Meteorology Part 1' Energy and Mass

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MeteorologyPart 1. Energy and Mass

• Chapter 2.
• Solar Radiation and the Seasons

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Introduction

• Energy is the ability to do work
• Solar energy in the form of radiation is
  transferred to Earths surface where it is
  absorbed
• Radiation provides energy for atmospheric motion
  and weather processes
• Energy transfer is always from hot to cold

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Forms of Energy

• Energy can be classified as either kinetic or
  potential energy
• Kinetic energy is energy of motion
• Potential energy is energy in reserve.
• Power is the rate at which energy is used or
  released
• Nearly all the energy available to the earth
  comes from the sun

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Examples of Kinetic and Potential Energy
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Examples of kinetic energy
Gas molecules have no bonds to other molecules
and move in random motion
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Conduction

• Heat moves by conduction, convection and
  radiation
• Conduction is energy transfer from molecule to
  molecule
• Heat moves though a metal spoon by conduction
• Heat moves into the ground by conduction
• Heat moves from a surface to the thin (1 to 2 mm)
  layer of surrounding air by conduction
• Even solids have internal movement

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Convection

• Convection involves the movement energy by means
  of a fluid
• Both liquids and gases can move energy by
  convection
• A radiator uses natural convection currents to
  circulate warm air through a room
• A forced-air furnace uses forced convection to
  move heat into a room
• Winds are natural convection currents
• Bulk movement of air is also called advection

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Radiation

• Energy transfer requiring no physical medium
• Propagates through vacuum of space
• Continually emitted by all substances
• Differ in wave properties
• Waves are perpendicular to each other
• The earth is warmed by radiation from the sun

Electromagnetic radiation. E electric wave M
magnetic wave
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Radiation quantity

• Refers to amount of energy transferred
• Expressed through wave amplitude
• Wave quality
• Relates to radiation wavelength
• Identifies the type of radiant energy
• Travels at a constant speed
• 300,000 km/sec (186,000 mi/sec), the speed of
  light

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Electromagnetic energy

• Comes in an infinite number of wavelengths
• Categorized into a few individual bands along
  the electromagnetic spectrum
• Visible light is a narrow band bounded by
  infrared and ultraviolet

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Intensity and Wavelengths

• All matter radiates energy over a wide range of
  electromagnetic wavelengths
• Physical laws defining amount and wavelength of
  emitted energy apply to hypothetical perfect
  emitters of radiation known as blackbodies
• The Earth and Sun are similar to blackbodies

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Stefan-Boltzmann Law

• The amount of energy emitted (intensity of
  radiation) proportional to temperature of the
  object
• Hotter objects emit more energy
• Energy emitted proportional to fourth power of
  emitters absolute temperature

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Weins Law

• Radiation emission is across a wide array of
  electromagnetic wavelengths
• Useful to determine the wavelength of peak
  emission, Weins Law ?max 2900/T
• Tells us that hotter objects radiate at shorter
  wavelengths than cooler bodies
• Largest portion of solar radiation emitted in
  visible spectrum
• Largest percentage of terrestrial radiation
  emitted as longwave radiation

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TEMPERATURE

• DEFINITIONS
• Average kinetic energy of molecular motion
• How hot or cold something is
• Indication of direction of energy flow hot to
  cold
• THERMOMETER
• Instrument that can show its own temperature

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TEMPERATURE

• TEMPERATURE SCALES
• Fahrenheit scale -- 32 to 212 OF
• Celsius scale-- 0 to 100 OC
• Kelvin scale -- 273 to 373 K
• Absolute zero-- 0 K or - 273 OC

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Solar Constant

• Intensity of electromagnetic radiation not
  reduced with distance through vacuum
• A reduction of intensity proportional to distance
  only as energy distributed over larger area
• Due to this, radiation intensity decreases in
  proportion to distance squared
• Calculating this inverse square law for Earths
  average distance from the Sun yields solar
  constant of 1367 W/m2
• Solar emission 3.865 x 1026W divided by
  distance surrounding the Sun 4?(1.5 x 1011m)2
  1367 W/m2

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Perihelion, closest in January
Orbit is elliptical
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Causes of the Earths Seasons

• Variations in incoming solar radiation
• Earth revolves the Sun in an ecliptic plane
• Distance varies
• Perihelion (Jan 3 147 mil km, 91 mil mi)
• Aphelion (July 3 152 mil km, 94 mil mi)
• Total variation is about 3
• Using the inverse square law, radiation intensity
  varies by about 7 between perihelion and
  aphelion
• The earth revolves once every 365.25 days

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Causes of the Earths Seasons

• Earth Rotation (length of day)
• Earth rotates once every 24 hours
• Axis of rotation offset 23.5o from the
  perpendicular plane
• Because axis of rotation is never changing,
  northern axis aligns with the star Polaris
• Hemispheric orientation changes as the Earth
  orbits the Sun
• A particular hemisphere aligns toward or away
  from Sun, or occupies position between the
  extremes

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Solstices

• Maximum axial tilt in relation to the Sun occurs
  on two dates for each hemisphere
• The June solstice occurs June 21
• The northern hemisphere axis of rotation fully
  inclined 23.5 o toward Sun
• This ensures maximum solar radiation absorption
  throughout hemisphere
• Opposite conditions occur relative to the
  December solstice (around Dec 21)
• First day of summer or winter

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Solstices

• Northern hemisphere summer solstice , June 21
• Subsolar point is at 23.5oN, the Tropic of
  Cancer
• Northern hemisphere winter solstice, Dec 21
• Subsolar point at 23.5oS, Tropic of Capricorn
• Exactly opposite conditions occur relative to
  these dates for the southern hemisphere
• Overall, the subsolar point fluctuates 47o
  between the Tropics (seasons main cause)

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• Equinoxes
• Temporally centered between the solstices
• Hemispheres get same amount of sunlight
• Day and night are equal length
• March equinox, on or about March 21
• September equinox, on or about Sept 21
• The subsolar point lies at the equator

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Solar Angle

• Incident radiation is directly proportional to
  solar angle
• Higher solar angles incorporate reduced
  radiational beam spreading which leads to greater
  heating
• Lower angles induce less intense illumination and
  heating per unit area

Demonstration of the effect of angle of incidence
on energy intensity
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Beam depletion is determined by the amount of
atmosphere the light must pass though on its way
to the surface
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Period of Daylight

• Axial tilt influences day length
• During alignment toward or away from Sun, lines
  of latitude illuminated unequally
• A hemisphere aligned toward the Sun has constant
  daylight poleward of 66.5o
• This line of latitude called the Arctic Circle
• Due to geometry, day length increases from the
  equator to pole of summer hemisphere
• Lines of latitude are equally split for both
  hemispheres on equinoxes ensuring equal day/night
  conditions everywhere

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Beam Depletion

• Solar radiation diminished relative to amount of
  atmosphere it passes through
• High solar angles result in little reduction in
  intensity as the path to surface is short
• Significant beam reduction occurs where energy is
  diffused through larger amounts of atmosphere
• The combined effects of solar angle, day length,
  and beam depletion cause winter hemisphere to run
  a deficit of energy, leading to cooler
  temperatures
• Summer hemispheres have a mean surplus of energy