Precipitation development; Warm and Cold clouds. >0 ° C. <0 ° C. Last lectures from me…. Cloud droplet formation (micro-scales) Cloud/fog formation processes (macro-scales) This lecture – return to the micro-scales. Cloud droplets and Raindrop sizes. r = radius in m
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r = radius in m
n = numberconcentrationper litre
v = terminalfall speedin cm/s
How do droplets grow and become raindrops?
Why doesn’t it always rain when there are clouds?
A: Updrafts can keep small cloud droplets suspended
Need stronger updraughts to support larger drops…
Max. size ~8-10 mm
What do rain drops look like?
equivalent diameter (mm)
of rain drop
turn into rain drops (1 mm) ?
Initial growth by condensation, but this is limited by diffusion…
They never get a chance to grow into raindrops by condensation alone – this process would take D A Y S . . .
collision and coalescence
in warm clouds
It takes about 106 small cloud
droplets (10 mm) to form one
large raindrop (1000 mm)
Stochastic model of collisions and droplet growth
Start with 100 drops
In 1 timestep, 10% grow
Next step, repeat…
End up with a logarithmic size distribution…
Actually, more complicated…
Raindrops reaching Earth’s surface rarely exceed 5 mm (5000 mm). Collisions or
glancing blows between large raindrops break them into smaller drops.
Also surface tension is too weak to hold the larger drops together
Drop diameter, D (mm)
Drop diameter, D (mm)
Distribution of raindrop sizes – raindrop spectra
No. of drops in each class size per m3
the Marshall-Palmer distribution
n(D) = noe-ΛD
no = 8 x 103 ; Λ= 4.1 Rh-0.21 where Rh is the rainfall rate (mm h-1)
Stratus – thin cloud (<500 m) and has a slow upward
movement (< 0.1 ms-1).
Growth by coalescence wouldn’t produce a droplet
more than about 200 mm.
If RH below the cloud is high, then the droplets will arrive at the ground as drizzle, defined as diameter of drop < 500 mm (0.5mm).
Thicker clouds, formed by convective motion, can have stronger updrafts and can keep larger cloud droplets aloft, permitting them to join (coalesce) with more droplets and grow to greater sizes.
but a 1000 mm (1mm) drop will not freeze until T ≈ -11 °C.
Cold clouds (temperate latitudes and polewards).
Does water always freeze at 0 °C ?
It depends … on its volume and the presence of ice nuclei.
For ice to form all the water molecules must align in the proper crystal structure – in a large volume there is a high chance a few of the molecules will line up in the proper manner whereas in a small volume of water the chances are reduced, simply because there are fewer molecules
cloud droplets frozen
at different temperatures
1 in 106
1 in 105
1 in 104
1 in 102
Ice or freezing nuclei aid the freezing process
c.f aitken nuclei (<0.2 mm) for condensation nuclei.
1 cm3 of pure water in a test tube wouldn’t freeze
until T was about -3 to – 5 °C.
- are less common than Aitken nuclei
- kaolonite (clay) minerals are effective ice nuclei
- are most effective at about -10 °C
If you look at the area in-between the two SVP curves you’ll see that an air parcel here would be unsaturated with respect to water but supersaturated with respect to ice. That means net evaporation will take place from the water but net condensation to the ice.
Bergeron process arises
so ice grows at the expense of
supercooled water droplets
you have to defrost
your freezer regularly…
Sheets of molecules – viewed from above
Why are snowflakes hexagonal? …it’s complicated!
Shape of H2O molecule and H-bonding gives rise to hexagonal crystals