Sun intensity refers to the amount of incoming solar energy, or radiation, that reaches the Earth’s surface. The angle at which the rays from the sun hit the Earth determines this intensity. The sun’s angle -- and hence intensity -- varies significantly depending on a particular spot’s geographic location, the time of year, and the time of day.
Angle of Incidence
The angle formed by rays of sunlight hitting the Earth is technically known as the angle of incidence. Rays striking the planet’s surface from directly overhead -- that is, at a 90 degree angle measured from the horizon -- are the most intense. At most times and locations, the sun forms an angle with the horizon less than 90 degrees -- that is, usually the sun sits lower in the sky.
The smaller the angle, the greater the surface area over which the sun’s rays spread. This effect reduces the sun’s intensity in any one place. For example, at a 45 degree angle of incidence, solar radiation covers a 40 percent greater area and is 30 percent less intense than at the maximum angle of incidence of 90 degrees.
Only locations lying along one line of latitude on the surface of the Earth can receive sunlight at a 90 degree angle on a given day. All other places receive sunlight at lesser intensities. In general, the sun’s rays are the most intense at the equator and the least intense at the poles. On an average yearly basis, areas north of the Arctic Circle receive only about 40 percent as much solar radiation as equatorial regions.
Relationship to Seasons
Fluctuations in the intensity and duration of solar energy in a particular area determine that area’s seasons. These fluctuations are dictated by the way the Earth is tilted on its axis. With respect to the plane of rotation around the sun, the Earth slants at a 23.5 degree angle, meaning that at certain points during its orbit, the Northern Hemisphere faces the sun moreso than the Southern Hemisphere, and vice versa. For example, at the summer solstice, the Northern Hemisphere faces the sun at maximum tilt, so the sun’s rays strike 23.5 degrees north latitude -- the Tropic of Cancer -- at a 90 degree angle.
Whichever hemisphere tilts further toward the sun receives a larger percentage of solar radiation than the opposing hemisphere. The former hemisphere experiences summer at this time, while the latter experiences winter. In the hemisphere experiencing summer, the sun rises higher in the sky and is more intense; its rays strike the ground at a higher angle than in the hemisphere experiencing winter. This explains why the risk of sunburn is greatest in summertime. It also explains why temperatures are warmest in summertime, since the sun delivers heat energy.
Time of Day
Regardless of latitude or time of year, the sun’s angle reaches closest to 90 degrees -- and is therefore at its most intense -- at the midpoint of the day: noon. At this time, the sun is said to have attained its zenith, or highest point. During daylight saving time, the sun is at its greatest angle and most intense at 1 p.m., due to the manmade one-hour offset from true solar time.