ATOC 181 Lecture Notes - Lecture 13: Wilson Bentley, Electric Field, Nimbostratus Cloud
LESSON 13
PRECIPITATION
Snow, Hail = precipitation
Definition of Precipitation
• Water falling from the clouds, transport of water to the grounds from the clouds
• From WHO: any form of water (liquid or solid) that falls from a cloud and reaches the ground
• In the tropics, in the summertime, have rain and it stops just before touching the ground
• To be precipitation, need to touch the ground
Condensation and Growth
• Clouds particles grow through condensation and deposition of water vapour onto appropriate nuclei.
• Before or as soon as RH = 100%. Will have accumulation of water around the condensation nuclei and
will grow. Ultimately, will grow enough and be big enough to fall.
• As long as RH is high enough, condensation and/or deposition exceed evaporation and the cloud droplet
grows
• Can the condensation-deposition be enough for precipitation?
• Condensation nuclei= ~0.2 micron. The size around which the water will start condensing and growing
• Cloud droplets = 20 micron.
o Volume is 1million times bigger.
• Drizzle: smallest rain drop (or large cloud droplet) = 200 micron
• Typical rain drop = between 2mm to 5mm.
o volume 1million times bigger than typical cloud droplets
Condensation and Deposition
• Few minutes for a droplet to grow to 5micron around an hygroscopic CCN = takes about 1hr
• Takes another hr to be 30micron in diameter
o So takes 2 more hrs to reach a diameter of 30 micron
• Will take days and weeks for a rain drop to fall.
• The minimum diameter for liquid precipitation is 100 micron (very fine drizzle)
• Droplets too small to fall as rain
o In suspension by upwards currents
• The droplets that fall slowly would evaporate before reaching the ground
• Therefore, precipitation is impossible because the entire process is too slow (several days to produce a
raindrop). But we know it is raining and snowing!
Terminal Velocity
• Velocity that an object falls towards the ground
• Cloud droplets = 0.01m/s. also have winds that will pick
up and prevents the droplets to fall. Will have plenty of
time to evaporate
• Raindrop = 16km/h
• Hailstone = 113km/h
Processes That Produce Rain
• Two important processes leading to rain have been identified
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o Collision-coalescence (warm-cloud process)
o Ice-crystal (Bergeron = Swedish scientist) (cold cloud process)
Collision-Coalescence
• Significant process for warm clouds (tops warmer than -15oC)
• Require large differences between size of droplets
o Require a number of large droplets among smaller ones
• Larger droplets fall faster (terminal velocity) and will collide with smaller ones
• If all the same size, fall at same speed and not collide
• Small droplets are captured by large droplets either from the front or the back (forward edge and
backside of the falling drop)
• When have collision, the 2 particles can bounce: smaller particles instead of being captured can just be
repelled
• Coalescence: merging of cloud droplets by collision with larger falling droplets. As they collide, big ones
capture the smaller ones
o Does not always have this
o Can be due to surface tension = have bouncing effect. Can prevent smaller droplets of the same
size to coalesce
o Have electrical charges, the process is enhanced if the 2 particles/colliding droplets have opposite
electrical charges
o Collision does not always lead to coalescence
• The amount of rain depends on the size of the final rain drops, which depends on the time spent inside
the cloud
• Depends also on the cloud thickness
o A 0.2mm droplet will spend 12min falling through a 500m thick cloud and over 1hr through a
2500m thick cloud (In still air)
• Presence of strong updrafts increase the time a falling drop spends colliding with the other droplets
Raindrop Break-Up
• Raindrop that fall as rain to the ground rarely exceed 5mm in diameter
• Collision between raindrops tend to break them
• When larger drops collide and combine, oscillations can break the raindrop apart
Raindrop Shape
• Not really like the one seen in cartoons
Factors In Warm Rain Production
• Liquid water content (main factor)
• Size distribution: range of droplet sizes
• Cloud thickness: determine how long the cloud droplets will stay in the clouds
• Vertical velocity inside the cloud (updrafts)
• Electrical charges of the droplets and the electric field in the cloud:
o if opposite, increase coalescence. Always have electric field.
Rise and Fall of A Droplet In A Warm Cloud
• Have rising phase: droplets become bigger and bigger
• At some stage, remain in suspension: suspension phase
• At some stage, will be big enough that will start falling especially if no updrafts
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Document Summary
In the tropics, in the summertime, have rain and it stops just before touching the ground. Condensation and growth: clouds particles grow through condensation and deposition of water vapour onto appropriate nuclei, before or as soon as rh = 100%. Will have accumulation of water around the condensation nuclei and will grow. Ultimately, will grow enough and be big enough to fall: as long as rh is high enough, condensation and/or deposition exceed evaporation and the cloud droplet grows, can the condensation-deposition be enough for precipitation, condensation nuclei= ~0. 2 micron. In suspension by upwards currents: the droplets that fall slowly would evaporate before reaching the ground, therefore, precipitation is impossible because the entire process is too slow (several days to produce a raindrop). But we know it is raining and snowing! Terminal velocity: velocity that an object falls towards the ground, cloud droplets = 0. 01m/s. also have winds that will pick up and prevents the droplets to fall.