Solar Radiation Basics
Solar radiation is a general term for the electromagnetic radiation emitted
by the sun. We can capture and convert solar radiation into useful forms of
energy, such as heat and electricity, using a variety of technologies. The
technical feasibility and economical operation of these technologies at a
specific location depends on the available solar radiation or solar
resource.
Basic Principles
Every location on Earth receives sunlight at least part of the year. The
amount of solar radiation that reaches any one "spot" on the Earth's surface
varies according to these factors:
- Geographic location
- Time of day
- Season
- Local landscape
- Local weather.
Because the Earth is round, the sun strikes the surface at different angles
ranging from 0º (just above the horizon) to 90º (directly overhead). When the
sun's rays are vertical, the Earth's surface gets all the energy possible. The
more slanted the sun's rays are, the longer they travel through the atmosphere,
becoming more scattered and diffuse. Because the Earth is round, the frigid
polar regions never get a high sun, and because of the tilted axis of rotation,
these areas receive no sun at all during part of the year.
The Earth revolves around the sun in an elliptical orbit and is closer to the
sun during part of the year. When the sun is nearer the Earth, the Earth's
surface receives a little more solar energy. The Earth is nearer the sun when
it's summer in the southern hemisphere and winter in the northern hemisphere.
However the presence of vast oceans moderates the hotter summers and colder
winters one would expect to see in the southern hemisphere as a result of this
difference.
The 23.5º tilt in the Earth's axis of rotation is a more significant factor
in determining the amount of sunlight striking the Earth at a particular
location. Tilting results in longer days in the northern hemisphere from the
spring (vernal) equinox to the fall (autumnal) equinox and longer days in the
southern hemisphere during the other six months. Days and nights are both
exactly 12 hours long on the equinoxes, which occur each year on or around March
23 and September 22.
Countries like the United States, which lie in the middle latitudes, receive
more solar energy in the summer not only because days are longer, but also
because the sun is nearly overhead. The sun's rays are far more slanted during
the shorter days of the winter months. Cities like Denver, Colorado, (near 40º
latitude) receive nearly three times more solar energy in June than they do in
December.
The rotation of the Earth is responsible for hourly variations in sunlight.
In the early morning and late afternoon, the sun is low in the sky. Its rays
travel further through the atmosphere than at noon when the sun is at its
highest point. On a clear day, the greatest amount of solar energy reaches a
solar collector around solar noon.
Diffuse and Direct Solar Radiation
As sunlight passes through the atmosphere, some of it is absorbed, scattered,
and reflected by the following:
- Air molecules
- Water vapor
- Clouds
- Dust
- Pollutants
- Forest fires
- Volcanoes.
This is called diffuse solar radiation. The solar radiation that reaches the
Earth's surface without being diffused is called direct beam solar radiation.
The sum of the diffuse and direct solar radiation is called global solar
radiation. Atmospheric conditions can reduce direct beam radiation by 10% on
clear, dry days and by 100% during thick, cloudy days.
Measurement
Scientists measure the amount of sunlight falling on specific locations at
different times of the year. They then estimate the amount of sunlight falling
on regions at the same latitude with similar climates. Measurements of solar
energy are typically expressed as total radiation on a horizontal surface, or as
total radiation on a surface tracking the sun.



