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GEORGE A ISAAC1 AND JOHN HALLETT2 1Cloud Physics and Severe Research Division, Meteorological Service of Canada, Toronto, ON, Canada 2Division of Atmospheric Sciences, Desert Research Institute, Reno, NV, US

Clouds and the precipitation that comes from them are important elements of the hydrological cycle. Clouds provide a blanket for our Earth, both shielding it from radiation from the Sun and trapping heat escaping from the surface. They also generate precipitation through the - mechanism, which involves only liquid drops, or through initiation leading to large ice particles and eventually or . Mean annual precipitation amounts reach a maximum near the equator, near 8 mm day−1 , and decrease poleward to about 1 mm day−1 . A good understanding of both cloud and precipitation processes is very important for and weather predictions. This paper outlines some of the most important processes and provides reference material where more detailed information can be obtained.

INTRODUCTION the earth in the tropical regions is much greater than that at the poles, which also accounts for a greater amount of Earth is covered with clouds as any satellite photo of our precipitation in that region. At the poles, the planet will show. Figure 1 shows a cloud from are low and the stratiform clouds that exist barely produce the GEWEX Surface Radiation Budget data set (Whitlock precipitation, less than 1 mm per day. et al., 1995). Almost everywhere, the annual mean cloud There are many uncertainties in our knowledge about cover is greater than 50% and in some large areas it is clouds and precipitation. However, much progress has been greater than 80%. These clouds literally provide a blanket made and this article will briefly summarize our current for our Earth, both shielding it from radiation from the Sun knowledge, and point to more comprehensive articles where and trapping heat escaping from the surface. Understanding additional information can be found. Earlier textbooks on clouds is extremely important for both climate and weather which provide useful information include predictions because their presence or absence can strongly those written by Fletcher (1962) and Mason (1971). More affect surface temperatures. Figure 2 shows that for a recently, a general textbook on cloud physics has been station in northern Canada, the presence of cloud in the written by Rogers and Yau (1989) and a more detailed wintertime makes the surface warmer, while it provides a book on the microphysics of clouds and precipitation was cooling effect in the summertime. published by Pruppacher and Klett (1997). Life cannot exist without and in most cases we get our water from precipitation. Figure 3 shows the annual mean precipitation rate as compiled by Xie and Arkin CLOUD FORMATION AND TYPES (1996, 1997) using gauge observations, satellite estimates and numerical model outputs. Precipitation amounts reach a Clouds can exist in many forms in the . See maximum near the equator, near 8 mm day−1, and decrease the World Meteorological Organization International Cloud poleward to about 1 mm day−1. The higher temperatures in Atlas (WMO, 1975, 1987), the AMS Glossary (Glickman, the tropical regions produce strong and more 2000) and Scorer (1972) for a full description of cloud precipitation, with cloud base temperatures being greater types. Cirrus clouds form at temperatures below −40 ◦C, than 20 ◦C. It should also be mentioned that the area of generally occur above 5 km, and they cover wide areas in a

Encyclopedia of Hydrological Sciences. Edited by M G Anderson.  2005 Canadian Crown Copyright 2 AND CLIMATOLOGY

Annual mean

60N

30N

EQ

30S

60S

0 60E 120E 180 120W 60W 0

0.2 0.4 0.6 0.8

Figure 1 Mean annual cloud cover as seen from satellite as a function of latitude and longitude. From the GEWEX surface radiation budget data set (Whitlock et al., 1995.  1995 American Meteorological Society). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs

Inuvik, January, 1961−1990 Inuvik, July, 1961−1990 −15 25 C) C) Scattered ° ° −20 Broken 20 Overcast All −25 15 −30

Scattered 10 −35 Broken Overcast Mean monthly ( All Mean monthly temperature ( −40 5 N NEE SE SE SW W NWCalm All NNEE SESESWWNWCalm All (a)Surface (b) Surface wind direction

Figure 2 January and July mean monthly temperature at Inuvik, Northwest Territories, Canada, as a function of cloud cover and surface wind direction. ‘‘Scattered’’ indicates 0–1 tenth sky, ‘‘broken’’ 2–8 tenths, and ‘‘overcast’’ 9–10 tenths sky coverage (Isaac and Stuart, 1996.  1996 American Meteorological Society) sheet form, often with a fibrous aspect (Figure 4a). Below vapor trails caused by high-flying jet aircraft (Figure 4b). this temperature, form even in the absence of These clouds generally do not create precipitation that insoluble nuclei in cloud drops and a few degrees lower reaches the ground and they are primarily formed of ice in diluted haze droplets. Cirrus can form from broadscale crystals. However, they can start to precipitate (Figure 4c) uplift, in the outflow of , or even from the and the falling ice crystals can “seed” lower layers of CLOUDS AND PRECIPITATION 3

Annual mean precipitation (mm day−1)

60 N

30 N

EQ

30 S

60 S

0 60 E 120 E 180 120 W 60 W 0

Figure 3 Annual mean precipitation (mm day−1) as compiled by Xie and Arkin (1996, 1997) using gauge observations, satellite estimates and numerical model outputs (Xie and Arkin, 1996.  1996 American Meteorological Society). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs cloud and initiate precipitation. Mid-level clouds such as sublimation. The liquid to phase change involves altostratus or altocumulus are also formed by broadscale the freezing process, while melting occurs when solid lifting, often produced by frontal systems. They can be water changes to liquid. Phase changes occur with the composed of either ice crystals or liquid water, but they corresponding release or uptake of latent heat. For example, do not account for much of our precipitation. Lower-level in order for water to change from the liquid to vapor state, cloud types such as stratus, and stratocumulus (Figure 4d, latent heat is required to break the hydrogen bonds between e) are associated with precipitation, either snow or rain. In water molecules in the liquid. This is called the latent midlatitudes, especially in , these cloud types account heat of vaporization. Similarly, thelatentheatoffusionis for most of our precipitation. Cumulus clouds (Figure 4f) required to change from the solid to liquid state. The latent and especially thunderstorms (Figure 4e) create most of our heat of sublimation is released when vapor changes directly precipitation in the at mid- and lower-latitudes. into ice. Normally, phase changes from liquid or solid to Clouds are generally formed in circulation around low vapor occur when the air is subsaturated with respect to pressure cyclonic systems, with precipitation tak- liquid water or ice, and the reverse happens when the air is ing place at fronts where higher temperature, moist air if supersaturated. lifted above cooler air. Mesoscale complexes and hurri- The Clausius–Clapeyron equation, one of the most canes form towards the and easterly waves form important in cloud physics, relates the saturation vapor convergence zones for precipitation in low-latitude tropical pressure with respect to water (es)orice(ei) to the latent regions. heat of either vaporization (Lv) and or sublimation (Ls) to temperature (T). For the saturation over water, the equation may be written: WATER PHASES, LATENT HEAT des = Lves Water in the atmosphere exists in three phases: vapor, dT RvT solid, and liquid. Cloud and precipitation formation involve −1 transforming vapor into the other two phases. A vapor where Rv is the gas constant for (461.5 J kg to liquid phase change occurs through condensation and K−1). the reverse process involves evaporation. A vapor to solid Table 1 shows the saturation vapor pressure over water phase change is called deposition, while the reverse is called and ice, and the latent heats of vaporization (condensation) 4 METEOROLOGY AND CLIMATOLOGY

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Figure 4 Some examples of cloud types such as: (a) , (b) cirrus clouds produced by high-flying aircraft (contrails), (c) cirrus clouds with falling ice crystal streaks, (d) stratus clouds below an inversion, (e) stratocumulus deck, (f) , (g) , and (h) mammatus. For full definitions see American Meteorological Society (AMS) Glossary (Glickman, 2000), World Meteorological Organization Cloud Atlas (WMO, 1975, 1987), and Scorer (1972). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs ( John Hallett) and sublimation as a function of temperature. The latent water is approximately 1.8, 3.8, 7.8 and 15.0 g of vapor heat of fusion, which is released when ice changes to liquid, per kg of air, respectively (List, 1968). This helps explain is the difference between Lv and Ls. The high values of why there is more precipitation in the Tropics than in the the latent heat of vaporization of ice and water, compared Arctic. with the latent heat of fusion (melting/freezing) show that water molecules are mostly bonded in both liquid and solid, ATMOSPHERIC STABILITY AND CLOUD and the bonding only changes by about 12% on melting. FORMATION It is clear from Table 1 that warm air can hold much more water than cold air. For example, at −10 ◦C, 0 ◦C, +10 ◦C In order to consider how clouds form in the atmosphere, and +20 ◦C and 1000 mb, the saturation mixing ratio over it is necessary to understand atmospheric stability and the CLOUDS AND PRECIPITATION 5

Table 1 The saturation vapor pressure over water and ice, about 1.5 km (a saturated parcel would keep rising through and the latent heats of vaporization (condensation) and buoyancy), showing the potential for convection. The sublimation as a function of temperature (from Rogers sounding on 9 January 1998 for Gray, Maine, was during an and Yau, 1989). Papers by Fukuta and Gramada (2003) and Marti and Mauersberger (1993) are examples of intense and very prolonged freezing rainstorm (DeGaetano, recent works that attempt to measure the saturation vapor 2000). The atmosphere is stable in this sounding but shows pressure over water and ice more accurately a deep saturated layer with a warm section reaching about ◦ ◦ −1 −1 +12 C near 1.5 km with a below freezing surface layer. T( C) es (Pa) ei (Pa) Lv (J g )Ls (J g ) −40 19.05 12.85 2603 2839 −30 51.06 38.02 2575 2839 −20 125.63 103.28 2549 2838 WARM RAIN PROCESS −10 286.57 259.92 2525 2837 0 611.21 611.15 2501 2834 10 1227.94 – 2477 – Water generally condenses in the atmosphere around hygro- 20 2338.54 – 2453 – scopic particles known as cloud condensation nuclei (CCN) 30 4245.20 – 2430 – Table 2 shows the relative sizes and fall velocity depen- 40 7381.27 – 2406 – dence on diameter of cloud nuclei (CN) which are only active at very high , CCN around which most cloud droplets form, and Ultra Giant Nuclei, (UGN) normal temperature profile of the atmosphere. Typically, which can form the nuclei of large cloud drops. Effec- as you go further up in height, the pressure falls and tive CCN are sodium chloride containing particles formed the temperature decreases. For a parcel moving upward, in spray or ammonium sulfate particles from anthro- without condensation occurring, its temperature cools down pogenic sources, possibly also contaminated further by following a dry adiabatic of approximately equally deliquescent organic materials. These CCN deli- 0.98 ◦C per 100 m. For a parcel where condensation has quesce at conditions less than about 80% relative , commenced, the release of latent heat slows the cooling of or when the air is subsaturated with respect to water, result- the parcel as it ascends and the parcel follows a wet adiabat ing in haze. More typically in clouds, at supersaturations (0.65 ◦C per 100 m at 0 ◦C and 1000 mb) approaching the of less than 1%, small droplets are quickly formed around dry adiabatic lapse rate at temperatures below −40 ◦C these CCN reaching sizes of 5 to 15 µm. The actual number where the amount of vapor is minimal. activated depends on the chemical composition of the CCN Figure 5 shows atmospheric soundings for two severe particles. More efficient CCN, as are typically found in weather events. On 3 May 1999, intense tornadoes were maritime environments, activate at lower supersaturations. generated by severe thunderstorms and killed 36 people Larger updrafts tend to create higher supersaturations, with near Norman Oklahoma (Thompson and Edwards, 2000; more CCN activated, and thus higher droplet number con- Brooks and Doswell, 2002). The sounding is unstable above centrations. Typically, maritime clouds have lower droplet

Table 2 Function relationships for terminal velocity for different particle types. As a first approximation, spheres in the range from 1 to about 100 µm follow Stokes law, with velocity proportional to diameter squared. Smaller and large particles follow a linear relation; particles larger than 0.5 cm follow a square root relationship (assuming a constant drag coefficient) up to 10 cm . Snow flakes and larger ice crystals follow Stokes law below about 100 µm but have constant fall velocity with size beyond a few mm. Brownian motion gives an inverse relation for molecular cluster size pollution particles, and inverse square root relation for particles greater than 0.02 µm. The fall velocity and Brownian displacement random direction velocity are comparable for particles of diameter near a fraction of a micron, depending on particle density. Terminal velocity or displacement Diameter (d) dependence Regime √ cm Rain, hail d Constant drag coefficient 500 µm Small rain, d 10 µm Cloud droplets d2 Stokes 10 µm–1µm UGN d2 Stokes–Cunningham 1 µm–0.1 µm CCN 10−2 µmCN √1 Brownian motion d 10−3 µm Molecular cluster 1 Stokes/Cunningham/self diffusion d Note: CN – cloud nuclei, CCN – cloud condensation nuclei, UGN – Ultra Giant Nuclei 6 METEOROLOGY AND CLIMATOLOGY

KM FT (×1000) KM FT (×1000) mb 300 mb 300 9 30 9 30

8 8 25 25 400 400 7 7

6 20 6 20 500 500 5 5 15 15 600 600 4 4 Pressure (mb) 700 3 10 700 3 10

800 2 800 2 5 5 900 1 900 1

1000 0 0 1000 0 0 1050 1050 −20 −10 0 10 2030 40°C MSL −20 −10 0 10 20 30°C MSL Temperature (°C) (a) (b)

Figure 5 Pressure versus temperature atmospheric soundings for 4 May 1999 at 00 UTC for Norman, Oklahoma (a) and 9 January 1998 at 00 UTC for Gray, Maine (b). The horizontal lines represent pressure, the lines running from the bottom up to the right represent constant temperature, the curved lines running upwards towards the left represent wet adiabat lines, while the approximately straight lines running up and to the left represent dry adiabats. The dark line on the right represents the temperature profile in the atmosphere while the dashed line on the left represents the dewpoint. The altitudes below the station elevation are shaded. As an example, the 9 January sounding has a surface temperature near −2 ◦C, and it reaches a maximum temperature at +12 ◦C near 850 mb (1.5 km), and then gets colder than 0 ◦C at about 750 mb (3.5 km). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs number concentrations and larger droplet sizes than conti- Although the warm rain process is commonly used to nental clouds (see Table 3). describe the process of condensation followed by coales- ◦ The cloud droplets that form in a cloud grow by con- cence in clouds >0 C, it also works in supercooled clouds ◦ densation in the presence of a persistent updraft that gen- downtoatleast−20 C and probably below (e.g. Ras- erates a small . As they grow, the droplets mussen et al., 1995; Cober et al., 1996; Kajikawa et al., also begin to collide with one another and grow by the 2000; Lawson et al., 2001). Such large supercooled drops collision-coalescence mechanism. This mechanism is most forming at cold temperatures are important for aircraft efficient when some of the droplets are larger than oth- icing processes and precipitation formation in tropical con- ers (20 to 40 µm). It is more efficient in maritime clouds vection. where for the same liquid-water content, because of the reduced droplet number concentration, the drops tend to be BERGERON–FINDEISEN OR ICE larger (see Table 3). As Rogers and Yau (1989) indicate PRECIPITATION FORMATION PROCESS “the central task of precipitation physics is to explain how Clouds exist in the atmosphere at temperatures colder raindrops can be created by condensation and coalescence than 0 ◦C. Many of these clouds contain liquid water in times as short as 20 min.” Rain appears to form in such in the supercooled state. When ice is also present, a a time interval in nature. There are considerable uncertain- precipitation formation process can commence whereby ice ties involved in this “warm rain” process but it is critical crystals grow by aggregation of ice particles or riming that the drop-size distribution widens, so that it permits the of cloud droplets. The resulting precipitation can fall to coalescence process to work effectively. It will take about the ground as either snow or rain, depending upon the 5 10 small cloud droplets to form a raindrop 1 mm in diam- . Mixed-phase clouds which contain both eter. Drops have to be larger than 500 microns in order to liquid and ice occur quite frequently in the atmosphere, be considered raindrops. Drops between 200 and 500 µm as the summary by Korolev et al. (2003) has shown. are considered as drizzle (AMS Glossary–Glickman, 2000). Liquid water typically will not freeze until about −40 ◦C Figure 6 shows the relative sizes of cloud aerosols, cloud unless it has some impurities in it known as ice nuclei droplets and raindrops that are involved in the warm rain (IN), such as clay mineral particles (Roberts and Hallett, process. 1968). Although ice melts at 0 ◦C, it does not form in the CLOUDS AND PRECIPITATION 7

Table 3 Cloud microphysical summaries, using 30 s or 3 km pathlength averages, for maritime and continental cases in terms of static temperature (Ta), droplet number concentration (Nd), total water content (TWC) and median volume diameter (MedVD). The ice crystal concentration is represented as I. As an example, 25% of the maritime droplet concentrations (Nd) were less than 16 cm−3. These data were collected as part of the Canadian Experiment (I and III) and the Alliance Icing Research Study I (from Isaac et al., 2001) and they represent stratiform mid latitude winter clouds. Reproduced from Isaac et al., 2001,  Canadian Aeronautics & Space Institute 1% 25% 50% 75% 99% − Maritime Points = 1154a Ta ≤ 0 ◦CI≤ 1L 1 TWC ≥ 0.005 g m−3 Ta (◦C) −20.6 −5.8 −4.1 −2.0 0.0 Nd (cm−3) 1 16 52 108 406 TWC (g m−3) 0.01 0.07 0.13 0.20 0.47 MedVD (µm) 10 18 24 34 527 1% 25% 50% 75% 99% − Continental Points = 4759a Ta ≤ 0 ◦CI≤ lL 1 TWC ≥ 0.005 g m−3 Ta (◦C) −24.7 −9.1 −6.2 −3.2 −0.2 Nd (cm−3) 2 55 121 233 643 TWC (g m−3) 0.01 0.05 0.11 0.21 0.49 MedVD (µm) 10 13 17 22 643 aLiquid and mixed-phase, in-icing conditions. atmosphere at any well-defined temperature. As described hurricane or an extensive occluded cold, deep, midlatitude in cloud physics texts, generally insoluble IN can activate low pressure system. the process by allowing vapor to go directly into the ice Small ice crystals have many shapes generally based on phase (deposition nuclei), by condensation onto the IN hexagonal or sixfold symmetry. These can include columns, followed by freezing (condensation freezing nuclei), by needles, plates, dendrites, and so on (Figure 6). Magono IN contact with a supercooled droplet (contact nuclei), and Lee (1966) discuss ice particle shapes as a function or by IN immersion into a droplet followed by freezing of temperature (0 to −40 ◦C) and supersaturation. Bailey (immersion nuclei). A typical concentration of IN might and Hallett (2004) have recently updated the work at tem- be 1 per liter near −20 ◦C, with a variability of X10 in peratures colder than −20 ◦C and extended the temperature concentration not being uncommon. Once ice is initiated, range to −70 ◦C. Much of this work on ice particle shape the concentration can increase via several ice multiplication was done in the laboratory under controlled conditions. mechanisms. For example, evaporation and melting of ice Measurements in natural clouds, especially lower-level lay- particles creates locally thinner regions leading to breakup, ered cloud, show that ice particles tend to have quite or during the riming process in the presence of and irregular shapes, either consisting of faceted polycrystalline large cloud drops, the freezing and shattering of favorably particles or sublimating (solid to vapor) ice particles with accreted drops can occur (Hallett and Mossop, 1974). smooth curving sides and edges (Korolev et al., 1999, 2000; Many have tried to show that ice nuclei concentrations Figure 7). More recent work on quantifying shapes has are dependent on temperature (Fletcher, 1962), or both found that ice particles tend to be “rounder” when they are temperature and supersaturation (Meyers et al., 1992). smaller, except for a small fraction which grow as pristine These relationships are often used in numerical models. single crystals. As the remainder grow larger, having many However, observations often show (e.g. Gultepe et al., crystal orientations, they tend to get more shaped (Korolev 2001) that ice particle concentrations in the atmosphere and Isaac, 2003). do not depend on temperature, and appear to be the same The saturation vapor pressure over ice is less than the in different geographic regions. Ice nuclei and ice particle saturation vapor pressure over water for all temperatures concentrations found in clouds are not easily related to each (Table 1), resulting from the greater bonding of water other because of possible ice multiplication mechanisms molecules in the ice lattice as compared with supercooled and the redistribution of ice particles within clouds. The liquid water. Consequently, in a supercooled cloud, once measurement uncertainties for both IN and small ice ice crystals begin to form, they grow rapidly by deposition crystals are also substantial. Ice initiation in the atmosphere while the surrounding droplets evaporate. If the cloud is is a complicated process that is not well understood. It not growing with a substantial updraft, the ice crystals should be noted that the existence of any extensive region will dominate and can quickly convert all the liquid to of supercooled drops depends on an absence of ice particles ice in a process called glaciation. Once the ice crystals and nuclei, while the universal presence of ice suggests a reach a certain size they begin to fall and accrete water remarkably efficient redistribution system, as in a mature droplets through a process known as riming. They can 8 METEOROLOGY AND CLIMATOLOGY

Cloud drop Drizzle µ Large haze drop 1 to 10 m 10 to 30 µm drop − 10 to 1000 cm 3 µ −1 200 to 500 µm Haze drop, CCN, 0.1 m 1 to 2 cm s − 1000 m 3 − 1 Pollution particle - 100 times less than 1 m s − this dot, 1000000 cm 3

Snow crystals 10 µm to 1 cm − Large, branched 1 to 10000 m 3 − mm dendrite arm 10−50 cm s 1 (1 of 6)

ed Capp column Dendrite mm to 5 cm 10000 to 1000000 crystals − − 100 to 1 m 3, 1 m s 1 Large hailstone, 5 to 10 cm 30−50 m s−1 − <0.0001 m 3 5 cm

− − Raindrop − 500 µm to 8 mm; 2 to 10 m s 1 graupel, small hail 5 mm to 1 cm; 2 to 5 m s 1

Figure 6 Relative sizes and shapes of cloud aerosols (haze particles, ice nuclei), cloud droplets, raindrops, ice crystals, snowflakes, graupel and hail (Adapted from Wallace and Hobbs, 1977; McDonald, 1958). Typical diameters, concentration, and fall velocities are given. For an additional sizing reference, a human hair is about 100 µm in diameter. The ice crystals shown are the classical hexagonal shapes, depending on the growth conditions. Columns form near −4 ◦C and plates at higher and lower temperatures, which sometimes lead to a capped column. Dendrite arm side branches may be symmetric or otherwise depending on the ambient turbulence (Hallett and Knight, 1994). Most ice particles have irregular shapes (e.g. Korolev et al., 1999, 2000) resulting from different nucleation and changing growth conditions also aggregate with other ice crystals to form snowflakes. snow, when such a warm layer is not present, is also formed Formation of precipitation via this mechanism accounts by the Bergeron–Findeisen process. for most of the precipitation that forms in mid- and high latitudes. This theory was proposed by Bergeron (1935) and further refined by Findeisen (1938). Because PRECIPITATION TYPES, DEW AND it builds on the early work of Wegener (1911), it is referred to as the Bergeron–Findeisen–Wegener theory, or it is Precipitation as Rain often shortened to the Bergeron–Findeisen process. As an illustration of this process, cirrus clouds often produce Precipitation may be defined as those particles falling from large ice crystals that begin to fall. If these ice crystals clouds in the atmosphere which reach the ground and fall into lower-level supercooled liquid clouds, they can remain for sufficient time to leave an observable residue initiate precipitation. Much of the physical basis for cloud of water. Table 2 shows how the fall velocities of different seeding or weather modification is to create the initial ice precipitation particles depend on their size. Precipitation it crystals by providing artificial ice nuclei, or through the use may be in the form of liquid-rain or drizzle as defined in of a coolant such as evaporating dry ice which can produce Sections “Warm rain process” and “Bergeron–Findeisen or temperatures well below −40 ◦C, near −80 ◦C. ice precipitation process” , either above 0 ◦C or supercooled This process produces rain because the resulting ice below 0 ◦C. Drizzle and rain drops are spherical for sizes crystals typically melt when they fall into warmer air <0.8 mm. Larger raindrops deform during fall with the near the surface, with melting usually being complete for lower part flattening by the airflow (Figure 8). In contrast, temperatures above +4or+5 ◦C. However, most of our cloud drops fall at a velocity of less than 100 cm s−1 CLOUDS AND PRECIPITATION 9

Spheres

(a) Irregulars (a)

(b) Needles

(b) (c) Dendrites Figure 8 Shapes of drops. Drops in free fall in the atmosphere are in balance between their weight and drag forces resulting from both viscosity of the air and a hydrodynamic effect. With increasing size above about 20 µm, flow around the drop becomes increasingly (d) asymmetrical, with a standing eddy wake and eventually shedding vortices, peaking by resonance with the drop 1 mm surface tension for a size near 1 mm. Much below this size drops are spherical (Figure 9). Near this size resonance Figure 7 An example of different cloud particles as occurs and the drops move sideways during fall. At seen by a Particle Measuring System 2D imaging probe larger sizes the drop base becomes increasingly flatter (Knollenberg, 1981) while flying through different clouds. (Figure 8a, 3 mm horizontal diameter), and eventually turn (From Korolev et al., 2000 by permission of The Royal inside out and a large bubble forms, becoming greater Meteorological Society). than a centimeter in diameter (Figure 8b). As it blows up further, the film breaks and forms many small drops with intermediate size drops being formed from the lower ring dependent on the square of the size. Beyond about 6 mm, remnant ( John Hallett) depending on ambient turbulence, the flat bottom may deform by the airflow, turning the drop inside out as a thin water film bag some 1–2 cm dimension with a ring breaks. Small drops and drops from the breaking jet fall at its base, which quickly breaks up into much smaller back into the water and penetrate to as much as 10 cm below drops (Figure 8). This process limits raindrop size to 6 mm the surface – but less in ocean water because freshwater has in more turbulent air, and up to >9 mm under quiescent buoyancy (Figure 10). conditions. The fall velocity of such particles increases to Optical phenomena, such as rainbows, corona, and glory a maximum between 9 and 10 m s−1. Figure 9 shows how yield information on the nature of liquid cloud and pre- the fall velocity of liquid particles increases from <1cms−1 cipitation particles from simple observation. Figure 11(a) for 10 µm diameter, approximately 100 cm s−1 for drizzle shows the transition of snow scatter of white light to a drops, >4ms−1 for >1 mm raindrops. For a size a little well-defined rainbow as melting proceeds. The rainbow less than 1 mm, the drop resonates with eddies shed from formed from large raindrops is a refraction process, but its rear and falls at angles as much as 40 degrees to the for smaller drops and cloud drops diffraction effects are vertical. Larger and smaller drops fall straight down in increasingly important. Table 4 summarizes observations quiescent air. of common optical phenomena leading to inference of the Drops splash and break on impact with the ground or form (drop size, crystal, shape, fall orientation) of the cloud vegetation, the splash depending on whether the ground is or precipitation particles responsible. Distinction is made dry or wet and the depth of any liquid film. This spreads between phenomena viewed toward the sun, as ice halo spores from fungi to considerable distances and aids their and cloud corona and to phenomena opposite the sun as dispersal (Levin and Hobbs, 1971). Splash is different in the rainbow or glory. Distinction is further made between fresh or ocean water and is highly complex (Hallett and phenomena having constant angular diameter as the pri- Christensen, 1984). The drop impact yields first a crater, mary and secondary rainbow (42, 50 degrees), rainbow and which subsequently retracts to an upward vertical jet which ice crystal halo (23 and 46 degrees) and variable diameter 10 METEOROLOGY AND CLIMATOLOGY d inction d a maximum size in the middle. This m. minsize mthick µ µ µ 20 20 20 > > attached vortices sun > from sun interference color 5 visible). Refraction by 90 degrees in hexagonal prisms Falling hexagonal crystals oriented horizontal by 46 degrees halo; red toward sun; blue away from Refraction by 120 degrees in hexagonal, ice crystals 22 degrees solar halo; red toward sun; blue away Colors closer to complementary, white less primary Toward sun/ Corona (as around sun or moon) =

innaert (1954), Greenler (1989), Tape (1994), Lynch an n > 10 degrees. > and changing orders of color. Turbulent cloud may also be colored e cloud leading and trailing edge an θ order of ring; sometimes = Viewing direction

n 1/2 degree, rings visible to = m. µ Brush Molecular scattering Polarized light all directions; Haidinger’s ending on particle size (corona, glory). See M Diffraction effects and spread of drop-size wash out colors 15 degrees from the sun, with limited dimension (5 degrees), the sun angle change is small compared with > half angle sun to ring, A diffraction effect giving a wide range of ring diameters and colors = m Sun dog; enhanced colors at low angles in haloes θ µ θ sin

d = m 2 µ λ/

n 100 droplet diameter, mm diameter > = Colored rings around sun or moon resulting from diffraction effects from cloud droplets (occasionally ice spheres) <

d 100um, > Inferences from common optical phenomena into the properties of atmospheric particulates. Distinction is made between phenomena viewed mdiameter µ wavelength, 100 = inside red; outside blue spherical raindrop > the drops in the cloud, which grow upstream and have a minimum size at th sun; red away from sun outside secondary bow, resultingdifferent from size diffraction drops of leadingbow to diameters. colors Intense and with different spread narrow drop-size colors missing) , both corona and glory may change angle from place to place around the viewing direction. leads to uniform colored regions tracing the cloud edge with constant d sin discharge causing the rainbow to shake head or aircraft) (mother of pearl clouds) but with changing patchiness. λ White rainbow; drop diameters 50–100 Separation by Alexander’s darkSecondary band rainbow 50 degrees away from antisun; Three internal reflections and refraction inside near Dark outside of halo Uniform droplet size givesColors intense not colors, uniquely a related spread to gives drop washed size out for colors. values Droplets near size few a few um to ( Two internal reflections inside near spherical raindrop towards the sun direction (right column, ice halo, corona), opposite the sun direction (left column, rainbow, glory), and at 90 degrees, middle. Dist As a special case, for lenticular clouds up to Table 4 Away from the sun direction (look atPrimary shadow rainbow: of 42 head) degrees from sun; blue toward At 90 degrees to sun direction Toward sun direction Supernumary rainbows, inside primary (brighter) and Colors closer to primary colors (two adjacent primary Optical interference for For spatially uniform droplets, colors are in a well-defined ring. For clouds with drop size changing with viewing angle, as in clouds growing over a is also made betweenphenomena phenomena having depending changing on anglesLivingston particle and (2001) colors shape dep having near constant angle to the sun or antisun viewing direction (rainbow, halo) and Smaller drops give larger diameter ring; sun angular diameter Drops may oscillate with changing electric fields ofAway a from sun/moon Glory (as around a shadow of CLOUDS AND PRECIPITATION 11

Drop fall speed (Gunn and Kinzer, 1949; Beard and Pruppacher, 1969) Snowflake, dendrite (Langleben, 1954) Snowflake, column & plate (Langleben, 1954) 10 10

8 1 ) ) 1 1 − − 6 0.1 Drop spherical 4 Drop deformed Fall speed (m s Fall speed (m s 0.01 2 Cloud droplets Drizzle Rain

0 0.001 0 2 4 6 0.01 0.1 1 10 (a)Drop diameter (mm) (b) Drop diameter (mm)

Figure 9 Fall velocity of drops, ice crystals and snow aggregates. Part (a) shows the fall velocity as a function of diameter on a linear scale, while part (b) shows it on a log scale in order to capture the full range of drops existing in the atmosphere. For the snowflakes, the melted equivalent diameter is used. Drops are spherical up to about 1 mm, increasingly deform by aerodynamic forces up to about 6–8 mm (depending on turbulence) and break up for larger size. Drops shed eddies and have a resonance with surface tension deformation with a resonance frequency of 320 hz for a diameter near 985 µm. They side slip during fall. Fall velocities decrease with deformation as compared with a frozen sphere as the corona and glory. The former rely on the spherical the particle size and shape through the drag coefficient and shape of smaller raindrops and fixed angle of an hexagonal the particle mass through its distribution of density. The ice crystal, whereas the latter rely on different diameters of density of snowflakes range from <0.1to0.4gml−1. cloud drops in the viewing direction. A corona is observed Figure 12(a–c) show individual crystals. Figure 12(d, e) looking toward the sun, and a glory is observed away from show composite crystals resulting from growth under the sun with a distinction in the perceived color in the form changing temperature. Snow flakes may be composed of of a ring for a uniform droplet size (Figure 11b and c). A pristine crystals or rimed crystals or mixtures of both. The smaller diameter implies larger cloud drops. Should cloud presence of pristine crystals is readily inferred from obser- drop size be related to location (as in a lenticular wave vation by the presence of specific optical effects, Crystals in cloud) well away from the sun, the contour of the cloud cirrus produce haloes of well-defined angles around sun or identifies the colors (Figure 11d and e). moon (Table 4, Figure 11f, g) and also sun dogs (parhe- lia) should the crystals become oriented during fall by Precipitation as Snow attached wake eddies (Figure 11h). Thin section analysis Precipitation as ice occurs in many forms. Ice forms indi- in polarized light reveals a complex growth regime from vidual crystals as hexagonal columns or dendrites. These the distribution of crystals and bubbles in small ice parti- are ideal, pristine forms. It also occurs as rimed snow, cles (Figure 12f, 12g). Fall velocities of ice particles are graupel, or hail, as frozen drops, or as aggregates of other also shown in Figure 9. Ice particles and snowflakes fall snow particles. Snow pellets and are variously much slower than liquid drops of the same size. defined (AMS Glossary – Glickman, 2000) and are more As snow falls to the surface, lower layers tend to compact complex particles of ice precipitation often formed from because of the weight of the snow above. With temperature ◦ multiple crystals grown from a frozen drop as an initial from some −15 to −20 C and below, crystals retain their nucleus. The vapor growth habit of individual and also the original habit for days or longer. Metamorphosis occurs by component crystals of polycrystals (defined as the relative vapor evaporation from edges and redeposition at contact lengths of hexagonal direction compared with that at right points. These processes eventually lead to bubbly ice, angles) is related to growth temperature and growth super- precursor to ice and its ability to flow under the saturation (relative humidity with respect to ice >100%, overlying stress. Bubbles shrink as the pressure increases up to about 140% for growth in cold supercooled clouds at and eventually disappear as the bubble air transforms to temperatures approaching −40 ◦C). The local supersatura- solid hydrate at hundreds of meters depth under pressures tion also increases with fall velocity, which is dependent on of many . At higher temperatures, this change 12 METEOROLOGY AND CLIMATOLOGY

100 is much more rapid, leading to the granular (fraction of mm) nature of recently fallen snow near 0 ◦C. Vertical 50 Splash L temperature gradients within the snow pack may lead to layers of vapor grown crystals close by layers of low- density snow, particularly near −4and−15 ◦C. Such events are preludes to avalanches along these layers of weakness )

1 50 − 10 in mountainous terrain.

5 10 Dew and Frost Formation 5 Coalesce Splash Beunce Dew forms at the surface as drops up to 1–2 mm diame- , 1 ter, and may or may not wet the surface. They differ from 1.0 guttation drops, typically formed on grass tips, formed by Impact velocity (m s Coalesce Coalesce, exuding water from the plant itself. In either case, a flux of 0.5 0.1 vapor from moist air above or from damp soil vegetation Skid below may be the source of dew or inhibit the evapora- tion of guttation drops. Dew formation follows radiative cooling of the surfaces when exposed to a clear sky or as 0.1 moist air is advected over a cooled surface. With tempera- 0.01 0.05 0.1 0.20.5 1 2 5 ◦ Drop diameter (mm) ture below 0 C, drops may supercool and nucleate over a temperature of several degrees below 0 ◦C. Most drops are frozen by −5 ◦C. With the initiation of ice, crystals grow Figure 10 Drop Breakup and Splash Characteristics. , from the vapor, in many ways having the same variability drizzle or rain drops falling into a flat water surface, as a as snow crystals, although in the case of frost the ventila- lake or ocean, behave in different ways depending on size and impact velocity. Assuming a terminal impact tion varies from zero to ambient wind speed. Higher wind velocity (Figure 9), a drop, diameter <0.6mm, forms a speed tends to mix out the cold surface layer producing the vortex ring which propagates into the liquid. The supersaturation. Most spectacular frost occurs with radiative for propagation is the impact kinetic energy and the drop surface cooling below 0 ◦C with moist air above, cooling surface energy; the former dominates for large drops, and supersaturation spreading upward from the surface to the latter for small drops. L = ratio of kinetic to surface energy. Larger drops (diameter between 0.6 and 1.1 mm) give crystals at varying levels on surrounding vegetation leave a crater which collapses and projects a jet upwards (Figure 13a–c). Figure 13(d) shows ice boules that formed which in turn breaks into smaller drops to fall back and over a stream with the air temperature below 0 ◦C but the reenter at a velocity well below terminal. Drops larger water temperature a few degrees above freezing. than 1.1 mm form a turbulent and wider jet which rises a shorter distance and also breaks before falling back. The jets are composed of liquid of the original drop and should Freezing Precipitation sea water drops be lofted and evaporate under suitable Freezing drizzle and rain is a significant weather hazard wind conditions, provide large and ultragiant nuclei for involving rain falling into a layer colder than 0 ◦Catthe cloud and drizzle drop formation. Drops diameter greater surface, and then freezing upon contact. It can form either than 2 mm produce a rising thin film crown outward from their entry periphery, which may break up and produce a through the warm rain process, entirely at cold temperatures multitude of smaller drops, which provide smaller salt (Huffman and Norman, 1988), or by snow falling into a nuclei for smaller cloud drop formation. Some larger warm layer, melting, and then the rain in drops (>3 mm) entering the surface, without any local the cold boundary-layer air. of perturbation, produce a crown which closes at the top have been compiled for the US and Canada (Stuart and resulting in an air pressure reduction from the growth of the crater below and ultimately leave air bubbles, Isaac, 1999; Cortinas et al., 2004). Stuart and Isaac (1999) some 2 cm diameter with a flat base. These are readily showed a maximum in freezing precipitation occurrence seen in puddles and larger bodies of water under heavy at St. John’s, Newfoundland, averaging about 150 h year−1. rain conditions and persist a few seconds before draining The 5–9 January 1998 , which covered portions of and breaking. The splash is modified for a water layer of northeastern US and southeastern Canada, yielding 90 mm thickness comparable with the drop size (no crater or jet); freshwater entering sea water produces buoyant vortex of rain across a wide area, caused $4.4 billion in damages rings which return toward the surface. Heavy rain on the (see Gyakum and Roebber, 2001). An atmospheric profile ocean has a calming effect by providing a light water layer during this storm is shown in Figure 5. over a dense water layer and also transporting momentum from the top layer downwards. Local winds influence the Graupel and Hail splash pattern in even more complicated ways as does drop impact on wet sloping land, which influences the Graupel and hail microphysical formation mechanisms have erosion of underlying soil been summarized in Pruppacher and Klett (1997). Graupel CLOUDS AND PRECIPITATION 13

(a)(b) (c)

(d) (e)

(f) (g)

(h)

Figure 11 Optical phenomena that can yield information on particle type, size, shape and orientation ( John Hallett). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs is simply a heavily rimed snow particle that can either be and microphysics is complex and not completely under- lumpy or conical in shape, and is often considered as small stood. However, hailstorms can cause large amounts of hail. It can form in weakly convective clouds. Hailstones are property and crop damage, mostly resulting from extensive greater than 5 mm in diameter up to the size of “grapefruit”, regions of modest 0.5 to 3 cm diameter hail. Changnon and and they form in large thunderstorms and convective com- Burroughs (2001) compared the US annual average of $445 plexes (Figure 14). Graupel particles have densities near million in damages the $1.9 billion caused by one storm 0.1to0.8gml−1, while hailstones have density approach- (2001 dollar values). ing pure ice (0.92 g ml−1). Marwitz (1972) recognized the Thin sections of hailstones reveal the presence of air different types of and attempted to categorize them bubbles of different size and concentration resulting from as (one large updraft), multicell, squall line, and so rejection of dissolved air as ice crystals grow from accreted on. It is clear that the formation mechanism of hail requires supercooled drops. Drop size and concentration changes strong updrafts to keep them aloft. The largest hailstones from place to place in the updraft environment; the bub- can have terminal velocities of approximately 50 m s−1 ble characteristics reflect these changes giving alternation which implies updrafts of similar magnitude (Pruppacher regions of clear and bubbly ice. Further information on and Klett, 1997). The interactions between the dynamics the growth process can be obtained by use of polarized 14 METEOROLOGY AND CLIMATOLOGY

(a) (b)

(c) (d)

(e)

(f) (g)

Figure 12 (a) Ice particles which formed in the atmosphere. (b) Growth is influenced by temperature, supersaturation and the accretion of cloud drops. (c) A dendrite crystal photographed by using the crystal as a mirror showing internal plates. (d) and (e) Composite crystals having combined plates and columns as conditions changed; (f) and (g) Part of a rimed crystal, in ordinary and polarized light. (b) Courtesy Yoshi Furukawa, Sapporo; (e) Courtesy Tom Henderson, Fresno ( John Hallett). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs light, distinguishing between wet growth when latent heat size distribution and fall velocity of the hailstone (Brown- evolution is sufficiently rapid to prevent complete freez- scombe and Hallett, 1967). Figure 14 shows thin sections of ing and ice grows in to a liquid surface layer, and dry two stones. The first (Figure 14a, b) formed originally on a growth when supercooled drops are captured and freeze conical graupel particle. The second (Figure 14c, d) formed as individuals. Under the former conditions, large crystals on a frozen raindrop, which grew first as graupel and then are formed as shown by uniform color of regions of the as a hailstone. The circular gaps around the core of the thin section; in the latter case bubbles and small crystals hailstone (Figure 14c) were regions grown as spongy ice, may be formed. This may also occur under simultaneous only partly frozen, during growth under high liquid-water accretion of mixed-phase ice and supercooled water drops. content. Water was lost prior to collection and storage at The correlation of crystals is complicated as it depends on low temperature. The contours of bubbles and changing ice several things, temperature, water and ice content, particle size result from changing cloud conditions for growth. The CLOUDS AND PRECIPITATION 15

(a) (b)

(d)

(c)

Figure 13 Examples of frost forming by radiative cooling with habit dependent on growth temperatures (a, b, and c) and ice boules formed over a stream by splashing of warmer water into cold air above (d) ( John Hallett). A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs larger the hailstone, the larger the fall velocity, and the Canadian precipitation-temperature relationships for differ- greater likelihood of its wet growth in high water content ent . clouds. Table 1 provides justification for why cloud water con- tent and precipitation amount should depend on tempera- ture. Warmer air has the capacity to hold more moisture CLOUD LIQUID-WATER CONTENT AND and the potential for greater precipitation is thus enhanced. PRECIPITATION AS A FUNCTION OF However, radiative influences may also be important in the TEMPERATURE precipitation-temperature relationship. For example, when convection occurs in the afternoon during the summertime, There have been several summaries of large data sets show- temperatures can drop because the incoming solar radiation is blocked. ing cloud properties as a function of temperature. Figure 15 shows how the cloud liquid-water content changes as a function of temperature (Gultepe and Isaac, 1997) for mid CLOUD AND PRECIPITATION PARTICLE SIZE latitude stratiform clouds. Similar results were obtained by DISTRIBUTIONS Feigelson (1978), Mazin (1995) and later again by Gultepe et al. (2002). There have been many studies of the particle size distribu- Similarly, precipitation amounts tend to be larger when tions in clouds and in precipitation. Figure 17 shows results the temperatures get warmer as shown in Figure 16 for of averaging 2037 spectra averaged over 3 km in all liq- some stations in the Mackenzie river valley (Isaac and uid stratiform clouds found at temperatures between 0 and Stuart, 1996). However, these relationships only hold for −34 ◦C.Themeasurementsweremadeusingfivedifferent simple stratiform clouds. For precipitation from convec- Particle Measurement System (PMS) probes (Knollenberg, tive clouds, which often occur during the summertime in 1981) and span a diameter range of a few microns to several the same area, the amounts get smaller when the temper- millimeters. Of most relevance to hydrology is the parti- atures are cooler. Evidence is found for this in Figure 16 cle size distribution of rain and snow at the surface. The when the temperature gets warmer than +10 ◦C. Isaac and early work of Marshall and Palmer (1948) and Gunn and Stuart (1992) came to the same conclusion in a study of Marshall (1958) assumed an exponential distribution of the 16 METEOROLOGY AND CLIMATOLOGY

(a) (b)

(c) (d)

(e)

Figure 14 Hailstones. (a, b) thin section of hailstone showing conical center as observed with normal light and under polarized light ( John Hallett); (c,d) thin section of hailstone with a drop center as observed with normal light and polarized light ( John Hallett); (e) an example of hailstones with rough surfaces ( George Isaac; Barge and Isaac, 1973). The scale on (a) and (c) is 1 cm wide. A color version of this image is available at http://www.mrw.interscience.wiley.com/ehs CLOUDS AND PRECIPITATION 17

100 108 FSSP 096 FSSP 124 6 ) 10 2D-C 1

− 2D-G

m 2D-P µ 4 3

−1 − 10 ) 10 3 − 102

LWC (20%) 0 LWC (50%) 10 LWC (g m −2 10 LWC (80%) LWC (95%) 10−2

Linear fit Drop concentration (m

10−4 10−3 −40 −30 −20 −10 01020 30 101 102 103 Temperature (°C) Diameter (µm)

Figure 15 Cloud liquid-water content as a function of Figure 17 Results from averaging 2037 spectra made in temperature for given percentile values. The solid line all liquid Canadian winter clouds using an instrumented is the best fit to the median values. (Gultepe and Isaac, aircraft (Cober et al., 2003)  2003 by Environment Canada. 1997.  1997 American Meteorological Society) Published by the American Institute of Aeronautics and Astronautics, Inc.

3 More recently, at least for raindrop-size distributions, both the gamma (Ulbrich, 1983) and lognormal distribu- Inuvik 257 mm Norman Wells 315 mm tions (Feingold and Levin, 1986) have been used. The Whitehorse 269 mm gamma distribution is given as: Fort Smith 352 mm µ Fort Nelson 449 mm N(D) = NoD exp(−D) 2 and it defaults to the exponential distribution when the cur- vature parameter, µ, goes to zero. The gamma distribution is a useful way of describing the raindrop size–distribution (e.g. Bringi et al., 2003), but the coefficients depend on climate regime and type of rainfall (e.g. convective or 1 stratiform). However, the older Marshall–Palmer param- eterization is still being used and can give a reasonable Average daily precipitation (mm) first approximation (e.g. Sheppard and Joe, 1994). Such distributions may only effectively describe extended data sets and thus are useful in a climatological sense. For specific short periods or small data sets, their use must be treated with caution because local turbulent regions may 0− − 40 20 020lead to particle sorting and physical processes of a specific ° Mean daily temperature ( C) scale (as the width of a cirrus trail) may lead to significant local deviations. Figure 16 Relationship between mean daily surface temperature and average daily precipitation amounts for several stations in the Mackenzie river valley (Isaac and Stuart, 1996.  1996 American Meteorological Society) CLOUD SYSTEMS The main purpose of this article is to describe cloud forma- form: tion and the microphysical processes leading to precipita- N(D) = Noexp(−D) tion. Other sections will handle storms and storm systems. However, Figure 18 shows the types of clouds and cloud 3 −3 −1 −0.21 with No = 8 × 10 m mm ( = 41R )forrainand systems that can generate precipitation around the world. In 3 −0.87 −3 −1 −0.48 No = 3.8 × 10 R m mm ( = 25.5R )for polar regions, as Figure 3 shows, little precipitation actu- melted diameters of snow where R is in mm h−1. ally falls, and, what does comes from stratiform clouds and 18 METEOROLOGY AND CLIMATOLOGY

Precipitation initiation in different systems worldwide

−80° C Homogeneous ice Polar stratospheric and hydrate clouds nucleation

Hurricane Homogeneous ice −40° C nucleation

Ice nuclei Midlatitude Meso-scale complexes Secondary ice Fontal systems Temperature 0° C Trade wind CCN (0.1 µm) cumulus Drop Large hydroscopic nuclei Arctic coalescence (1− >10 µm) Ocean 20° C stratus stratus

Polar low

90° 60° 30° 0° Longitude

Figure 18 Examples of precipitation mechanisms, as a function of latitude and altitude, on a worldwide basis (Hallett and Isaac, 2001.  2001 American Meteorological Society) associated systems such as polar lows and frontal systems. may result in a radiative forcing of about −2.1Wm−2 Frontal systems and convective storms dominate the mid in the heat budget of the atmosphere. In comparison, latitude belt, with the occasional hurricane. In the trop- the Intergovernmental Panel on (2001) ics, deep convective clouds and their associated systems estimated that the radiative forcing due to increases in account for the large amount of precipitation that falls greenhouse gases from preindustrial times to 1998 was within this latitude zone (see Figure 3). The dynamics of 2.4 W m−2. such systems is very complicated but obviously very impor- It should be emphasized that the greatest driving force tant for cloud development and precipitation formation. for the atmospheric engine is generated in the tropics, since this is the region where cloud bases have the highest temperature and water contents aloft, and therefore most CONCLUDING REMARKS of the atmospheric latent heat is released in the convection The driving engine for most clouds systems is the latent heat occurring in this area. The transport of this energy away release that occurs through phase changes. For example, from the tropics drives the atmospheric system. for each mm of precipitation, 25 W m−2 of latent heat However, there are many uncertainties in how clouds and is released into the atmosphere. It is essential to know precipitation form. Hallett and Isaac (2001) summarized a how such precipitation is formed from clouds in order to Panel discussion at the 13th International Conference on completely understand the energy cycle of the atmosphere. Clouds and Precipitation (Reno, 2001), which considered Our climate is particularly sensitive to cloud properties. this topic. We need to extend our knowledge of CCN For example, Slingo (1990) stated that the radiative forcing for larger size particles, and determine the geographic at the top of the atmosphere due to a doubling of carbon variability of such particles. Our current knowledge of dioxide concentrations could be balanced by relatively how ice forms in the atmosphere from ice nuclei to ice modest increases of 15–20% in the amount of low clouds multiplication needs further study. The development of and 20–35% in liquid-water path, and by decreases of precipitation through the warm rain process also has gaps ∼ 15–20% in mean drop radius. Rotstayn (1999) suggested in our understanding. A better insight into the physical that a 1% increase in cloudiness, a 6% increase in liquid- (particle growth) and dynamical (air motions) processes is water path, and a 7% decrease in droplet effective radius required. We also need to gain more insight through studies CLOUDS AND PRECIPITATION 19 that compare model simulations to observations. Finally, Fletcher N.H. (1962) Physics of Rain Clouds, Cambridge we need a multidisciplinary approach to these questions University Press. p. 386. because they require a balance between, theory, modeling, Fukuta N. and Gramada C.M. (2003) Vapor pressure measurement laboratory experimentation, instrument design and field of supercooled water. Journal of the Atmospheric Sciences, 60, observational work. 1871–1875. Glickman T.S. (Ed.) (2000) , Second Edition, American Meteorological Society, p. 745. REFERENCES Greenler R. 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