Precipitation development;
Warm and Cold clouds
>0°C
<0°C
Last lectures from me…
• Cloud droplet formation (micro-scales)
• Cloud/fog formation processes (macroscales)
• This lecture – return to the micro-scales
Cloud droplets and Raindrop sizes
r = radius in m
n = number
concentration
per litre
v = terminal
fall speed
in 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
Radius
(m)
Terminal Velocity
(cm s-1)
Type of Particle
0.1
0.0001
Condensation (Aitken)
nuclei
10
1
Typical cloud droplet
100
70
Large cloud droplet
1000 = 1 mm
650
Typical raindrop
2500 = 2½ mm
900
Large raindrop
Need stronger updraughts to support larger drops…
What do rain drops look like?
equivalent diameter (mm)
of rain drop
Drops break up for larger sizes;
Max. size ~8-10 mm
How do cloud droplets (radius = 10 m)
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 . . .
There are 2 main processes:
1. In ‘warm’ clouds with cloud top T > -15 °C
2. In ‘cold’ clouds with cloud top T <-15 °C
Raindrop formation by
collision and coalescence
in warm clouds
It takes about 106 small cloud
droplets (10 m) to form one
large raindrop (1000 m)
Stochastic model of collisions and droplet growth
‘Statistical’
Start with 100 drops
In 1 timestep, 10% grow
Next step, repeat…
End up with a logarithmic size distribution…
Actually, more complicated…
Cascade process
Raindrops reaching Earth’s
surface rarely exceed 5 mm
(5000 m). Collisions or
glancing blows between large
raindrops break them into
smaller drops.
Also surface tension is too
weak to hold the larger drops
together
No. of drops in each class size per m3
Distribution of raindrop sizes – raindrop spectra
6000
10000
5000
1000
different
rain rates
4000
100
3000
10
2000
1
1000
1
2
3
4
5
6
Drop diameter, D (mm)
1
2
3
4
5
6
Drop diameter, D (mm)
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)
Depth of cloud influences type of rain
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 m.
If RH below the cloud is high, then the droplets will
arrive at the ground as drizzle, defined as diameter of
drop < 500 m (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.
1 Low – Nimbostratus (Ns)
3 Cumulonimbus (Cb)
Supplementary feature: virga
Cold clouds (temperate latitudes and polewards).
Does water always freeze at 0 °C ?
It depends … on its volume and the presence of ice
nuclei.
Ice in your freezer in an ice tray – it’ll freeze at 0 °C.
but a 1000 m (1mm) drop will not freeze until T ≈ -11
°C.
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
Ice nuclei
Ice or freezing nuclei aid the freezing process
c.f aitken nuclei (<0.2 m) for condensation nuclei.
1 cm3 of pure water in a test tube wouldn’t freeze
until T was about -3 to – 5 °C.
Proportion of
cloud droplets frozen
at different temperatures
0
-10
-20
-30
-35
Proportion
frozen
none
1 in 106
1 in 105
1 in 104
1 in 102
-42
all
T (°C)
Ice nuclei
- are less common than Aitken nuclei
- most effective ones have the same crystal shape
as ice crystals hence ice can form around and on them easily.
- kaolonite (clay) minerals are effective ice nuclei
- are most effective at about -10 °C
- because of the relative sparseness of ice nuclei, ice crystals
and supercooled water can coexist at the same time.
- this last point is crucial in the formation of precipitation
in cold clouds as it gives rise to the Bergeron process.
vapour pressure
Bergeron process arises
since svpice<svpwater
so ice grows at the expense of
supercooled water droplets
Super-cooled
water
ice
0 °C
temperature
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.
One of the reasons
you have to defrost
your freezer regularly…
Bergeron
process
Lab ice crystal growing from
super-cooled water drops
Why are snowflakes hexagonal? …it’s complicated!
Angle ~104°
+
+
-
Sheets of molecules – viewed from above
http://www.uwgb.edu/dutchs/PETROLGY/Ice%20Structure.HTM
Shape of H2O
molecule and Hbonding gives rise to
hexagonal crystals
Melting and
re-freezing
gives rise to
vast variety
of snow
flakes
Clouds can be a mixture of water droplets and ice
Summary
• Cloud particle size limited to a few mm by fall
velocity
• Droplets (μm) grow to raindrops (mm) by two
main routes:
– Warm clouds: condensation, collision, coalescence
(then break-up)
– Cold clouds: super-cooled water freezes on ice nuclei
– producing larger ice particles – often melt en route
to surface
• Precipitation can evaporate en route
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