Chapter 10 Cloud and Precipitation Lesson 32

1. Chapter 10Cloud and PrecipitationLesson 32

2. 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.

3. 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.

4. Coalescence Inside a Warm Cloud Droplets grow by collision and entrainment.

5. 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.

6. 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.

7. 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

8. Ice Crystal Theory NO ICING RIME ICE CLOUDY ICE CLEAR ICE

9. 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.

10. 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.

11. 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.

12. Types of Precipitation • Rain Showersfall 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.

13. Types of PPN cont’d • Virga is precipitation which evaporates before reaching the ground and is most frequently observed in summer.

14. 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.

15. 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.

16. Types of PPN cont’d • Freezing Rainoccurs 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.

17. 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.

18. Ice Pellets

19. 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.

20. 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.

21. 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.

22. 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.