Energy Flow Emitted by the Sun and Albedo Effect

The Sun is a star that emits energy in the form of radiation called electromagnetic waves. The Earth actually absorbs the short wavelengths emitted by the Sun to keep warm. Only part of the visible light, infrared rays, radio waves, and a very small amount of ultraviolet rays reach the Earth's surface.

A number of factors can affect insolation.

1. The time of day directly impacts insolation, since solar radiation is zero if the Earth’s surface is on the opposite side from the Sun, that is, at night.

2. The seasons also affect insolation. Since the axis of rotation is tilted, a surface can receive a greater amount of solar energy depending on its position relative to the Sun.

3. Latitude also influences insolation, since regions further north do not receive as much solar energy as other regions closer to the Equator.

Other factors, such as the presence of the ozone layer or the presence of air pollutants, can also have an impact on insolation.

Some of the radiation that reaches the surface is absorbed by certain objects or surfaces (such as asphalt, rocks, or bricks) to be eventually released in the form of heat. What remains is the unabsorbed energy that is reflected by other surfaces (such as water or snow) and remains in the atmosphere.

Here are some examples of albedo.

Matter that is white (cloud, snow, white sand) has a very great reflective power on the Sun’s rays, while darker matter is less likely to reflect it. This finding is alarming, because polar caps are rapidly melting and decreasing in surface area, thereby reducing the general albedo of the Earth, thus causing an increase in the Earth's temperature.

As for the rays of the Sun, they are absorbed by a surface (or an object) to heat this surface (or this object). In other words, light energy is transformed into heat by the surface. Earth’s surface is first heated by solar radiation and then the ground heats the surrounding air.

The Earth's atmosphere receives its energy from the Sun mainly in the form of heat.

It may seem that the Earth's core is heating up the surface, but it is not the case. The rocks forming the Earth's crust are very poor heat conductors. Aside from active volcanoes, heat transfer from the Earth's core to the surface is virtually zero. The Sun is therefore the Earth’s only source of energy: it is essential for the presence of human life on Earth. The temperature is not the same everywhere on our planet, because several factors affect the amount of heat received from the Sun.

• Time of day

• Latitude

• Seasons

• Ground surface

• Altitude

• Clouds

When the Sun rises, the Sun's rays do not strike the Earth's surface perpendicularly. By striking at a certain angle (|30| degrees in the diagram below), they illuminate a larger area. However, this surface receives a small amount of heat.

When the Sun is at its peak, the Sun's rays strike the Earth's surface perpendicularly. The heat is therefore maximum, although on a smaller surface. The Sun therefore heats up more if it is at its peak rather than at the horizon.

Temperature is impacted by latitude. The Earth is far enough away from the Sun that solar rays can be said to be parallel when they reach the Earth's surface. As shown in the diagram below, the same amount of solar energy will heat a much smaller area at the Equator than at the poles, as the rays strike the Earth's surface perpendicular to the Equator. Thus the temperature will be higher at the Equator than at the poles.

The temperature will also be influenced by the seasons. The Earth revolves around the Sun in a nearly circular path. The Earth also rotates around an axis that is tilted relative to the plane of its revolution.

This tilt ensures that some areas of the Earth are not exposed to the Sun's rays for as long as others. Thus, when the Northern Hemisphere is tilted towards the Sun, the amount of solar rays received will be greater than in the Southern Hemisphere. The hours of daylight are also more numerous. This means it is summer in the Northern Hemisphere.

The surface of the ground is another factor which influences the temperature at a given place. This difference can especially be seen with soil and water. Soil has low thermal conductivity and only the surface layer is heated. If the surface of the ground heats up and cools down quickly, then temperature changes will be slower to modify the temperature of the soil in depth.

Water also has a greater heat capacity, it is transparent and it can move easily due to sea currents. The oceans are therefore immense heat reservoirs and they greatly reduce the amplitude of temperature variation.

Altitude is a factor that influences temperature. The atmosphere is primarily heated near the Earth’s surface. The temperature therefore decreases with altitude.

It is possible to observe an increase in temperature with altitude; this is referred to as an inversion phenomenon. An inversion can be caused by a soil which cools a lot at night (ground inversion), by the passage of a front (frontal inversion), by the presence of an anticyclone (subsidence inversion) or by the transition from the troposphere to the stratosphere (tropopause inversion).

The temperature is affected by the cloud layer. Clouds block the solar rays from reaching the ground. This is why a lower temperature can be observed when clouds cover the sky. Even condensation trails left by planes in the sky can cause the same effect as a cloud layer.