Download Clouds and radiation

MET 12 Global Climate Change - Lecture 4
Clouds and global climate
Shaun Tanner
San Jose State University
 By the end of this chapter you should:

Understand the role of water in the Earth system

Know the roles of clouds

Understand how high clouds differ from low clouds in terms of albedo
and absorption
1
Questions
 What role do clouds play on the Earth’s
climate?
 What would happen to our climate if clouds
were to increase/decrease?
2
3
4
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!)
5
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
7
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.
8
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?
Explain your answers.
9
10
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?
11
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
(sun 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
12
Changes in clouds
 Increases in low level clouds will:
–
 Increases in high level clouds will:
13
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)
14
15
16
17
18
19
20
How would the earth’s climate change as
a result of aircraft contrails.
21
22
Definitions
 Insolation – Incoming solar radiation
 Solstice – day of the year when the sun shines
directly over 23.5°S or 23.5°N
 Equinox –
days of the year when the sun shines
directly over the equator
24
25
26
What influences incoming solar
energy?
 The Sun’s angle of incidence:
– Lower sun angle, less incoming energy
– Higher sun angle, more incoming energy
 Length of time the Sun shines each day:
– Summer season, more sun hours
– Winter season, less sun hours
27
What month do you think this graph represents?
a) December b) March c) June d) September
28
Review questions
 On June 21st, at what latitude is the sun directly
overhead at noon?
 On September 22nd, at what latitude is the sun
directly overhead at noon?
 How many hours of daylight are present at the
South Pole on February 20th?
 Where would you expect to have longer days;
45 ° N on June 21st or 50°S on Dec 21st?
30