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Add
(remove) heat to (from) the air parcel (diabatic processes) |
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(1)
Conduction: requires touching |
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(2)
Convection: Hot air rises |
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(3)
Advection: horizontal movement of air |
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(4)
Radiation: exchanging heat with space |
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(5)
Latent heating: changing the phase of water |
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Conduction is the process of heat transfer from
molecule to molecule. |
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This energy transfer process requires contact. |
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Air is a poor conductor. (with low thermal
conductivity) |
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Conduction is not an efficient mechanisms to
transfer heat in the atmosphere on large spatial scales. |
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Convection is heat transfer by mass motion of a
fluid (such as air or water). |
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Convection is produced when the heated fluid
moves away from the heat source and carries energy with it. |
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Convection is an efficient mechanism of heat
transfer for the atmosphere in some regions (such as the tropics) but is an
inefficient mechanism in other regions (such as the polar regions). |
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Radiation is heat transfer by the emission of
electromagnetic waves which carry energy away from the emitting object. |
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The solar energy moves through empty space from
the Sun to the Earth and is the original energy source for Earth’s weather
and climate. |
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Virtually all the exchange of energy between the
Earth and the rest of the universe takes place by radiation transfer. |
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Radiation transfer is also a major way of energy transfer
between the atmosphere and the underlying surface and between different
layers of the atmosphere. |
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Radiation energy comes in an infinite number of
wavelengths. |
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We can divide these wavelengths into a few
bands. |
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Solar Energy Absorbed = Terrestrial Energy
Emitted |
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All objectives radiate energy, not merely at one
single wavelength but over a wide range of different wavelengths. |
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The sun radiates more energy than the Earth. |
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The greatest intensity of solar energy is
radiated at a wavelength much shorter than that of the greatest energy
emitted by the Earth. |
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The single factor that determine how much energy
is emitted by a blackbody is its temperature. |
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The intensity of energy radiated by a blackbody
increases according to the fourth power of its absolute temperature. |
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This relationship is called the Stefan-Boltzmann
Law. |
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The absolute zero temperature is the temperature
that the molecules do not move at all. |
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This temperature occurs at –273°C. |
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The Kelvin Scale (K) is a new temperature scale
that has its “zero” temperature at this absolute temperature: |
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K = °C
+ 273 |
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Sun |
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Es
= (5.67 x 10-8 W/m2 K4) * (6000K)4 |
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= 73,483,200 W/m2 |
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Earth |
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Ee
= (5.67 x 10-8 W/m2 K4) * (300K)4 |
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= 459 W/m2 |
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Sun emits about 160,000 times more radiation per
unit area than the Earth because Sun’s temperature is about 20 times higher
than Earth’s temperature. |
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è 204
= 160,000 |
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Wien’s law relates an objective’s maximum
emitted wavelength of radiation to the objective’s temperature. |
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It states that the wavelength of the maximum
emitted radiation by an object is inversely proportional to the objective’s
absolute temperature. |
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Sun |
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lmax
= 2898 mm K / 6000K |
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= 0.483 mm |
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Earth |
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lmax
= 2898 mm K / 300K |
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= 9.66 mm |
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Sun radiates its maximum energy within the visible portion of the
radiation spectrum, while Earth radiates its maximum energy in the infrared
portion of the spectrum. |
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The hotter the objective, the shorter the
wavelength of the peak radiation. |
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Solar radiation is often referred to as
“shortwave radiation”. |
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Terrestrial radiation is referred to as
“longwave radiation”. |
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Solar
Luminosity (L) |
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the
constant flux of energy put out by the sun |
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L = 3.9 x 1026 W |
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Solar
Flux Density (Sd) |
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the
amount of solar energy per unit area on a sphere centered at the Sun with a
distance d |
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Sd = L / (4 p d2) W/m2 |
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Solar
Constant (S) |
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The
solar energy density at the mean distance of Earth from the sun (1.5
x 1011 m) |
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S
= L / (4 p d2) |
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= (3.9 x 1026 W) / [4 x 3.14 x (1.5 x 1011 m)2] |
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= 1370 W/m2 |
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Revolution |
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Earth revolves about the Sun along an ecliptic
plane |
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Total variation is about 3% |
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Using the inverse square law, radiation
intensity varies by about 7% between perihelion and aphelion |
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This variation in radiation intensity is small
and is not the cause of seasons. |
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At present-day, the axis is tilted at an angle
of 23.5°, referred to as Earth’s “obliquity”, or “tilt”. |
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The Sun moves back and forth through the year
between 23.5°N and 23.5°S. |
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Earth’s 23.5° tilt also defines the 66.5°
latitude of the Artic and Antarctic circles. No sunlight reaches latitudes
higher than this in winter day. |
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The tilt produces seasons!! |
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Seasons |
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Solstices: mark the longest and shortest days of the years (June 21
and December 21 in the northern hemisphere, the reverse in the southern) |
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Equinoxes: the length of night and day become equal in each
hemisphere. |
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At the present-day orbit, the winter and summer
solstices differ from the aphelion and perihelion by about 13 days. |
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Period
of Daylight |
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Solar
Angle |
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Atmospheric Bean Depletion |
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Solar zenith angle is the angle at which the
sunlight strikes a particular location on Earth. |
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This angle is 0° when the sun is directly
overhead and increase as sun sets and reaches 90 ° when the sun is on the
horizon. |
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The larger the solar zenith angle, the weaker
the insolation, because the same amount of sunlight has to be spread over a
larger area. |
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Solar angle = 90° - (solar zenith angle) |
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Solar angle = (90° - latitude of the location) +
solar declination |
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Example for Toronto (located at 44°N) |
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In
spring equinox (March 21) |
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solar declination = 0° |
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solar angle = (90° - 44°) + 0° = 46° |
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In
summer solstice (June 21) |
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solar declination = 23.5° |
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solar angle = (90° - 44°) + 23.5° = 69.5° |
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In
winter solstice (December 21) |
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solar declination = -23.5° |
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solar angle = (90° - 44°) + (-23.5°) = 22.5° |
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When the zenith angle is large, sunlight has to
pass through a thicker layer of the atmosphere before it reaches the
surface. |
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The thinker the atmospheric layer, more sunlight
can be reflected or scattered back to the space. |
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