Clouds and
Precipitation
Earth Science
Ms. Kurtzweil
Unit Objectives
 6.
Describe what happens when the
temperature of air drops to or below the dew
point.
 7. Identify the conditions that must exist for a
cloud to form.
 8. Describe the various types of liquid and
solid precipitation.
Basics
 Evaporation
is the changing of water
from a liquid to a gas

Liquid water on Earth entering the atmosphere
as water vapor (when energy is added aka
sunlight)
 Condensation
is the changing of
water from gas to a liquid

Water vapor in the atmosphere condensing into
liquid water droplets in a cloud (when energy is
released = cooled)
* When water reaches the dew point, or saturation point,
it will begin to condense into water droplets that form
a cloud.
Four ways that air may cool enough below the dew
point to begin condensation:
1. Contacting a colder surface (blade of grass, cold glass)
2. Radiating heat
3. Mixing with colder air
4. Expanding when it rises
“Expanding when it rises” is KEY to
cloud formation!
Frost

If the temperature is
below the freezing,
the dew point is called
the frost point.
 If the surface
temperature falls
below the frost point
water vapor is
deposited directly as
ice crystals
Before a true cloud can form, water vapor still needs
something to condense upon!
Condensation nuclei: tiny particles on which water vapor
condenses.
-The foundations of water droplets in a cloud
Examples of possible condensation nuclei
in our atmosphere:
1. Salt
2. Dust
3. Ice
4. Smoke
5. Sulfate/nitrate particles
Cloud Seeding during droughts
How Do Clouds Form?
1. Water leaves the Earth’s surface via one of 2 ways:
2. Warm, moist air is less dense; it rises high up into the
atmosphere
3. At high altitudes, lower air pressure allows the air mass to
expand and cool… eventually to the dew point
4. Water condenses on condensation nuclei in the
atmosphere, and…
Voila!!!
A cloud is born!
I’ve heard that fog is just a cloud
near to the ground. Is this true?
 For
the most part, yes. There are different
types of fogs, radiation or advection, but
overall, fog is formed nearer to the ground
than a cloud. This occurs when moist air close
to the ground cools to its dew point.
 But clouds are thicker than fog, you say? As
you fly through a cloud in a jet, you will see
that clouds only appear as thick as cotton
balls from far away!
Radiation Fog





Forms under similar conditions as dew
On calm, clear nights, the ground will quickly lose heat
via radiation
Cold air sinks and cools
A whole layer of air is cooled below the dew point and
fog forms
Common in humid valleys, near rivers or lakes
OK, back to
clouds…
Cloud Types

The shapes of clouds show how air is moving
through them.


Rising? Horizontally? Stable?
3 main cloud types, based on their location in the
sky

Cirrus “curly”
• Highest family of clouds in the sky

Stratus “sheetlike” or “layered”
• Mid-level clouds

Cumulus “piled” or “heaped”
• Lowest clouds, closest to Earth’s surface
Cloud Type
Cirrus
What kind
Description
Picture
of weather?
Thin, feathery, Just before
snowfall or
ice crystals
rainfall
Stratus
Low sheets or
layers
Cumulus
Fair weather,
Thick, puffy
masses, “cotton maybe leading to
abrupt storms
balls”
Little
precipitation to
heavy rains or
snowfall
 Video:
Storm Clouds
Precipitation
 The
falling of any form of water from the air
to Earth’s surface
 When will water leave a cloud and fall to the
ground as precipitation?

Basically, when gravity tells it to. Like thick beads
of condensation on a glass, when cloud droplets
grow into drops heavy enough to fall to Earth,
precipitation occurs.
4 major types of precipitation

Rain


Large droplets (up to
0.25 cm in diameter)
that are far apart and
fall rapidly
Snow


Clumps of 6-sided
crystals that grow by
collision
The most common type
of solid ppt
4 major types of precipitation

Sleet


Ice pellets that form
when rain falls through
a layer of freezing air
What about “black ice”
or “glare ice”?
Sometimes in an ice
storm, supercooled rain
drops will freeze
instantly as they come in
contact with a cold
surface such as roads,
roofs, and power lines
4 major types of precipitation

Hail
 Solid ppt in the form of lumps of
ice
 Begins as raindrops falling from a
cumulonimbus cloud
 Convection currents toss the
droplet high up into the cloud
where it freezes. It then falls to a
lower level and water condenses
on it as a liquid. The tossing and
freezing process repeats itself and
the hailstone keeps adding layers
and growing larger.
How Does Hail Form

http://www.ehow.com/video_4872174_whatcauses-hail.html
Experience a Hailstorm
 http://www.komonews.com/weather/blog/3
1359739.html