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Precipitation
What is precipitation?
• Why, it is any form of water that falls
from clouds and reaches Earth’s
surface.
• But not all clouds produce precipitation.
• For precipitation to occur, cloud
droplets or ice crystals must grow
heavy enough to fall through the air.
When you listen to the weather report and hear
that there is a chance for light rain or that it is
currently drizzling, there is actually a scientific
method for defining the terminology.
Now, if the person making the report is just a
local broadcaster looking out the window to
render an opinion, then the definitions listed in
the next slide are not guaranteed to apply, but if
the forecast is coming from a certified
weatherman, then there is a pretty good chance
that the number of drops per second per square
foot really does fall within the ranges listed.
Number of Drops
(per ft2 per sec)
Diameter of Drops
(mm)
Intensity
(in. per hr.)
113
2.85
4.0
Excessive Rain
76
2.4
1.6
Heavy Rain
46
2.05
0.6
Moderate Rain
46
1.6
0.15
Light Rain
26
1.24
0.04
Drizzle
24
0.96
0.01
Mist
2,510
0.1
0.002
Fog
6,264,000
0.01
0.005
Cloudburst
One interesting tidbit…
What is the technical difference between
rain and a shower?
Rain refers to participation from clouds like
stratus and altostratus, and tend to occur
over a large area for a long time. Showers
originate from cumulus and cumulonimbus
clouds and cover a relatively small area for
a short time.
Types of Precipitation
•
•
•
•
•
Rain
Sleet
Freezing Rain
Snow
Hail
• In cold air way up in the sky, rain
clouds will often form. Rising
warm air carries water vapor high
into the sky where it cools, forming
water droplets around tiny bits of
dust in the air. Some vapor freezes
into tiny ice crystals which attract
cooled water drops. The drops
freeze to the ice crystals, forming
larger crystals we call snowflakes.
Rain
• When the snowflakes become
heavy, they fall. When the
snowflakes meet warmer air on the
way down, they melt into
raindrops. In tropical climates,
cloud droplets combine together
around dust or sea salt
particles. They bang together and
grow in size until they're heavy
enough to fall.
Sleet.
• Sometimes there is a layer of air in the
clouds that is above freezing, or 32
degrees F. Then closer to the ground
the air temperature is once again below
freezing. Snowflakes partially melt in
the layer of warmer air, but then freeze
again in the cold air near the ground.
This kind of precipitation is called
sleet. It bounces when it hits the
ground.
Freezing Rain
• However, if snowflakes completely melt in
the warmer air, but temperatures are below
freezing near the ground, rain may freeze on
contact with the ground or the streets. This
is called freezing rain, and a significant
freezing rain is called an ice storm. Ice
storms are extremely dangerous because the
layer of ice on the streets can cause traffic
accidents. Ice can also build up on tree
branches and power lines, causing them to
break and our lights to go out.
Snow
• Snowflakes form when water vapor
freezes into ice crystals in cold
clouds. The ice crystals attract cooled
water droplets to form various
shapes. They get heavy and fall. If the air
is cold enough, the snow falls all the way
to the earth without melting. If the ground
is freezing, the snowflakes stick to the
ground.
Hail
• There is another kind of precipitation that comes
from thunderstorms called hail.
• Hail is formed through a sort-of roller coaster ride
through intense thunderstorms. Strong convection
currents lift small ice pellets high into the middle and
upper portions of a cumulonimbus cloud. This is
where super-cooled water droplets collide and the
ice pellet grows through a process called accretion.
Once the pellet is too heavy for the updrafts to keep
it within the cumulonimbus, it begins to fall, and if it
does not melt completely before reaching ground
level, it comes out as hail.