CLOUD PHYSICS COVERAGE General aspects of cloud precipitation

CLOUD PHYSICS

COVERAGE General aspects of cloud & precipitation formation Condensation Nuclei Growth of water droplets Microphysical properties of clouds Condensation plus Coalescence Ice phase Nucleation, Ice Nuclei, Diffusional growth on ice. nuclei, further growth by accretion & aggregation Types of precipitation processes Weather Modification 10 -Sep-20 Cloud Physics 2

CLOUD PHYSICS Cloud physics is the study of physical processes that lead to the formation, growth and precipitation of clouds Clouds are composed of microscopic droplets of water, tiny crystals of ice, or both Under suitable conditions, the droplets combine to form precipitation, where they may fall to the earth 10 -Sep-20 Cloud Physics 3

CLOUD PHYSICS Precise mechanics of how a cloud forms and grows is not completely understood, but scientists have developed theories explaining the structure of clouds by studying the microphysics of individual droplets Advances in weather radar and satellite technology have also allowed the precise study of clouds on a large scale 10 -Sep-20 Cloud Physics 4

CLOUD Defined as a visible aggregate of minute particles of water or ice or both, in the free air Clouds form in the sky, develop, take different shapes and dissolve Each process is an indication of some physical state or process in the atmosphere Size of water droplets is about 0. 02 mm in diameter 10 -Sep-20 Cloud Physics 5

COMPOSITION OF CLOUDS Clouds below freezing level are fully composed of water droplets Clouds just above freezing level are composed of super-cooled water droplets Clouds at further higher level are composed of super-cooled water drops and ice crystals Clouds above - 40ºC are composed wholly of ice crystals 10 -Sep-20 Cloud Physics 6

CLOUD FORMATION Free Convection Topography Low-level Convergence Fronts 10 -Sep-20 Cloud Physics 7

FORMATION OF CLOUD DROPLETS • Phase changes of water are basic to cloud microphysics • Possible changes are as follows: Vapour liquid (condensation, evaporation) Liquid solid (freezing, melting) Vapour solid (deposition, sublimation) 10 -Sep-20 Cloud Physics 8

FORMATION OF CLOUD DROPLETS • Changes in the direction from left to right in this pattern correspond to increasing molecular order • A droplet to form by condensation from the vapour, surface tension must be overcome by a strong gradient of vapour pressure 10 -Sep-20 Cloud Physics 9

FORMATION OF CLOUD DROPLETS • When the air reaches saturation, condensation of water vapor into tiny cloud droplets may begin • In order for water vapor to condense, particles in the atmosphere called cloud condensation nuclei (CCN) are required • Without CCN, a relative humidity of several hundred percent would be needed to keep a tiny cloud droplet from evaporating away 10 -Sep-20 Cloud Physics 10

CLOUD CONDENSATION NUCLEI • Cloud condensation nuclei or CCNs (also known as cloud seeds) are small particles (typically 0. 2 µm) • Common types : § Dust or Clay, § Soot or black carbon from grassland or forest fires § Sea salt from ocean wave spray § Soot from factory smokestacks or internal combustion engines § Sulfate from volcanic activity § Organic particles from land surfaces § Gas-to-particle conversion particulates 10 -Sep-20 Cloud Physics 11

CCN • Nuclei activated at super saturations less than a few per cent (S < 1. 02) are called CCN • Sea Salt is the principal CCN and Sulfurous and Nitrous acids take the second place • Size 0. 2 -1 µm 10 -Sep-20 Cloud Physics 12

FEATURES OF CCN’s • Typical diameter of : Raindrop ~ 2 mm Cloud droplet ~ 0. 02 mm Cloud condensation nucleus ~ 0. 0002 mm • Number of cloud condensation nuclei in the air can be measured and ranges between around 100 to 1000 per cubic cm • Total mass of CCNs injected into atmosphere has been estimated at 2 x 1012 kg over a year's time • Large concentrations of particulates are also responsible for haze in areas with lower humidity 10 -Sep-20 Cloud Physics 13

CLOUD DROPLETS TO RAINDROPS 106 bigger Raindrop is 106 bigger than a cloud droplet 106 bigger 10 -Sep-20 Cloud Physics 14

FEATURES OF CCN’s • Ability of these different types of particles to form cloud droplets varies according to their size and also their exact composition, as the hygroscopic properties of these different constituents are very different • Sulfate and sea salt, readily absorb water whereas soot, organic carbon and mineral particles do not 10 -Sep-20 Cloud Physics 15

FEATURES OF CCN’s • This is made even more complicated by the fact that many of the chemical species may be mixed within the particles (in particular the sulfate and organic carbon) • Additionally, while some particles (such as soot and minerals) do not make very good CCN, they do act as very good ice nuclei in colder parts of the atmosphere 10 -Sep-20 Cloud Physics 16

TYPES OF CLOUDS Clouds formed in nature can be grouped into different categories depending on the process by which they develop Two types of processes by which clouds form: - 10 -Sep-20 Warm Clouds Cold Clouds Cloud Physics 17

WARM CLOUDS Temperature -10ºC Temperature 0ºC 10 -Sep-20 Warm Cloud Physics 18

WARM CLOUD CHARACTERISTICS ØCloud tops below freezing level ØComposition (Microphysics) - Consists of water droplets ØParticle radius - ~ 0. 01 mm ØMain process – Collision Coalescence ØFew large drops form => then the cloud grows 10 -Sep-20 Cloud Physics 19

COLD CLOUDS Temperature -10ºC Temperature 0ºC 10 -Sep-20 Cold Cloud Physics 20

COLD CLOUD CHARACTERISTICS Û Cloud tops above freezing level Û Composition (Microphysics) - consists of Ice, crystals and super cooled water droplets Û Particle radius - ~ 0. 25 mm Û Main process – Bergeron process Û Ice crystals grow at expense of cloud droplets 10 -Sep-20 Cloud Physics 21

COLD CLOUD CHARACTERISTICS Cold Cloud - Cloud extending above the 0°C isotherm Super Cooled droplets - Water droplets at temp < 0°C Mixed Cloud - Cloud consisting of both ice particles & super cooled droplets Glaciated Cloud - Cloud consisting of ice only 10 -Sep-20 Cloud Physics 22

COLD CLOUD CHARACTERISTICS In mixed cloud, equilibrium vapour pressure over ice is less than that over water at same temperature Therefore, ice crystal grows by deposition of vapour provided by the evaporation of water droplets Ice crystals can also grow by colliding with super cooled droplets which then freeze onto them - accretion or riming Heavily rimed ice crystal is known as Graupel 10 -Sep-20 Cloud Physics 23

COLD CLOUD CHARACTERISTICS Hailstones represents an extreme case of growth by riming Frozen water particles that can be "re-cycled" through vigorous convection during which they collect super cooled water droplets and freeze them When ice crystals collide with one another, they can "stick" together to form larger crystals - Aggregation 10 -Sep-20 Cloud Physics 24

COLD CLOUD CHARACTERISTICS Precipitation from cold clouds form first by the process of deposition and then through the riming and aggregation processes so that the ice crystals can grow to sizes heavy enough to fall through the upward current (updraft) 10 -Sep-20 Cloud Physics 25

CLOUD FORMATION • Cloud formation: As air parcels rise, cool and condense • In warm clouds, droplets can grow by : – condensation in a supersaturated environment and – colliding and coalescing with other cloud droplets • Cloud droplets can also form with aid of cloud condensation nuclei in an unsaturated environment (RH > 90%) • If a cloud extends above 0°C line, it is called cold cloud 10 -Sep-20 Cloud Physics 26

CLOUD FORMATION Water droplets can exist in clouds as super-cooled droplets even below 0°C Cloud temperatures frequently need to get below -10°C for any significant number of ice particles to form Cloud containing both ice particles and super-cooled droplets: Mixed cloud Cold cloud consisting entirely of ice, is said to be glaciated Since cloud droplets are small, they are hardest to freeze Condensation nuclei help the cloud droplets to freeze 10 -Sep-20 Cloud Physics 27

CONCEPTS OF CLOUD FORMATION Condensation acts too slow to produce rain Several days required for condensation Clouds produce rain in less than 1 hour Warm clouds (no ice) Collision-Coalescence Process Cold clouds (with ice) Ice Crystal Process Accretion-Splintering-Aggregation 10 -Sep-20 Cloud Physics 28

GROWTH OF CLOUD DROPLETS • Ordinary cloud droplets are extremely small – Average size is 20 microns (0. 002 cm) – If the cloud droplet is in equilibrium with its surroundings, the size of the droplet does not change because the water molecules condensing onto one droplet will be exactly balanced by those evaporating –If it is not in equilibrium, the droplet size will either increase or decrease depending on whether condensation or evaporation predominates 10 -Sep-20 Cloud Physics 29

MECHANISM OF GROWTH OF CLOUD DROPLET • Condensation • Collision & Coalescence • Bergeron Process 10 -Sep-20 Cloud Physics 30

PROCESS OF PRECIPITATION • Clouds do not necessarily mean precipitation • Many clouds never produce precipitation • So why do some clouds produce precipitation while others do not? 10 -Sep-20 Cloud Physics 31

CONDENSATION • At a given temperature air can hold a certain amount of water vapour and no more • When the air holds the maximum amount of water vapour which it can hold at that temperature, the air is said to be saturated 10 -Sep-20 Cloud Physics 32

CONDENSATION • Temperature at which saturation occurs is called the dew point • When air is cooled below the dew point temperature, condensation takes place • Conversion of water vapour into liquid water is called condensation 10 -Sep-20 Cloud Physics 33

CONDENSATION • Curvature Effect • Solute Effect 10 -Sep-20 Cloud Physics 34

CURVATURE EFFECT • Figure shows a cloud droplet and a flat water surface both in equilibrium • More vapor molecules surround the small droplet, therefore it has a greater equilibrium vapor pressure • This occurs because water molecules are less strongly attached to a curved water surface, so they evaporate more readily 10 -Sep-20 Cloud Physics 35

CURVATURE EFFECT Smaller the droplet, greater its curvature, higher the super saturation required for equilibrium 10 -Sep-20 Cloud Physics 36

CURVATURE EFFECT • Saturation water vapor pressure depends on the curvature of the water surface • Larger the curvature the easier it is for the water molecules to leave the surface of the liquid water • Saturation vapor pressure for small droplets is higher therefore they require more vapor to keep their size 10 -Sep-20 Cloud Physics 37

CURVATURE EFFECT • Small droplets don’t make it as raindrops • When air is saturated with respect to a flat surface, it is unsaturated with respect to a curved droplet of water, and the drop evaporates • Bottom line: the smaller the droplet, the more difficult it is to grow 10 -Sep-20 Cloud Physics 38

PROCESS OF PRECIPITATION • To prevent evaporation, the air surrounding the cloud droplets must be supersaturated (RH > 100%) • For droplets near 20 microns, the RH must be near 100. 1% to keep it from evaporating • The figure shows that a droplet near 1 microns will grow larger as the RH approaches 101% • However RH, even in clouds, is rarely greater than 101% • Question: How do small cloud droplets of less than 1 microns grow to the size of an average cloud droplet? 10 -Sep-20 Cloud Physics 39

SOLUTE EFFECT • For the cloud droplets to grow, it is necessary for the water droplets to form around condensation nuclei rather than a pure water droplet • Most cloud condensation nuclei (CCN) are hygroscopic and therefore have a connection to water vapor • These CCN reduce the saturation vapor pressure needed for condensation to occur • Therefore, a droplet containing salt can be in equilibrium with its environment when the atmospheric RH is much lower than 100% • However, RH closer to 100% mean water vapor molecules attach to the droplet at a faster rate 10 -Sep-20 Cloud Physics 40

SOLUTE EFFECT • When CCN is dissolved in the water droplet, a solution is formed • We know that the equilibrium vapour pressure reduces when salt is dissolved in liquid water The reduction is expressed by : - e = 1 - CM es If water vapour condenses on the solution, M will decrease and e will approach es 10 -Sep-20 Cloud Physics 41

SOLUTE EFFECT • Therefore, if an average sea salt CCN is dissolved in a spherical drop of water of size > 1µ, the solution will be diluted and the effect is insignificant • Therefore Solute effect is dominant in smaller droplets 10 -Sep-20 Cloud Physics 42

SOLUTE EFFECT • The solute effect and the curvature effect both work together to dictate how a droplet will grow • At small sizes, the solute effect dominates • At large sizes, the solution becomes more diluted and the curvature effect dominates 10 -Sep-20 Cloud Physics 43

COMBINED EFFECT: SOLUTE & CURVATURE

PROCESS OF PRECIPITATION • When droplets are small they grow slowly because the curvature effect dominates • As the droplets grow, they eventually reach the point where the solute effect dominates and then the droplet grows rapidly • Over land masses there a lot of CCN and each of new droplets compete for existing water vapor • Over the ocean there are fewer CCN and there is a lot of vapor for each droplet • In either case we now have large number of droplets that 45 Physics are cloud sized and we have a. Cloud cloud 10 -Sep-20

PRECIPITATION PROCESSES CONDENSATION Condensation process by itself is entirely too slow to produce rain If condensation was sole process, it would take several days to grow raindrop sized droplets It takes a nucleus about 1 sec to grow to 10µ 2 min to grow to 100µ 3 hrs to grow to 1 mm a day to grow to 3 mm 10 -Sep-20 Cloud Physics 46

PRECIPITATION PROCESSES CONDENSATION Observations show that clouds can develop and begin to produce rain in less than an hour. There must be some other process or processes that accounts for the growth of cloud droplets into raindrops Based on radar, photographs, and aircraft observations there are two methods that can account for the growth of raindrops from cloud droplets 10 -Sep-20 Cloud Physics 47

PRECIPITATION PROCESSES: COLLISION & COALESCENCE • Noted in southern US and in tropics that cloud tops have temperatures rarely below 0 C • Process starts when just one large cloud droplet forms • This large droplet reaches its terminal velocity • Velocity where gravity and air friction balance each other out and the droplet no longer accelerates • As this one large droplet falls, it collides with other droplets 10 -Sep-20 Cloud Physics 48

COALESCENCE Ü Coalescence is the process by which two or more droplets or particles merge during contact to form a single droplet (or bubble)Its role is crucial in the formation of rain Ü As droplets are carried by updrafts and downdrafts in a cloud, they collide and coalesce to form larger droplets. Ü When the droplets become too large to be sustained on the air currents, they begin to fall as rain Ü Adding to this process, the cloud may be seeded with ice from higher altitudes either by the cloud tops reaching − 40 C or cloud being seeded by ice from cirrus clouds 10 -Sep-20 Cloud Physics 49

COLLISION-COALESCENCE Area swept is smaller than area of drop small raindrop • Big water drops fall faster than small drops • As big drops fall, they collide with smaller drops • Some of the smaller drops stick to the big drops • Drops can grow by this process in warm clouds with no ice • Occurs in warm tropical clouds Collection Efficiency 10 -50% 10 -Sep-20 Cloud Physics 50

COLLISION-COALESCENCE • Theory explaining how the behavior of individual droplets leads to the formation of clouds is “collision-coalescence” process • Droplets suspended in the air will interact with each other, either by colliding and bouncing off each other or by coalescing -- combining -- to form a larger droplet • Eventually, the droplets become large enough that they fall to the earth as precipitation • Collision-coalescence process does not make up a significant part of cloud formation for the same reason that water droplets have a relatively high surface tension, which prevents them from coalescing on a large scale before they eventually fall to the earth 10 -Sep-20 Cloud Physics 51

COLLISION-COALESCENCE • As droplets fall they collide with smaller droplets and coalesce • After collecting ~1 million cloud droplets the particle is large enough to fall without evaporating • Because there a large number of collisions needed, clouds with great vertical extent are typically produce precipitation by this process 10 -Sep-20 Cloud Physics 52

PRECIPITATION FORMATION -Cloud droplets are tiny (20 micrometers) -Many condensation nuclei are present -Tiny particles fall more slowly than large ones -Cloud droplet’s diameter must grow 200 times to reach a raindrop’s diameter -To attain volume of a rain droplet, cloud droplet increases a million times in volume 10 -Sep-20 Cloud Physics 53

WARM CLOUD PRECIPITATION Ø As cloud droplet ascends, it grows larger by Collision. Coalescence Updraft (6. 5 m/s) Ø Cloud droplet reaches height where updraft speed equals terminal fall speed Ø As drop falls, it grows by Collision- Coalescence to size of a large raindrop 10 -Sep-20 Cloud Physics 54

BERGERON PROCESS • Primary mechanism for the formation of ice clouds was discovered by T Bergeron. • Bergeron process notes that the saturation vapor pressure of water, or how much water vapor a given volume can hold, depends on what the vapor is interacting with. • Specifically, the saturation vapor pressure of air with respect to ice is lower than the saturation vapor pressure with respect to water. • Air interacting with a water droplet may be saturated (at 100% RH) when interacting with a water droplet, but the same air would be supersaturated when interacting with an ice particle. 10 -Sep-20 Cloud Physics 55

BERGERON PROCESS • Air will attempt to return to equilibrium, so the extra water vapor will condense into ice on the surface of the particle. • These ice particles end up as the nuclei of larger ice crystals. • This process only happens at temperatures around -40 °C. • Surface tension of the water allows the droplet to stay liquid well below its normal freezing point. • When this happens, it is now supercooled liquid water. • Bergeron process relies on supercooled liquid water interacting with ice nuclei to form larger particles. • A process whereby scientists seed a cloud with artificial ice nuclei to encourage precipitation is known as cloud seeding. 10 -Sep-20 Cloud Physics 56

ICE CRYSTAL PROCESS As SVP for a water droplet is higher than for ice crystal, vapor next to droplet will diffuse towards ice. Ice crystals grow at the expense of water drops, which freeze on contact. Ice crystals grow, they begin to fall. Effect maximized around -15 o. C 10 -Sep-20 Cloud Physics 57

ACCRETION-AGGREGATION PROCESS Small ice particles will adhere to ice crystals Supercooled water droplets will freeze on contact with ice snowflake ice crystal Accretion (Riming) Splintering Aggregation Known as Bergeron Process after meteorologist who first recognized importance of ice in precipitation process 10 -Sep-20 Cloud Physics 58

SVP OVER LIQUID & ICE SVP over ice is less than over water because sublimation takes more energy than evaporation. If water surface is not flat, but instead curves like a cloud drop, then the SVP difference is even larger. So at equilibrium, more vapor resides over cloud droplets than ice crystals 10 -Sep-20 Cloud Physics 59

SVP NEAR DROPLETS & ICE SVP is higher over supercooled` water drops than ice 10 -Sep-20 Cloud Physics 60

IDEALISED MODEL OF PRECIPITATING CLOUD 10 -Sep-20 Cloud Physics 61

10 -Sep-20 Cloud Physics 62

MIXED WATER-ICE CLOUDS Glaciated region Clouds that rise above freezing level contain mixture of water-ice. Mixed region exists where Temps > -40 o. C. Only ice crystals exist where Temps < -40 o. C. 10 -Sep-20 Cloud Physics 63

ICE NUCLEUS • Ice particles can have a significant effect on cloud dynamics. • They are known to be important in the processes by which clouds can become electrified, which causes lightning. • They are also known to be able to form the seeds for rain droplets. • Many different types of particulates in the atmosphere can act as ice nuclei, both natural and anthropogenic, including those composed of minerals, soot, organic matter and sulfate. • However, the exact nucleation potential of each type varies greatly, depending on the exact atmospheric conditions. 10 -Sep-20 Cloud Physics 64

ICE NUCLEUS • Ice nucleus is a particle which acts as the nucleus for the formation of an ice crystal in the atmosphere. • Presence of ice nuclei increase temperature that ice will form in atmosphere from around − 42°C to − 10°C. • Simpleset process that can take place in the atmosphere to form ice particles is by water vapor depositing directly onto solid particle. • Presence of an ice nucleus can also cause a previously supercooled water droplet to freeze through contact, immersion or dissolution within water that would otherwise have stayed in the liquid phase at a given temperature. 10 -Sep-20 Cloud Physics 65

NUCLEATION Process by which the phase change of a substance to a more condensed state (condensation, deposition, fusion) is initiated within the less condensed state. 10 -Sep-20 Cloud Physics 66

NUCLEATION TYPE PROCESS Homogeneous or Spontaneous Nucleation Formation of: Pure water droplet by condensation of supersaturated water vapour, or Pure ice by freezing of water droplets or deposition of water vapour without the existence of aerosol Heterogeneous Nucleation Formation of: Water droplets through condensation of water vapour, or Ice through freezing of water droplets or deposition of water vapour upon aerosols 10 -Sep-20 Cloud Physics 67

NUCLEATION Warm cloud Liquid water droplets generally form through process of Heterogeneous Nucleation of water vapour upon aerosols (CCN) which must be wettable (i. e. water vapour can adhere to them). 10 -Sep-20 Cloud Physics 68

NUCLEATION Cloud cloud: Nucleation of ice particles • Homogeneous or Spontaneous Nucleation - freezing of pure water droplets • Heterogeneous Nucleation - freezing of a water droplet which contains a freezing nucleus 10 -Sep-20 Cloud Physics 69

PRECIPITATION TYPES Ý Precipitation types can include character or phase of precipitation which is falling to ground level. Ý Three distinct ways that rain can occur: Convective Stratiform Orographic Ý Convective precipitation is more intense, and of shorter duration, than stratiform precipitation. 10 -Sep-20 Cloud Physics 70

PRECIPITATION TYPES Û Precipitation can fall in two phases: Liquid Solid Û Liquid forms of precipitation include rain and drizzle. Û Rain or drizzle which freezes on contact within subfreezing air mass gains the preceding word of freezing, becoming known as freezing rain or freezing drizzle. Û Frozen forms of precipitation include: Snow Ice needles Sleet Hail Graupel. 10 -Sep-20 Cloud Physics 71

PHASES OF PRECIPITATION Ý Liquid precipitation: • Drizzle (DZ) • Rain (RA) Ý Freezing precipitation: • Freezing drizzle (FZDZ) • Freezing rain (FZRA) Ý Frozen precipitation: • Snow (SN) • Snow grains(SG) • Ice pellets (PL) • Hail (GR) • Snow pellets/Graupel (GS) • Ice crystals (IC). 10 -Sep-20 Cloud Physics 72

PRECIPITATION TYPES 10 -Sep-20 Cloud Physics 73

SIZE OF PRECIPITATION less than 0. 5 mm 0. 5 – 5 mm 0. 005 -0. 05 mm Mist 10 -Sep-20 Drizzle Cloud Physics Rain/Sleet 74

LIQUID PRECIPITATION 10 -Sep-20 Cloud Physics 75

TEMP PROFILES FOR PRECIPITATION Snow - Temp colder than 0 o. C everywhere (generally speaking!) Sleet - Melting aloft, deep freezing layer near ground Freezing Rain - Melting aloft, shallow freezing layer at ground Rain - Deep layer of warmer than 0 o. C near ground 10 -Sep-20 Cloud Physics 76

TYPES OF CLOUDS Ö Main characteristics for classifying clouds are depth and altitude. Ö Cloud classification scheme was first proposed back in 1803 by Howard: - Cumulus (Heap or Pile) Stratus (Layer) Cirrus (Filament of hair) Nimbus (Rain clouds) Ö Other names : Alto (Mid-level) Lenticularis (Lens-shaped clouds) Castellanus (Turrets) 10 -Sep-20 Cloud Physics 77

TYPES OF CLOUDS 10 -Sep-20 Cloud Physics 78

TYPES OF CLOUDS CONVECTIVE CLOUDS LAYER CLOUDS OROGRAPHIC CLOUDS Formation mechanism Local ascent of warm, buoyant air parcel in a conditionally unstable environment Forced ascent of stable air Forced lifting of air over hills or mountains Location in the vertical Vertically extending from cloud base; Can reach up to Tropopause All altitudes (ground level to tropopause) Top of hill or mountain Diameter ~ 0. 1 - 10 km 103 km Varies Vertical velocity ~ 1 - 5 m s-1, Max: 20 m s-1 in Thunderstorms 10 cm s-1 1 -5 m s-1 depending on speed and direction of wind and height of barrier Lifetime Minutes to Hours 10 hours < 1 hour but can last for a long period under favourable conditions 10 -Sep-20 Cloud Physics 79

WEATHER MODIFICATION ☻ On 08 August 2008, China ensured that there is no rain on their opening ceremony of the Games of the XXIX Olympiad by firing 1140 rain dispersal rockets around Beijing to disperse storm clouds. ☻ Rockets were launched from twenty-one sites and prevented the ceremony from receiving precipitation in the range of 25 to 100 millimeters. ☻ Government authorities in Beijing had mobilized all its science and engineering capability, including satellite monitoring and cloud seeding, to prevent downpour from spoiling the extensively expected Olympic opening ceremony. 10 -Sep-20 Cloud Physics 80

WEATHER MODIFICATION • Weather modification refers to willful manipulation of the climate or local weather. • If human activities could change climate, why not change it for purpose, to suit us better? • Vincent Schaefer : discovered the principle of cloud seeding in July 1946. First attempt to modify natural clouds in the field through cloud seeding began during a flight in New York on 13 Nov 1946. Schaefer was able to cause snow after he dumped six pounds of dry ice into the target cloud from an aircraft. 10 -Sep-20 Cloud Physics 81

CLOUD SEEDING Vincent J. Schafer in 1946 discovered that dry ice dropped into super cooled clouds spurred the growth of ice crystals which either induced precipitation or dispersed fog or clouds. 10 -Sep-20 Cloud Physics 82

BASIC PROCESSES OF WX MODIFICATION • Altering the available solar energy by introducing materials to absorb or reflect sunshine. • Adding heat to the atmosphere by artificial means from the surface. • Altering air motion by artificial means. • Influencing the humidity by increasing or retarding evaporation. • Changing the processes by which clouds form & causing precipitation by using chemicals or inserting additional water into the clouds. 10 -Sep-20 Cloud Physics 83

CLOUD SEEDING Þ In order for cloud seeding to trigger precipitation, conditions must be just right: - ◘ Clouds must be present; seeding cannot create clouds. ◘ A portion of the clouds must contain supercooled water. Þ One method assumes that the clouds are lacking in freezing nuclei and adding them will stimulate precipitation by the Bergeron process. Þ One must be careful not to over seed as this will produce too many, too small ice crystals. 10 -Sep-20 Cloud Physics 84

DESIGN TO MODIFY WEATHER • Total weather-modification process would be a real-time loop of continuous, appropriate, measured interventions, and feedback capable of producing desired weather behavior. • Essential ingredient of the weather-modification system is the set of intervention techniques used to modify the weather. • Number of specific intervention methodologies is limited only by imagination, but with few exceptions they involve infusing either energy or chemicals into the meteorological process in right way, at the right place and time. • Intervention could be designed to modify the weather in a number of ways, such as influencing clouds and precipitation, storm intensity, climate, space, or fog. 10 -Sep-20 Cloud Physics 85

DESIGN TO MODIFY WEATHER Dissipation/Generation of Fog 8 Field experiments with lasers have demonstrated capability to dissipate warm fog at an airfield with zero visibility. 8 Smart materials based on nanotechnology are currently being developed with gigaops computer capability at their core. 8 UAVs could be used to deliver and distribute these smart materials. 8 Recent army research lab experiments in USA have demonstrated the feasibility of generating fog. 8 Used commercial equipment to generate thick fog in an area 100 meters long. 10 -Sep-20 Cloud Physics 86

DESIGN TO MODIFY WEATHER Generation/Suppression of Precipitation 8 Exploitation of the solar absorption potential of carbon black dust can be used to enhance rainfall on the meso scale, generate cirrus clouds, and enhance cumulonimbus (thunderstorm) clouds in otherwise dry areas. 8 If UAV technology is combined with stealth and carbon dust technologies, the result could be a UAV aircraft invisible to radar. 8 If clouds are seeded using chemical nuclei before their downwind arrival to a desired location, the result could be a suppression of precipitation. 10 -Sep-20 Cloud Physics 87

DESIGN TO MODIFY WEATHER Tropical Storms 8 Focus of the weather-modification effort would be to provide additional conditions that would make the atmosphere unstable enough to generate cloud and eventually storm cell development. 8 In summary, the ability to modify battle space weather through storm cell triggering or enhancement would allow us to exploit the technological weather advances. 10 -Sep-20 Cloud Physics 88

DESIGN TO MODIFY WEATHER Artificial Weather 8 While most weather-modification efforts rely on the existence of certain preexisting conditions, it may be possible to produce some weather effects artificially, regardless of preexisting conditions. 8 For instance, virtual weather could be created by influencing the weather information received by an end user. 8 Nanotechnology offers possibilities for creating simulated weather. 8 One major advantage of using simulated weather to achieve a desired effect is that unlike other approaches, it makes what are otherwise the results of deliberate actions appear to be the consequences of natural weather phenomena. 10 -Sep-20 Cloud Physics 89

NEED TO MODIFY WEATHER Weather-modification is a force multiplier with tremendous power which can be exploited across the full spectrum of warfighting environments. 10 -Sep-20 Cloud Physics 90

CLOUD SEEDING Precipitation is a complicated process. After more than fifty years of cloud seeding experimentation & study scientific community is still lacking a full understanding of precipitation processes. 10 -Sep-20 Cloud Physics 91