Circle of illumination – imaginary line that divides the globe into a night-half and a day half.
Subsolar Point – the point at which the sunÕs rays strike the earth directly (at a 90o angle).
Plane of the Ecliptic – the imaginary surface the passes through the center of the earth and the sun and which contains the orbit of the earth around the sun (and most of the orbits of the other planets).
One day in the year (June 20) when the subsolar point lies at the Tropic of Cancer, every latitude north of 66 1/2N receives 24 hr of light and a greater fraction of the northern hemisphere is light.
One day in the year (Dec 20) when the subsolar point lies at the Tropic of Capricorn, every latitude north of 66 1/2N receives 24 hr of darkness and a greater fraction of the northern hemisphere is in darkness.
Two days in the year when the subsolar point lies at the equator, the circle of illumination passes through the north and south poles and there are 12 hours of darkness and light everywhere on the globe.
Electromagnetic Radiation--the form of energy emitted by any object that has a temperature greater than 00K. (Often called radiation.). Characterized by wavelength and intensity (amount).
Emission--produced by or given off by...
Laws of Radiation--physical principles that govern the amount and characteristics of the electromagnetic energy emitted by an object.
Electromagnetic Spectrum--the radiation possible in the universe, defined by the intensity (amount) and wavelength of emitted energy.
Solar Spectrum--the collection of wavelengths and intensity of radiation emitted by the sun.
Terrestrial or EarthÕs Spectrum--the collection of wavelengths of radiation emitted by the earth.
Solar constant--the amount of energy that arrives from the sun at the top of the atmosphere on a plane perpendicular to the sun’s rays. For the earth, this averages out to be around 2 cal per cm2 per min. This is often written 2 cal/cm2/min.
Insolation--incoming solar radiation or the amount of radiation that after passing through the atmosphere makes it to the surface of the earth.
Infrared or Atmospheric Window--the transparent portion of the abosrption spectra of water vapor and carbon dioxide through which long wave radiation is allowed free passage to space.
Greenhouse Effect--a popular term for the phenomenon in which the atmosphere is mostly transparent to the short wave energy from the sun (lets the energy get to the surface, where the energy is absorbed and warms the planet) but largely opaque to the long wave energy emitted by the earth (prevents the energy from reaching space) thus keeping the atmosphere much warmer than it would have been had carbon dioxide and water vapor NOT been present. The analogy is that the glass that makes up the walls of a greenhouse allow light from the sun in, at which time it is absorbed by surfaces in the greenhouse, and converted to internal energy, but that glass is opaque to the long wave radiation emitted by the warmed surfaces. This keeps the greenhouse warmer than it would have been, and usually much warmer than the external temperature.