ATOC 181 Lecture Notes - Lecture 13: Wilson Bentley, Electric Field, Nimbostratus Cloud
7 Jun 2018
School
Department
Course
Professor
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
find more resources at oneclass.com
find more resources at oneclass.com
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
find more resources at oneclass.com
find more resources at oneclass.com
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.
