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Chapter 10Cloud and Precipitation

Lesson 32

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Cloud Droplet Formation

• Coalescence Theory– Droplets grow by merging with one another.– Occurs in ‘warm clouds’ forming below the

freezing level.

• Ice Crystal Theory– Droplets are formed mainly by formation of ice

crystals which melt before reaching ground as rain droplets.

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Coalescence Theory

• The greater the cloud depth the larger the droplets.

• The greater the updraft velocity the larger the droplet size that can be held in suspension.

• Droplets only descend when the terminal velocity of the droplet exceeds the updraft velocity.

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Coalescence Inside a Warm Cloud

Droplets grow by collision and entrainment.

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Ice Crystal Theory

• Droplets are carried well above the freezing level.

• Freezing nuclei are scarce above the freezing level.

• Droplets remain as droplets but become super-cooled.

• Few ice crystals exist between 0°C and -10°C.

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Ice Crystal Theory, cont’d

• The vapour pressure over ice is less than over water.

• Water vapour migrates to the ice crystals and causes them to grow at the expense of the water droplets.

• As the droplets ascend they decrease in size and the ice crystals grow at the expense of the droplets.

• Between 0°C and -10°C large droplets give clear icing.

• Below -15°C down to -40°C rime ice forms.

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Bergeron/Ice Crystal Theory

As the droplets ascend in the cloud the ice crystals grow at the expense of the super cooled water droplets.

High Vapour Pressure Low Vapour Pressure

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Ice Crystal Theory

CLEAR ICE

CLOUDY ICE

RIME ICE

NO ICING

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Precipitation Summary

• Droplet size depends on extent of vertical motion and depth of cloud.

• Stratus with little vertical motion produces drizzle.• Stratocumulus produces a mixture of rain and drizzle.• Nimbostratus usually produces moderate continuous

rain.• Towering CU produces light shower.• CB and TS produce moderate to heavy rain showers.

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Addendum to Precipitation

• Layer cloud produces intermittent or continuous rain with no clearances between rain events.

• Intermittent rain lasts less than 1 hour

• Continuous rain lasts 1 hour or more.

• Rain showers fall from cumuliform cloud with clearances in between the showers.

• Rain showers last for less than 1 hour.

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Types of Precipitation

• Drizzle is produced in cloud with little vertical motion such as stratus or stratocumulus.

– The lack of vertical up currents and shallow depth of the cloud do not allow the droplets to grow sufficiently and thus they fall from the cloud as drizzle.

• Rain falls from layer clouds which which have stronger vertical motion.

– Layer clouds such as Nimbostratus have a large horizontal extent such as along a warm front and produce light to moderate or heavy continuous rain.

• Continuous means for a periods of longer than one hour. Less than one hour means the rain is described as intermittent.

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Types of Precipitation

• Rain Showers fall from cumuliform cloud such as towering cumulus or cumulonimbus. – These have large vertical extent but limited horizontal extent with

strong up currents producing large droplet sizes. – Showery precipitation is intermittent lasting on average for less than

one hour with clearances between showers when blue sky may be seen.

• Showers are described as being light, or moderate to heavy.

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Types of PPN cont’d

• Virga is precipitation which evaporates before reaching the ground and is most frequently observed in summer.

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Types of PPN cont’d

• Snow may fall from layer cloud or cumuliform cloud and reaches the ground as snow flakes when the surface temperatures are at least below about 4°C.

– It will not lie but melt quickly. Surface temperature have to be at or below freezing for the snow to lie for any length of time.

– If as in summer when the freezing level is well above the surface the snow flakes will melt forming droplets as described in the Bergeron process.

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Types of PPN cont’d

• Sleet is a mixture of rain and snow and falls when the surface temperatures are above 0°C (5°C)

– it is still cold enough for some of the larger snow flakes to reach the surface as large wet flakes before melting very rapidly.

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Types of PPN cont’d

• Freezing Rain occurs when the surface below the cloud is at or just below freezing when the supercooled water droplets fall from the cloud and freeze instantly into a veneer of ice after striking the surface. – If the droplet size is less than 0.5 mm the precipitation is

described as freezing drizzle.

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Types of PPN cont’d

• Ice Pellets are formed when melted rain drops refreeze into ice particles by falling into a sub-zero layer before reaching the ground. – The next diagram illustrates the conditions favourable for their

formation.

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Ice Pellets

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Types of PPN cont’d

• Snow Grains are very small white and opaque grains of ice usually less than 1 mm in diameter. – The grains are usually flat or fairly elongated.

– They fall in small quantities from stratus clouds and never in the form of a shower.

• Snow Pellets are white or opaque pellets of ice with a diameter of 2 to 5 mm.

– They tend to be round or conical in shape and are sometimes confused with snow grains.

– Snow pellets are brittle, crunchy and bounce or break apart when hitting a hard surface.

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Types of PPN cont’d

• Graupel(Soft Hail) is formed when ice crystals collide with super-cooled water droplets which freeze onto the crystal as rime ice, containing many air spaces. – This keeps the density low and gives it an opaque appearance. – By the time it reaches the bottom of the cloud it has grown in size and with the accumulation

of rime ice it cannot be recognized as an ice crystal or a snow flake. – Since the freezing level is at a low elevation, the graupel reaches the surface as a light,

round clump of snow-like ice - a snow pellet. – In summer, when freezing levels are high, the graupel melts before reaching the surface. – In vigorous convective clouds, the graupel may develop into a fully fledged hailstone.

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Types of PPN cont’d

• Hail is formed from pieces of ice ranging in size from from that of peas to that of golf balls or larger. Some are rounded, others are of irregular shape.

– The largest authenticated hailstone fell on Coffeyville, Kansas, USA in September, 1970. This weighed 757 grams and had a measured diameter of over 14 cm.

– The violent upcurrents and down currents combine to give the hailstones many cycles within the storm before falling to the ground.

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Types of PPN cont’d

• Large damaging hail is not a feature of winter thunderstorms, mainly because the convective activity is not as severe and probably due to the lower water vapour concentrations at winter temperatures.

• Tropical thunderstorms also do not produce damaging hail as the freezing levels are much higher and the stones have largely melted by the time they reach the surface.

• Most damaging hail occurs in summer thunderstorms in temperate latitudes in summer over land.