Chapter 8 – Precipitation
It’s harder than you might think to produce raindrops in a cloud!
Problems:
1) Small water droplets (~1µm) evaporate much more readily than a flat water
surface.
2) A small drop of pure water freezes at far below 0°C.
First, why are (1) and (2) true, and second, why do they affect precipitation?
(1) For a small droplet, the latent heat released by each condensing water vapor molecule
has a greater effect on the temperature of the droplet. Also, large curvature means
bonding of water vapor molecules to the droplet is smaller. So, higher than 100%
relative humidity is required for small cloud droplets to grow.
(2) Without ice nuclei present, it is difficult for water to freeze. Ice embryos are formed
and destroyed by thermal motion. In a large drop, the probability is higher that one
will survive. Then drop rapidly freezes.
Example: Say on average, an ice embryo gets large enough at –30 °C every
100µm distance. Then drops of 20 µm will only rarely freeze at that temperature.
Effect of (1) and (2) on Precipitation:
How do large drops form?
Note: Small drops won’t fall out of clouds because of small terminal velocity.
Vt = terminal velocity = constant x
m
ρA
m = mass of falling object
ρ = density of the air through which the mass falls
A = cross sectional area of falling object
(take horizontal slice at largest point)
Mass M
Area A
drag force
v=velocity
Drag force, Fd ,
increases with
increasing speed .
When Fd = Fg ,
v = constant.
gravity
Vt smaller for water than air (ρw > ρair)
Vt larger for larger m
Vt larger for smaller A
2 water drops, which
has a higher Vt ?
d1
d2 > d1
d2
mass ∝ d3
A ∝ d2
(m/A) ∝ d
⇒ Vt ∝
d
Large drops form as they fall through a fog of slower small drops and absorb them. This
is coalescence.
In addition to the above collision and coalescence process, in colder regions of clouds,
rain can form by the ice crystal process :
A few small droplets freeze and are surrounded by thousands of small water
droplets.
Recall: at temperatures below freezing, the vapor pressure of water is greater than that of
ice {see Figure 5.10 (insert)}.
Therefore, water diffuses from water drops to ice crystals and the ice grows while the
water drops shrink.
Eventually, the ice crystals are large enough to fall through the cloud. They may reach
the ground as rain or snow.
Precipitation Types:
Snow, sleet, freezing rain, rain – see Fig 8.22
Rime (rhymes with rhyme!) is the accretion of tiny supercooled water droplets on
something frozen. Usually roughly spherical & not dense.
Rime that gets to be 2-5mm in diameter is called Graupel.
Graupel that reaches ground can be:
1) Soft hail – if ground is above freezing
2) Snow pellet – if ground is below freezing (they bounce and crunch).
Size < 5mm
3) Hail – if graupel stays in cloud more than ~5 min & is carried around by
strong air currents. Hail is larger than snow pellets.
Cumulus Cloud
In wetter areas,
liquid water collects
on surface &
later freezes –
this forms hard,
clear ice layer.
In colder, dryer areas of cloud, a layer of opaque rime forms with many air bubbles
- see fig 8.26 (167lb, 5.5” diameter hailstone).
Instruments:
Rain Gauge
Radar (RAdio Detection And Ranging)
Transmitter
Receiver
rain cloud
Microwave pulses are sent out & some are scattered back to receiver.
-
Brightness (intensity of signal) indicates how much rain
Time of arrival in receiver indicates how far away rain is.
Wavelength is 1 – 20 centimeters
1 centimeter → cloud droplets
Larger waves → rain drops, ice pellets, hail, etc