Transcript Radiation and Climate_Earth`s Energy Balance

Radiation and Climate
Earth’s Energy Balance
Earth’s Energy Balance
• The mild average temperature (15°C, or 59°F) at Earth’s
surface is determined partly by a balance between the
inward flow to Earth of the Sun’s energy and the outward
flow into space of solar energy following its interaction with
the Earth and its atmosphere.
• Certain properties of Earth’s surface and atmosphere help
determine how much thermal energy our planet can hold
near its surface—where, of course, terrestrial life resides—
and how much energy Earth radiates back into space.
• The combination of these two factors helps establish a
balanced energy flow, leading to a hospitable climate here
on Earth.
Solar Radiation Reflection
• 30% of incoming solar radiation never reaches
Earth’s surface, but is reflected directly back
into space by clouds and atmospheric
particles.
• Solar radiation is also reflected when it strikes
materials, such as snow, sand, or concrete on
Earth’s surface.
– In fact, visible light reflected in this way allows
Earth’s illuminated surface to be seen from space.
Solar Radiation Transmission
• Of the remaining 70% of incoming solar radiation
that actually reaches Earth’s surface, about 2/3 is
absorbed, warming the atmosphere, oceans and
continents.
• The other 1/3 of this energy powers the
hydrologic cycle.
– Solar energy causes water to evaporate from the
oceans and land masses.
– The water condenses to form clouds, which then
release water back to Earth as precipitation.
1/3 of solar energy that reaches Earth powers
the hydrologic cycle. Imagine how much thermal
energy was generated as water vapor
condensed in this developing thunderhead.
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Reradiation
• All objects with temperature above zero Kelvins radiate
energy.
• The quantity of this radiated energy is directly related to an
object’s Kelvin temperature.
• Specifically, Earth’s surface reradiates most absorbed solar
radiation, but usually at longer wavelengths (lower energy)
than that of the original incoming radiation.
• Certain types of molecules in the air do not absorb the
Sun’s UV and visible radiation, allowing it to reach Earth’s
surface, but absorb any IR radiation that is reradiated from
Earth’s surface, thus holding warmth in the atmosphere.
Greenhouse Gases
• Carbon dioxide and water readily absorb IR
radiation, as do methane (CH4), nitrous oxide
(N2O), and halogenated hydrocarbons such as
CF3Cl and other chlorofluorocarbons (CFCs).
• Because clouds are composed of droplets of
water or ice, they absorb IR radiation.
– Energy absorbed by these molecules in the
atmosphere is reradiated in all directions.
– Thus, energy can pass back and forth between Earth’s
surface and molecules in the atmosphere many times
before it finally escapes into outer space.
Greenhouse Effect
• This trapping and returning of infrared
radiation by carbon dioxide, water, and other
atmospheric molecules is known as the
greenhouse effect because this process
resembles, to some extent, the way thermal
energy is held in a greenhouse (or in a closed
car) on a sunny day.
• Atmospheric gases that effectively absorb IR
radiation are classified as greenhouse gases.
Solar Radiation Energy Balance
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Venus’ Atmosphere
• Without water and carbon dioxide molecules in the atmosphere to
absorb and reradiate thermal energy back to Earth, scientists
estimate that our planet would have an average temperature of a
frigid -18°C (0°F).
• At the other thermal extreme is the planet Venus, demonstrating a
runaway greenhouse effect.
– The Venusian atmosphere is composed of 96% carbon dioxide (and
clouds made of sulfuric acid), which prevents the escape of most IR
radiation.
– The average surface temperature on Venus (450°C) is much higher
than on Earth (15°C).
– Although some of this difference is due to their planetary positions
relative to the Sun, Venus (the second planet from the Sun) is actually
hotter than Mercury (the planet nearest to the Sun).
Climate Connections
• In addition to maintaining a habitable average
temperature on Earth, the interaction of solar radiation
with Earth’s atmosphere is a major factor in
determining climates and weather.
• Radiant energy from the Sun warms Earth’s land and
water surfaces.
– Earth’s warm surfaces, in turn, warm the air above them.
– As warmer air expands, its density decreases. This warmer
air becomes displaced by colder, denser air, causing the
warmer air to rise.
– These movements of warm and cold air masses help create
continuous air currents that drive the world’s weather.
This hang glider depends on strong air
currents.
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creation/hanggliding.shtml
Weather Conditions
• The average or prevailing weather conditions in a
region, commonly referred to as climate, are
influenced by other factors:
– Earth’s rotation, which causes day and night and also
influences wind patterns
– Earth’s yearly revolution around the Sun
– Earth’s tilt on its axis
• The combination of these factors causes uneven
distribution of solar radiation, which results in
four distinct seasons in Earth’s mid-latitudes, a
climatic pattern.
HOMEWORK
1) Compare infrared, visible, and ultraviolet
radiation in terms of how well they are
absorbed by the atmosphere.
2) Describe two main effects of the solar
radiation that reaches Earth’s surface.