Transcript Introduction - San Jose State University

MET 112 Global Climate Change - Lecture 4
Clouds and Global Climate
Dr. Craig Clements
San Jose State University
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MET 112 Global Climate Change
In class assignment #2
How does this situation affect the climate?
(2-3 sentences)
Questions
 What role do clouds play on the Earth’s
climate?
 What would happen to our climate if clouds
were to increase/decrease?
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Water in the atmosphere
 Definitions:
– Evaporation: Process where a liquid changes into a gas
– Condensation: Process where a gas changes into a liquid
– Precipitation: Any liquid or solid water that falls
from the atmosphere to the ground.
(i.e. RAIN!)
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Water freely
evaporating and
condensing
Since more water
molecules are
evaporating than
condensing, then net
evaporation is
occurring.
Lid on:
Now, evaporation
and condensation are
equal. The air above
water is now called
‘saturated’.
The humidity is now
100%
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Condensation

The process by which water vapor changes to a
cloud droplet

Water vapor molecules may ‘stick’ to
condensation nuclei and grow (billions) to
eventually form cloud droplet.

Examples of condensation nuclei include:
a. Dust
b. Salt
c. Smoke
 Condensation occurs primarily as temperature cools:
-colder the molecules more likely they are to ‘stick’ to
other molecules
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http://www.ssec.wisc.edu/data/comp/cmoll/cmoll.html
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Clouds and radiation
Cloud - Climate Interactions
Albedo effect - COOLING
 Clouds reflect incoming solar radiation.
 The cloud droplet size and total water content
determine the overall reflectivity.
Greenhouse effect - WARMING
 Clouds are good absorbers (and emitters) of long
wave (infrared) radiation.
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Clouds and day to day
temperatures
Imagine that you are going camping in the Sierras
with your friends. On the first day (and evening) it
is cloudy, while on the second day (and evening) it
is clear. Based on this information alone:
Which day would be warmer?
Which evening would be warmer?
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Which day would be warmer?
1. First day (clear)
2. Second day (cloudy)
3. Both the same
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Which evening would be warmer?
1. First day (clear)
2. Second day (cloudy)
3. Both the same
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Low and High clouds
Consider two types of clouds:
1. Low levels clouds
2. High levels clouds
Q: How is the Earth’s surface energy budget
different for low clouds compared to high
clouds?
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Clouds and climate
Cloud A: Low level, (dark, thick)
Excellent reflector of incoming radiation;
good absorber/emitter of infrared radiation
Cloud B: High level, light
(sub visible or thin)
Fair/poor reflector of incoming radiation;
good/excellent absorber/emitter of infrared radiation
 So, clouds both warm and cool the earth.
 Overall, though, clouds act to cool the
earth
Changes in clouds
 Increases in low level clouds will:
–
 Increases in high level clouds will:
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Changes in clouds
 Increases in low level clouds will:
– cool the surface (cooling outweighs
warming)
 Increases in high level clouds will:
– warm the surface (warming outweighs
cooling)
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Explain how the earth’s climate would
change as a result of aircarft contrails.
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Questions
1. What percentage of the sun’s radiation is
a) absorbed by the Earth’s surface?
b) absorbed by the atmosphere
c) reflected out to space?
2. What percentage of the energy received by the earth’s surface
comes directly from greenhouse gas emissions?
3. If the sun’s radiation was to increase by 10%, how would the
following energy units change (increase, decrease or stay the
same)
a) Energy gained by the Earth’s surface.
b) Energy lost by the Earth’s surface.
c) Energy emitted by greenhouse gases.
d) Energy lost to space.
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