solar radiation and the seasons - universitetet i oslo · solar radiation and the seasons chapter 2...

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Solar Radiation

and the Seasons

Chapter 2 Lecture

Redina L. Herman

Western Illinois University

Understanding

Weather and

Climate

Seventh Edition

Frode Stordal, University of Oslo


Energy

• Energy is traditionally described as “the ability to do work.”

• About one two-billionth of the energy emitted by

the Sun is transferred to Earth as

electromagnetic radiation.

• Some electromagnetic radiation is absorbed by the

atmosphere and some by the Earth’s surface.


• Kinds of Energy – Energy can be classified as either kinetic or potential.

• Kinetic energy is energy in use or motion.

• Potential energy is energy in reserve or stored.

– Power is the rate at which energy is used, released, transferred, or received.

Energy


• Kinetic

Gas molecules have no bonds to other

molecules and move in random motion.

Energy


• Heat Transfer Mechanisms – Energy can be transferred from one place to another by

three processes:

• Conduction

• Convection

• Radiation

Energy


• Conduction – Conduction is the movement of heat through a substance

without the movement of molecules in the direction of the heat transfer (from molecule to molecule).

• Heat moves to the handle of a warmed pot and this is conduction.

• Heat moves into the ground by conduction.

• Conduction is most effective in solid materials.

Energy


• Convection – Convection is the transfer of heat by mixing of a fluid.

– Both liquids and gases can move energy by convection.

• A pot of boiling water is an example of convection.

• Convection in the atmosphere occurs when the heating of the Earth’s surface warms the 1 mm layer of air in contact with the surface.

• Winds are natural convection currents (forced convection).

Energy


• Radiation – Radiation is the transfer of energy that requires no

physical medium (can occur through empty space).

• Continually emitted by all substances

Energy


Characteristics of Radiation

• Radiation Quantity and Quality – Radiation quantity

• Refers to the amount of energy transferred

• Associated with wave height, or amplitude

– Radiation quality

• Relates to radiation wavelength, or the distance between the wave crests

• Identifies the type of radiant energy


• Radiation Quantity and Quality

Electromagnetic radiation

E = electric wave

M = magnetic wave



• Intensity and Wavelengths of Emitted Radiation

– Categorized into a few individual “bands” along the

electromagnetic spectrum, visible light is a narrow band

bounded by infrared and ultraviolet.



• Intensity and Wavelengths of Emitted Radiation – 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.



• Energy radiated by substances occurs over a wide range of wavelengths.




– The intensity of radiation depends on the temperature

raised to the fourth power (Stefan-Boltzmann law):

– The surface of the Sun is about 5800 K (5500°C or 9900°F)

and emits about 64 million watts per square meter.

– Most liquids and solids are graybodies, meaning they emit

some percentage of the maximum amount of radiation

possible at a given temperature.


I = σ T4

I intensity

T temperature

σ Stefan Bolzmann’s constant



– Emissivity refers to the percentage of energy radiated by a

substance relative to that of a blackbody.

– Radiation intensity is a function of both emissivity and

temperature

– Most natural surfaces have emissivities above 0.9 (that is,

above 90 percent of blackbody).


I = ε σ T4

ε emissivity



– For any radiating body, the wavelength of peak emission

(in micrometers) is given by Wien’s law.

– Warmer objects radiate energy at shorter wavelengths

than do cooler bodies.

– Wavelengths less than 4 µm are considered shortwave

radiation.

– Wavelengths longer than 4 µm are considered longwave

radiation.

– Warmer bodies radiate more energy than do cooler bodies

at all wavelengths.




Wiens law

λmax = c/T

λ wavelength

C constant


The Solar Constant

• Intensity of electromagnetic radiation is not depleted or reduced as it moves toward Earth.

• The intensity is reduced as a result of radiation being distributed over a large area, not because of the distance from the Sun.

– Radiation intensity decreases in proportion to the distance

squared.

– Calculating this inverse square law for Earth’s average

distance from the Sun yields a solar constant of 1367 W/m2.

– Solar emission = 3.865 x 1026 W/distance surrounding the

Sun = 4 (1.5 x 1011m)2 = 1367 W/m2.


The Solar Constant


The Causes of Earth’s Seasons

• Earth’s Revolution

– Earth revolves around the Sun once every 365.25 days.

– Earth revolves the Sun in an ecliptic plane annually,

known as the revolution.

– Distance from the Sun varies.

• Perihelion (Jan 3; 147 million km

• Aphelion (July 3; 152 million km

– Using the inverse square law, radiation intensity varies by

about 7 percent between perihelion and aphelion.




– The length of a day is defined by Earth’s rotation, which

occurs every 24 hours.

– Axis of rotation is offset 23.5° from the perpendicular plane.

– Northern axis aligns with the star Polaris.

– As Earth orbits the Sun, the hemispheres are impacted

seasonally.

– A particular hemisphere aligns toward or away from the Sun

or occupies a position between the extremes, creating our

solstices and equinoxes.



• Earth’s Revolution and Rotation



• Summer and Winter Solstices



• Equinoxes: March 21 and September 21


Effects of Earth’s Changing Orientation

• Solar Angle

– Solar radiation is directly related to solar angle.

– Higher solar angles reduce beam spreading, which leads to

warming.

– Lower angles induce less intense warming.



• Solar Angle



Changes in Energy Receipt with Latitude and Season

solar radiation and the seasons - universitetet i oslo · solar radiation and the seasons chapter 2...

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