ESCI 241 Meteorology
Lesson 10 Precipitation
Reading: Meteorology Today, Chapter 8
GENERAL
l Cloud
droplets are very small (diameter of about 0.02 mm), and do not fall very fast.
o
Terminal velocity The maximum speed of fall of an
object
l The
terminal velocity for cloud droplets is usually smaller than the updraft
velocity, so cloud droplets remain suspended in air.
l In order to
form precipitation, the cloud droplets must somehow grow heavy enough to
fall. A typical raindrop has a diameter
of 2 mm.
l The cloud
droplet must somehow grow from .02 mm to 2 mm.
This represents an increase of a million times in volume!
DIFFUSIONAL GROWTH
l Cloud
droplets initially grow by diffusion.
o
Water vapor diffuses from the air to the cloud
droplet.
l In order
for diffusional growth to occur, the vapor pressure of the air must exceed the
saturation vapor pressure.
l The
saturation vapor pressure over a curved surface is much greater than that over
a flat surface (the curvature effect).
o
This effect decreases exponentially with increasing
radius
o
The smaller the droplet, the greater that saturation
vapor pressure.
o
This means that we need relative humidities greater
than 100% or the droplet will evaporate!
o
Since relative humidity is rarely much greater than
100%, there must be something else we need to consider for droplet growth.
l It turns
out that by adding a non-volatile compound (such as a salt) to a liquid will
lower the saturation vapor pressure (the solute effect).
o
This effect decreases with decreasing radius.
o
For small droplets the saturation vapor pressure can
be much less than 100%
l The solute
effect more than offsets the curvature effect for small radii, and allows small
droplets to grow at relative humidities less than 100%.
o
This is how haze is formed
l Droplets
cannot grow infinitely large via diffusion
o
There is competition for available water among the
droplets. The more droplets, the smaller
each droplet will be.
o
Diffusional growth is slow.
l Diffusion
can only grow a droplet to up to 50 mm, which is too small to be precipitation.
FORMATION OF PRECIPITATION FROM
CLOUD DROPLETS
l Growth of a
cloud droplet into precipitation is accomplished through two main ways.
o
The Bergeron process
‘ The
formation mechanism in cold clouds
‘ Liquid
water exists in the atmosphere at temperatures as low as 40°C.
‘ Freezing
doesnt occur unless there are freezing nuclei present.
‘ Freezing
nuclei are not very abundant.
‘ The
saturation vapor pressure over ice is less than that over liquid water.
‘ Once ice
forms in the presence of supercooled liquid water, the ice crystals will
grow at the expense of the cloud droplets.
‘ The ice
crystals continue to grow until they are heavy enough to fall to the ground.
‘ If the
temperature at the ground is near freezing, the snowflakes will reach the
ground as snow.
‘ If the
temperature at the ground is above about 39 degrees F, the snowflakes will have
melted to form rain.
‘ The
Bergeron process is the primary mechanism for forming precipitation in storms
in the middle latitudes.
o
The collision-coalescence process
‘ How
precipitation is formed in warm clouds.
‘ For the
collision-coalescence process to work, at least a few cloud droplets must be as
large as 20 microns (.02 mm).
‘ These
larger drops fall and collide with smaller droplets. Some of the smaller droplets will just bounce
off, but some will stick, or coalesce.
‘ The drop
continues to grow through collisions. As
it reaches a size of greater than 4 mm, it becomes unstable and breaks apart
into smaller droplets. These smaller
droplets can continue the process of collision with even smaller droplets, and
can continue growing.
‘ The
collision-coalescence process is the main means of precipitation formation in
the Tropics, as well as in stratus clouds.
FORMS OF PRECIPITATION
l Rain Drops of
water falling from a cloud, and having a diameter of greater than 0.5 mm
l Drizzle Liquid
water drops having a diameter of less than 0.5 mm. You can often tell the difference between
rain and drizzle because drizzle usually doesnt cause ripples in standing
water puddles.
l
Snow Ice crystals or aggregates of
ice crystals. The shape of snowflakes
varies with the temperature at which they are formed.
l
Snow grains frozen equivalent of
drizzle (from stratus clouds). Diameter
< 1 mm.
l
Sleet (also called ice pellets)
Sleet is frozen raindrops. If greater than
5 mm in diameter, it is called hail (see note under hail).
l
Snow pellets (also called graupel)
larger than snow grain, but have diameter < 5 mm. Snow pellets are crunchy and break apart when
squeezed. Usually fall in showers from cumulus
congestus clouds. If greater than 5
mm they are called hail (see note under hail).
l Hail Hail
begins as a snowflake that partially or completely melts, and then
refreezes. But, instead of immediately
falling to the ground, it gets caught in an updraft and can make several trips
up and down through the cloud, each time accumulating more ice. Hail is only formed in very strong
thunderstorms (cumulonimbus clouds).
Hail has diameters > 5 mm. If
smaller, it is either snow pellets or ice pellets, depending on its hardness
and crunchiness (see note below).
o
U.S. record hailstone fell in Aurora, Nebraska on June
22, 2003. It was at least 7 inches in diameter and had a circumference of 18.75 inches!
U.S. record hailstone. NOAA
Photo used with permission.
o
NOTE: It is
sometimes difficult to differentiate between hail, snow pellets, and
sleet. Here are some rules to follow:
‘
If it has a diameter larger than 5 mm it is hail.
‘
If it has a diameter less than 5 mm, and is
transparent and solid it is sleet.
‘
If it has a diameter less than 5 mm, is not
transparent, and crunches when squeezed, it is snow pellets.
l
Glaze Also called freezing rain,
glaze forms when supercooled raindrops strike an object and instantly freeze on
impact.
l
Rime Forms in a manner similar to
glaze, only it is caused by the freezing of supercooled cloud droplets rather
than supercooled raindrops. It often
forms feathery ice crystals on trees.
MEASURING
PRECIPITATION
l Rainfall is
measured in inches (or millimeters) per hour.
Any flat bottomed, vertically sided container can be used as a rain
gage.
l Rainfall
rate is classified as
o
Trace less than 0.01 inches per hour
o
Light between 0.01 and 0.1 inches per hour
o
Moderate between 0.1 and 0.3 inches per hour
o
Heavy greater than 0.3 inches per hour
l Snowfall is
measured in one of two ways. Either the
depth of the snow, or the depth of the liquid water content.
o
Liquid water content is measured by melting the snow
and then measuring the height of the resulting water.
o
On average, 10 inches of snow is equivalent to 1 inch
of rain. But, heavier snow has more
water, and light, powdery snow has less water.
o
The temperature at which the snowflakes formed
determines how much water they contain.
l Radar can
also be used to estimate precipitation rates and amounts.
WEATHER MODIFICATION
l Attempts
have been made to seed cold clouds with dry ice or silver iodide crystals in
order to enhance the Bergeron process.
This is done to either enhance precipitation, or to disperse fog. It can be somewhat successful on a small scale
(such as trying to clear fog at an airport).
l Warm clouds
are seeded with salt particles in an effort to get a few large drops to form to
initiate the collision-coalescence process.
l Inadvertent
weather modification occurs when mans activities seed clouds. There is evidence that this is occurring near
industrial areas.
l The jury is
still out on how effective cloud seeding is.
There are also legal and ethical issues involved.