How Light Varies at Different Times of Day, Seasons and Cloud Cover

Screenshot 2016-07-24 at 3.48.17 PMThe intensity of light varies with latitudes and seasons, which means at different times of the year, the amount of light changes. All solar radiation comes directly from the sun, which according to research has a surface temperature of approximately 5,500 degrees Celsius. This temperature plays a significant role in determining the characteristics of the solar radiation that we receive on earth.

The amount of radiation we receive at the surface of the atmosphere that is closest to the sun is 1370 joules of energy per second per square meter. If we are close to the equator on a cloud-free day, the amount of direct solar radiation we receive on the earth’s surface is about 75 percent. This is because 13% is absorbed by the atmosphere while another 13% is scattered. In terms of joules, direct radiation therefore measures about 1000 joules per square meter per second.

Quality and Quantity of Energy

Inasmuch as the amount of energy we receive is important, the quality of the energy is significant as well. The amount of energy per unit wavelength of solar radiation peaks at a value of approximately 500 nanometers but ranges to about 3000 nanometers. This radiation consists of short wavelengths. The heat emitted by the earth back into space has a wavelength which is greater than 3000 nanometers and thus referred to as long wave radiation.

Effect of Rotation

Because the earth rotates, the full effect of the 1370 joules per square meter per second is not felt at all times of the day. Since the area of a sphere is calculated as 4 x the area of a disc, the average radiation across the earth atmosphere at any one given time is about 340 joules per square meter per second.

This means, on a diurnal basis, an area close to the equator should expect zero radiation on average on a clear day before sunrise and after sunset. At noon, the area should expect about 1000 joules of shortwave radiation per second.

Effect of Revolution

In addition to rotation, the earth also revolves around the sun in a clockwise motion. This revolution happens at an angle of 23.5 degrees relative to the sun. At autumn and spring equinoxes, each section of the earth receives 12 hours of daylight. However, during winter in the northern hemisphere, the daylight hours are substantially reduced. During summer, daylight hours tend to be greater than any season of the year.

Because of a slight offset in the earth orbit, the winters in the northern hemisphere are slightly milder compared to the southern hemisphere for places along the same latitude.

The Effect of Latitude

The farther away a particular latitude is from the equator, the larger the area over which the solar beam spreads. This means, an area lying on a latitude 50 degrees north receives a solar radiation spread that is 35% larger than what the equator receives. Because of the distance solar radiation travels through the atmosphere to reach these latitudes, the amount of solar radiation also decreases.

The Effect of Cloud Cover

Cloud cover has a profound impact on the actual radiation received on the earth’s surface. Even on a day with clear skies, solar radiation loss can measure up to about 25%. The presence of light cloud increases the loss to about 50% while during thicker cloud cover, losses of about 75% can be experienced.

Therefore, the light received from the sun varies with different aspects amongst them the clarity of the skies, earth rotation, and revolution.