Download Lecture 7

Seasons
Mid-chapter review
• The
Earth absorbs solar radiation only during the daylight hours;
however, it emits infrared radiation continuously, both during the
day and at night.
• The
Earth’s surface behaves (almost) as a blackbody, making it a
much better absorber and emitter of radiation than the
atmosphere.
• Water
vapor and carbon dioxide are important atmospheric
greenhouse gases that selectively absorb and emit infrared
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radiation, thereby keeping the Earth’s average surface temperature
warmer than it otherwise would be.
Cloudy, calm nights are often warmer than clear, calm nights
because clouds strongly emit infrared radiation back to the Earth’s
surface.
It is not the greenhouse effect itself that is of concern, but the
enhancement of it due to increasing levels of greenhouse gases.
As greenhouse gases continue to increase in concentration, the
average surface air temperature is projected to rise substantially
by the end of this century.
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RECAP
The Sun is the ultimate energy source for our atmosphere
♦ The peak of the solar emission is in the visible
wavelengths
♦ As the solar light travels through the atmosphere it is
Reflected (albedo) by clouds and particles
Absorbed by air molecules and clouds
Scattered by molecules and particles
Transmitted to the surface of the Earth
♦ The latter two portions heat the Earth’s surface,
which, in turn, warms the air above
The maximum of the Earth emission is in the infrared (IR)
part of the spectrum
The Earth is close to thermal equilibrium: the amount of
received energy is equal to the amount of emitted energy
♦ Jupiter, for example, is not in thermal equilibrium
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The solar energy budget
The Earth Energy Balance
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Seasons
The long-term (monthly) weather on the Earth varies:
♦ With time (summer, winter…)
♦ With latitude (equator versus middle latitudes)
♦ With the geographic location (coastal areas versus
continental areas)
These variations of the climate are a result of:
♦ Different length of the day (the time during which a
given latitude receives energy from the sun)
♦ Different amount of solar energy that is received at a
given location at a given time of the year
♦ The different response of the Earth surface to the
incoming solar energy (oceans and continents)
How much solar energy do we get?
• The
amount of solar energy
received at the Earth’s surface
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depends on several factors
♦ Distance to the sun (not
important, varies a few %)
♦ How long the sun shines
♦ The angle at which the sun
rays strike the Earth’s surface
All three factors depend on the
season
The orbit of the Earth around the Sun
The Orbit of the Earth
The Orbit of the Earth
• The Earth orbits the Sun on a slightly elliptical (almost circular) orbit.
• The
Earth is closer to the Sun during the northern hemisphere (NH)
winter than during the NH summer
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♦ One would conclude that during the winter the temperatures should
be higher since the Sun is closer???
♦ The change of the distance to the Sun is less than 4%
♦ This has NO significant impact on the seasonal climate!
The Earth axis of rotation is tilted with respect to the ecliptic at 23.5
degrees.
♦ The inclination of the Earth axis is the reason for the changing
weather during a year.
The duration of the day changes
The angle at which the sun rays hit the surface of the Earth at
a given latitude changes.
♦ What would happen if the tilt was larger? What if it was smaller?
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The solar energy at different latitudes
Seasonal Variations of the Received
Solar Energy
Seasonal Changes on the Earth:
vegetation and seawater temperature
Summer in the Northern Hemisphere
• The noon Sun shines straight overhead at 23.5 N.
• The length of the day increases moving north
• North of the Arctic Circle (66.5 N) the sun does not set down (Jun 21)
• Insolation = Incoming solar radiation
• The maximum solar insolation is at the pole (sun shines all day!).
• The
maximum surface temperature is at about 30N (Why?) At higher
angles the path of the solar light through the atmosphere is longer and
more of the light is absorbed, scattered or reflected in the atmosphere.
The albedo of the Earth at high latitudes (snow) is also higher.
Midnight Sun!
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The path of the sun above the horizon around midnight
during the polar summer north of the Arctic Circle.
Winter in the Northern Hemisphere
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• The noon sun is lower on the horizon -the average in•
The length of the day decreases to the north.
coming solar energy is less than during the summer.
The polar region is not illuminated for about 6 months.
Seasons in the Southern Hemisphere
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• The thermal response to the incoming solar energy is
The seasons in the SH are just the opposite to the
seasons in the NH.
somewhat different:
♦ A larger percentage (81%) of the SH is covered with
oceans.
♦ The specific heat capacity of water is higher than
that of land (soil).
♦ Using the same amount of solar energy the
temperature of the oceans will increase less than the
temperature on the continents.
♦ The temperatures during the winters and the
summers in the SH are on average somewhat less
extreme than in the NH.
The Energy Balance
• Blue – IR energy emitted by the Earth
• Red - solar energy received by the Earth
• Low latitudes receive more energy than they emit- warming
• Higher latitudes receive less energy than they emit- cooling
• The
energy in the atmosphere is redistributed by heat transfer
(convection, conduction, radiation).
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The Sun’s position in the sky.
In the continental US the
sun is never directly
overhead
In the winter
♦ the sun is rising south
of east
♦ The noon sun is very low
In the summer
♦ The sun is rising north
of east
♦ The noon sun is high
The Sun’s position in the sky