Science 3360 Lecture 5: The Climate System
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Transcript of Science 3360 Lecture 5: The Climate System
Science 3360 Lecture 5: The Climate System
The Earths Orbit and Effective Temperature A Brief Review Energy
depends inversely on wavelength
Intensity of radiation decreases as 1/r^2 The intensity of
radiation reaching the Earths surface depends on the angle (beam
spreading) All objects with T>0K emit radiation The Earths Orbit
Earth rotates about its axis 24 hours and about the sun every days.
Note that the tilt or obliquity remains constant. Solstice: Day of
the year with the longest amount of daylight Equinox:Day of the
year when daytime and nighttime are equal Solstice At each
solstice, 1 pole is always illuminated, one is always dark From
physicalgeography.net Equinox Sun is directly overhead at the
equator.
Why would the obliquity of the Earths orbit cause seasons? There
are 3 different reasons From physicalgeography.net The
Seasons:Reason 1 The intensity of radiation is affected by the
angle the beam makes with the horizontal surface.Thehigher the
angle of incidence, the less the beam is spread and the more energy
is received per unit area. The Seasons: Reason 2 As light passes
through the atmosphere it is scattered, diffused, and reflected.The
larger the angle of incidence, the less of the atmosphere the light
must pass through to reach the surface.Thus a higher angle means
less loss so more energy reaches the surface. The Seasons:Reason 3
The length of day changes depending on the day of the year. During
the Equinox the day is 12 hours long everywhere.However, on the
Solstice the length of day can be anywhere from 0 hours (the dark
pole) to 24 hours (the illuminated pole) The Seasons:Recap So
during the boreal summer, the north hemisphere is warmer because
Sunlight hits at a large incident angle so less beam spreading
Higher incident angle so less of the atmosphere to travel through
Day is longer in the boreal summer so longer period of solar
heating Keplers Laws Johannes Kepler formulated 3 laws that
describe how the planets move in the early 1600s The First law
states:Orbit of every planet is an ellipse with the sun at 1 foci
From Wikipedia Why isnt one summer a lot hotter than the other?
Keplers Laws Because the Earths orbit is almost circular Planetary
Energy Balance
Rough Idea:Amount of energy absorbed by the Earth must equal the
amount being emitted Aside: If Global Warming is occurring, are we
in balance? Energy In = Energy In Start with the concept of
Effective Temperature Effective Temperature
Think of it as the temperature of the Earths surface if no
atmosphere was present Also the temperature that a body radiates it
energy Remember: Energy Balance: An idealized Case
Calculate the temperature of a metallic cube orbiting the Sun at
the same distance as the Earth What can we assume? One face of the
cube is perpendicular to the sun Ein=Eout (energy is balanced)
Assume albedo of cube is 0 Energy Balance: An idealized Case
Re Sun L What section of the cube receives solar insolation?What is
its area? What part of the cube radiates energy? Energy Balance: An
idealized Case
System is in radiative balance so Ein=Eout Ein is the energy
received by the cube. Previously found that flux that Earth
receives is 1370 W/m2/second We can ignore time due to radiative
balance If Albedo=0, everything is absorbed so Energy Balance: An
idealized Case
Eout is the energy radiated by the cube. Stefan-Boltzmann says Box
radiates from all sides So we can calculate the energy radiated as
Energy Balance: An idealized Case
We need to solve for T.We have: and Which yields T=252K Energy
Balance: An idealized Case
What if the albedo of the cube wasnt 0? Remember that absorptivity
= 1 A Would Eout be affected by albedo? Energy Balance What about
for a spherical planet?
The area for absorbing radiation is simply a 2-d circle facing the
sun so area receiving sunlight is just Energy Balance What about
the area radiating?The surface area of a sphere is Again using
Ein=Eout we can solve for the effective temperature: or Effective
Temperature of the Earth
Solving for effective temperature yields the equation What is Teff
for the Earth? S=1370 W/m2; = 5.67 x10-8 ; A = .3 Yields Teff of
255K or 0F.Too cold!The average temperature of the Earth is
actually about 60F.Why the discrepancy?The Greenhouse Effect!