This document discusses clouds and precipitation. It defines evaporation and condensation, and explains how clouds form when air rises and cools to the dew point, allowing water vapor to condense on condensation nuclei. The main cloud types - cirrus, stratus, and cumulus - are identified based on their location in the sky and the weather they predict. The four major types of precipitation - rain, snow, sleet, and hail - are described based on their characteristics and formation processes.

1. Clouds and Precipitation
Earth Science
Ms. Kurtzweil

2. Unit Objectives
 1. Describe what happens when the
temperature of air drops to or below the
dew point.
 2. Identify the conditions that must exist for
a cloud to form.
 3. Name the main types of the clouds and
the weather they predict.
 4. Describe the various types of liquid and
solid precipitation.

3. 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 at the 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)

4. Water Cycle

5. * 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!

6. 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

7. 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

8. How Do Clouds Form?
1. Water leaves the Earth’s surface.
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!

9. “Fog is just a cloud near to the ground.” Is
this true?
 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.
 Fog can be considered a cloud at ground level. Like clouds, fog is made up
of condensed water droplets which are the result of the air being cooled to
the dewpoint, where it can no longer hold all of the water vapor it contains. For
instance, rain can cool and moisten the air near the surface until fog forms.

10. 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

11. OK, back to clouds…

12. Cloud Types
 The shapes of clouds show how air is moving
through them.
 Rising? Horizontally? Stable?
 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

13. Cloud Type Description
What kind of
weather?
Picture
Cirrus High-level
clouds.
Thin, feathery,
ice crystals
Just before
snowfall or rainfall
Stratus Low sheets or
layers
Little precipitation
to heavy rains or
snowfall
Cumulus Thick, puffy
masses,
“cotton balls”
Fair weather,
maybe leading to
abrupt storms
Nimbus Thick, grey
masses
Rain, snow and
thunderstorms

14. Nimbus and Stratus are the clouds
that bring rain, heavy rain or snow.
Thunderstorms are usually
associated to the cumulonimbus .

15. 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.

16. 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

17. 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

18. 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.

20. Cloud Type Description
What kind of
weather?
Picture
Cirrus High-level
clouds.
Thin, feathery,
ice crystals
Stratus Little precipitation
to heavy rains or
snowfall
Thick, puffy
masses,
“cotton balls”
Fair weather,
maybe leading to
abrupt storms
Nimbus Thick, grey
masses