Solar Radiation and the Earth's Energy Balance.

Electromagnetic Waves

Electromagnetic waves are the self-propagating, mutual oscillation of electric and magnetic fields. Electromagnetic waves move electromagnetic energy through space (either empty or filled with transparent matter) and affect regions far remote from the origin of the energy. The propagation of electromagnetic energy is often referred to as radiation. The concept of the connection between magnetic and electrical energy evolved in the 19th century and into the early 20th century through experimentation and mathematical reasoning by scientist such as Hans Christian Oersted, Michael Faraday, and James Clerk Maxwell, Gustav Kirchoff, Josef Stefan, Ludwig Boltzmann, Wilhelm Wien, and Max Planck.

Properties of electromagnetic waves

As in any wave motion, the are a few physical properties that characterize the waves (see figure below). One is the wavelength, which is the distance between one wave peak to the next. In the ranges typical to climate the wavelength is usually measured in microns (or micro meters μm with 1 μm = one millionth, or 10-6 of a meter) and is denoted by the Greek letter lambda (λ). Another wave property is its frequency denoted by the Greek letter nu (ν). The frequency is the number of waves occurring within a unit of time (usually per second). The product of wave number and frequency has units of speed and is referred to as the wave speed, denoted by the latin letter c. Thus we can write:

λν = c

The speed of electromagnetic waves is a constant equal to 3 x 108 m/s (also known as the speed of light).

Another wave property is the wave period τ (Greek letter tau). The period is equal the time elapsing during the passage of one full wavelength and is measured in seconds. The period is related to the frequency by the following relation:

ν = 1 / τ

Finally there is the wave amplitude (A), the distance between the peak and trough of the wave. The wave energy is proportional to the square of the wave amplitude.

Electromagnetic wave energy flux

Electromagnetic energy is continuously propagating through empty space and matter (unless the latter is opaque). We often depict the electromagnetic energy as rays emanating from their source. The intensity of the radiation is expressed in terms of an energy flux (hereafter denoted by I), which is the amount of energy impinging in one time unit on a unit area placed perpendicular to the electromagnetic rays. The units normally used to measure energy flux are W/m2 (Watts per square meter - where 1 W = 1 Joule/sec).

The electromagnetic wave spectrum

There are two types of sources of electromagnetic wave energy, those who emit monochromatic radiation - with all the energy concentrated in a narrow band surrounding a set of well defined frequency - and those who emit over a spectrum of frequencies (continuous or discrete). The range of the entire known spectrum of electromagnetic radiation is shown in the figure below.

Text by Yochanan Kushnir, 2000.