wet snow pack suddenly and is usually intense and short lived. – Colder conditions provide for less adhesion (riming) and a “powder”

Snow Snow

Ice crystals may group together to form larger ice • Precipitation composed of white or crystals. This process is called aggregation and translucent ice crystals, chiefly in may lead to the snowflakes we see on the ground. complex branched hexagonal form and often agglormerated into snowflakes. As the ice crystals fall, they may fall into warmer air. If so, then the ice crystals will melt into rain • Dendrites -- Fernlike, branched drops. • Columns -- Taller than thin If the crystals never melt, they • Plates -- Flat will fall as snow. • Other types

Snow Kar kristalleri • Snow shapes, or habits, are temperature dependent.

Temperature (C) Crystal Habit

0 to -4 Thin Plates -4 to -10 Columns -10 to -12 Plates -12 to -16 Dendrites -16 to -22 Plates

Much of the snow we see are aggregates of these crystal habits.

17 Kar örtüsü Snow

• The warmer the air, but still below freezing, often the heavier the snowfall. – Warmer temperatures have a higher saturation vapor pressure so there are more vapor molecules that can allow crystals to grow. – Greater chance of large aggregates of crystals forming in warmer temperatures.

Showers Snow

• Shower - Rain that occurs briefly and • Most precipitation begins as snow sporadically due to downdrafts in a • In summer the freezing level is above 3600 convective cloud. m • Cloudburst - A heavy shower. • Snowflaces will melt by the time they fall 300 m below the freezing level. • Continuous rain usually comes from layered clouds that have small vertical air currents. • Snow scatters light better than rain; therefore, the bottom of rain clouds are darker than snow clouds.

Snow (cont.) Snow Fall

• Snowflakes falling through humid air • Flurries - Light shower of snow. slightly above freezing will stick together • Snow Squall - Intense shower of snow. forming very large snowflakes. – A heavy snow will reduce visibility to 1/2 km. • Dendrite (snowflake) - the most common • Blizzard - Low temperatures and >30 knot form of ice crystal and forms when the winds which bear large amounts of small difference between water and ice saturation snow particles. vapor pressure is the largest. – Ice crystal type depends on humidity and temperature conditions during formation. • Why does a layer of snow make it quieter?

18 US Snowfall Other Frozen Precipitation

• Snow grains - Opaque grains of ice – D < 1 mm • Snow pellets - White opaque grains of ice which bounce and possibly break apart upon impact. – D < 5 mm • These are the result of riming and have many small air bubbles.

Snow grains, pellets and hail Hail – contd.

• Snow grains: Small, opaque grains of ice (equivalent of • Graupel : Ice particles between 2-5 mm in diameter that drizzle); fall from stratus clouds form in a cloud often by the process of accretion • Snow Pellet: White, opaque grains of ice of the size of • For a hail to grow to the size of golf ball, it must remain rain drop for 5-10 minutes in the cloud • Hail: Pieces of ice either transparent or partially opaque, • Ice crystals of appreciable size that can’t be supported by ranging in size from that of small peas to that of golf balls or larger; biggest size in US 757 g & 14 cm diam .; rising air, begin to fall – Hail • Single hailstorm can damage in minutes; annual loss • Largest form of precipitation occurs during the warmest hundreds of millions of $ in US; time of the year (due to strong updraft that keeps the • Hail is produced in a cumulonimbus cloud when large crystal to become bigger) frozen raindrops that grow by accumulating supercooled • Preventing hailstorm --- cloud seeding --- excessive liquid droplets nuclei prevents from growing

Accumulation of small hail after a Graupel thunderstorm

• When riming coats an ice crystal and its 6 - sided sharp edges are lost into a milky - white spongy texture. • Graupel can fall to the ground or remain in the cloud providing a nucleus for hail.

19 Coffeyville Hailstone (Sept. 3, 1970), Kansas: When updrafts are tilted, ice particles are swept Layered structure indicates travel through a cloud of horizontally through the cloud, producing the optimal varying water content and temp. trajectory for hailstone growth

Hail Hail

• Hail - Pieces of ice ranging from the size of • Precipitation in the form of balls or small peas and larger. irregular lumps of ice, always produced • Grow by accretion in cumulonimbus clouds. by convective clouds, nearly always • Growth bands on a hailstone reflects the cumulonimbus clouds. different conditions within the cloud • Growth by accretion. through which the hail passes. • Embryo is the origin of the hailstone. • Clear sections indicate where liquid water on surface froze slowly. • Various paths in a thunderstorm.

Hail Dolu Oluşumu

• Ice pellets forms in roughly concentric layers. • Formed by a repetitive sequence, where ice falls into a region with liquid droplets and provides a liquid coating, upon lifting the ice crystal higher the coating freezes and the process restarts. (Figure 6 -13 in text book)

20 Hail Sleet

• Raindrops freeze in the air while falling to the surface. • Must have a layer in the lowest portion of the atmosphere with a temperature below 0°C. • Requires an inversion aloft.

Sleet -- Ice Pellets Sleet Cold: Warm: T < 0
C T > 0
C • Sleet - Tiny ice pellets that are translucent – D ≤ 5mm – Sleet bounces when it hits the ground.

• A melted snowflake or raindrop freezes when it Rain freezes hits a cold surface layer of air and forms sleet. before reaching Deep • If the cold surface layer is shallow, the raindrops the surface. freezing will not freeze before hitting the ground giving layer freezing rain.

Sleet Freezing Rain

• Light rain or drizzle of supercooled water droplets fall through air at or slightly below 0°C. It freezes when it hits the ground.

21 Freezing Rain Freezing Rain

• Precipitation that falls to the ground as a liquid then freezes upon impact with the Cold: Warm: ground or exposed objects. Forms a T < 0
C T > 0
C Rain freezes glaze of ice on the objects and ground. upon contact with below freezing Shallow surface. freezing layer

Clouds

• Clouds , visible aggregates of minute droplets of water or tiny crystals of ice, are one form of condensation. • Clouds are classified on the basis of two criteria: form and height . • The three basic cloud forms are:  cirrus (high, white, and thin),  cumulus (globular, individual cloud masses), and  stratus (sheets or layers).

Cloud Types

22 Stratocumulus - low level layer cloud - mainly water gentle updraft - slow droplet growth - drizzle Cumulonimbus Strong vertical motion rapid droplet and crystal growth by condensation then collision heavy precipitation

Cirrus Ice crystals beneath cirrus clouds

very high altitude all ice crystals

Virga : Streaks of Falling precipitation evaporates before reaching the ground Fog

• Fog , generally considered an atmospheric hazard, is a cloud with its base at or very near the ground. • Fogs formed by cooling include:  radiation fog (from radiation cooling of the ground and adjacent air),  advection fog (when warm and moist air is blown over a cold surface), and  upslope fog (created when relatively humid air moves up a slope and cools adiabatically ).

23 Fog (cont.)

• Those formed by evaporation are:  steam fog (when rising water vapor over warm water condenses in cool air) and  frontal fog (when warm air is lifted over colder air along a front).

Cloud Seeding & Precipitation Dew and White Frost • Cloud Seeding: Inject a cloud with small particles that will act as nuclei, so that cloud particles will grow large enough to fall to the surface as precipitation • Dew is the condensation of water vapor • Silver iodide is used: has a crystalline structure similar on objects that have radiated sufficient to ice crystal, as it acts as an effective ice nucleus at heat to lower their temperature below temp. of -4°C (25 °F) and lower the dew point of the surrounding air. • Important factors in cloud-seeding experiment: Type of cloud, its temperature, moisture content, droplet size • White frost (hoar frost ) forms when distribution, and updraft velocities in the cloud the dew point of the air is below • Cloud seeding in certain instances may lead to more freezing. precipitation; in others, to less precipitation, and in still others, to no change in precipitation amounts; • Can avoid hail storms --- very important use

Cloud Seeding

• What is the purpose of cloud seeding? Cloud seeding • Actual effect – Use silver iodide to provide CCN’s – If too many CCN’s , you get many small • CO2 (dry ice) droplets, which remain in the cloud. • Use this concept to suppress the formation • AgI (silver iodide) of hail.

24 Intentional Weather Modification Weather Modification Techniques • Weather modification is deliberate human • The focus of intentional weather intervention to influence atmospheric processes modification using modern weather that constitute the weather. technology is on: • Weather modification falls into three categories:  cloud seeding ,  (1) the use of energy to forcefully alter the weather,  fog and cloud dispersal ,  (2) modifying land and water surfaces to change their natural interaction with the lower atmosphere, and  hail suppression , and  (3) triggering, intensifying, or redirecting atmospheric  frost prevention processes.

Natural seeding by cirrus clouds may lead to Dağ dalgaları, bulut tohumlama, precipitation downwind kar yağışı, lens bulutları...

Measurement of Precipitation

• Rain Gauge: Instrument to collect & measure rainfall Rain Measurement • Tipper Bucket rain gauge: Receiving funnel leading to two small metal collectors; bucket below the funnel collects • Rain, the most common form of precipitation, the rain water; each time a bucket tips (with 1/100”), an is probably the easiest to measure. electric contact is made – recorded; each ‘tip; it loses some • The most common instruments used to rainfall – limitation; Automated weather stations use this measure rain are: technique  the standard rain gauge , which is read directly, • Weighing -type rain gauge: Precipitation is caught in a and cylinder & accumulates in a bucket; special gears translate  the tipping bucket gauge and weighing gauge , both of which record the amount of rain. weight of rain (or snow) into mm or inch of precipitation; info can be transmitted to satellites or land -based stations

25 Measuring Precipitation Measurement

• Rain Gauge • Either the volume or weight of the collected – Standard precipitation is measured. – Weighing Gauge – < 1/100 in. of rain is called a trace . – Tipping Bucket – Radar • For fresh snow the equivalent amount of – Optical Rain Gauge rain is 1/10 as much. – (10 in. of snow = 1 in. of rain)

Measuring Rainfall Rain Gauges

• Raingauge – A instrument which collects • Standard Rain Gauge - A funnel that the rain and enables its measurement collects rain water into a long measuring – Tipping Rain Gauges tube. – Weighing -Bucket Gauges • Tipping Bucket Rain Gauge - A bucket beneath the funnel tips and empties for each 1/100 in. or rain. Each tip is recorded. • Weighing -type Rain Gauge - Rain is collected and weighed on a scales.

A standard rain gauge

26 Standard rain gauge – surface area = 10 x Snow Measurement area of the cylinder

• The two most common measurements of snow are depth and water equivalent . • Although the quantity of water in a given volume of snow is not constant, a general ratio of 10 units of snow to 1 unit of water is often used when exact information is not available.

Rain/snow conversion & Doppler Radar Tipping bucket rain gauge: 1/100” bucket tips • 10 cm of snow ~ 1 inch of water • Fresh snowpack : water equivalent 10:1 • Useful about spring runoff and potential for flooding • Radar (RAdio Detection And Ranging): Gathers info about storms and precipitation in previously inaccessible regions • A transmitter sending short, microwaver signals --- Fraction of the energy is scattered back by the ‘target’ to the Transmitter & detected by a Receiver – Returning signal provides info about target’s distance & intensity of the rainfall

Snow Measurement Measurement Errors

• Measure depth • Rain gauges only represent a point • Water equivalency – depth of water which measurement. would result if all of the snow was melted. • Errors: • Conversion factors – 10:1 – Wind generated turbulence can deflect precipitation from entering the gauge • Snow pillows – big mattresses filled with antifreeze liquid and connected to a – Splashing pressure regulator. – Snow drifts

27 Radar Estimates Radar

• Radar - RAdio Detection And Ranging • The returned signal from a target is called an echo. • Radar can show distance to precipitation and intensity of precipitation.

Measuring Precipitation by Radar

• Intensity of backscattered radiation to the Doppler radar radar indicates the size of the particles (droplets, snow flakes, graupel or • Radio detection and ranging (microwaves) hailstones)

• Doppler shift used to detect direction of movement

Doppler radar display of precipitation Doppler Radar intensity –Oklahoma, April 24, 1999 • Doppler Radar: Provide information on: distance, amount of rainfall and whether the rain/cloud is stationary or moving

• Concept of Doppler Shift

• Doppler Radar allows scientists to peer into a tornado - generating thunderstorms and observe its wind

28 Doppler radar display of 1-hr rainfall Key Terminology amounts - Oklahoma, April 24, 1999

Condensation Cirrus Cumulus Weather modification Radiation fog Advection fog Upslope fog Steam fog Frontal fog Bergeron Process White frost Dew Collision -coalescence process Rain gauge Stratus

Pertinent Web Sites (cont.) Pertinent Web Sites Moisture, Atmospheric (Definitions) Cloud Classification This site has a discussion of atmospheric moisture from the Univ ersity of Wisconsin. Here are some photographs of various types of clouds from Lakeland Community College, Illinois.

Clouds and Precipitation: On -line Meteorology Guide National Drought Mitigation Center This site contains an educational module that introduces a number of cloud classifications, different types of precipitation, and the mechanisms responsible for producing them. The National Drought Mitigation Center helps people and institut ions develop and implement measures to reduce societal vulnerability to drought. Cloud Cover, Current United States This site has current United States cloud cover statistics. National Snow and Ice Data Center (NSIDC) Clouds (NASA) Here is a good explanation of the dynamics of clouds. The National Snow and Ice Data Center (NSIDC) is a data and info rmation resource for those studying snow and ice, and their importance to the Earth system. Clouds On-line Tutorial This site offers an excellent on-line tutorial featuring cloud classification and cloud descripti ons with photographs. Precipitation Condensation This site provides a discussion of condensation: dew, fog, and c louds from the University of Wisconsin. This site offers a discussion of precipitation from the Universi ty of Wisconsin.

Cooperative Institute for Meteorological Satellite Studies The Cooperative Institute for Meteorological Satellite Studies ( CIMSS) is a cooperative institute formed through a Precipitation Map, Current United States Memorandum of Understanding between the University of Wisconsin-Madison (UW -Madison), the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics a nd Space Administration (NASA). Current radar map of the United States show areas of precipitati on.

Current Weather Data (University of Illinois) An excellent source for current weather data, including printable maps. Stability and Cloud Formation, Atmospheric

Dew points, Current United States Here is a discussion of atmospheric stability and cloud formatio n from the University of Wisconsin. This site contains current United States dew point temperature s tatistics.

Extreme Science Weather Data and Imagery, Current (American Meteorological Socie ty) Extreme Science is a great place for all kinds of science information. The site includes sections on time, weather, space, Earth science, biology, and maps. An excellent source for current surface, satellite, upper air, a nd forecast information from the American Meteorological Society (AMS).

Summary

• Adiabatic process; dry adiabatic & moist adiabatic rate • Environmental lapse rate • Conditions for Stable and unable atmosphere • What cloud type is formed in stable air • Condensation nuclei, cloud seeding • Rain shadow, orographic uplift • Coalescence, accretion • Rain, drizzle, virga, shower, fallstreaks, flurries, snow squall, sleet, freezing rain • Blizzard, hailstone, standard rain gauge • Doppler radar • Water equivalent

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