Solar Insolation, Meaning, Intensity and Factors Affecting Insolation

What is Insolation

Insolation is defined as the total amount of heat energy received from the sun during the day. It is measured by the amount of solar energy received per square centimeter per minute. Insolation affects temperature, the highest insolation is received atnoon, hence the more the intensity of insolation, the higher the temperature.

Over 99% of all energy in the earths atmosphere has its origin in radiation from the sun or insolation. A minute fraction of atmospheric energy is supplied by heat from the earth itself, either by volcanic eruptions or by the decay of radioactive minerals and by the burning of organic material. All movements and changes in the atmosphere are ultimately caused by variations in the amount of insolation received. During transmission, a considerable amount of this energy is absorbed, scattered and reflected by the atmosphere so that the visible rays of the spectrum U-V and infra-red gives rise to the characteristic blue sky that we see above us.

If we say that at a certain time, the intensity of insolation at one place is twice than at another, it means that during equal and very short periods of time, the former place receives twice as much heat as the latter.
Radiation from the sun is made up of three parts which include:

• The visible white light- this is the light that we see when the sun shines, this visible light has a huge influence on our climate.
• The ultra violet rays- the ultra violet rays is less visible, it can affect our skin and cause sun burn when our bare body is exposed to them for a very long time.
• The infra-red rays- the infra-red rays can penetrate even dust and fog and are widely used in photography. However, only the part of suns radiation that reaches the earth is what is referred to as Insolation.

In the clear sky, about 80 per cent of this energy can reach the earth through the atmosphere and the rest of 20 per cent is lost in the space. Even the total energy that the earth receives from the sun is not distributed equally throughout the world. The amount of energy received by the equatorial region, in comparison to this, loses less amount of energy. On the other hand, the condition is just the opposite in the polar regions, in other words, the polar regions lose more energy to space by dust, water vapour, clouds and air molecules than what they receive. The solar energy tries to bring equilibrium of this unequal distribution of insolation through the wind movement and the ocean currents.

These variations can be in time or in place. They are the main cause of climatic differentiation. The amount of insolation received at the outer limit of the atmosphere, at normal incidence and when the sun is at its mean distance from the earth is about 2 gram calories per square centimeter per minute. This is called the solar constant.

What is solar constant?

Solar constant is the measure of the intensity of insolation per unit area received at the outer limit of the atmosphere. Variations of the solar constant caused by changes in the radiative activity of the sun, rarely exceeds 2% of the average value.

The total amount radiation received at any place on earth depends on certain factors, these factors are controlled by the movements of the earth: its rotation around its axis and its annual orbit around the sun.

Factor Affecting Insolation

1. The intensity of insolation
2. Length of the day in comparison to night and the solar radiation.
3. Latitude
4. Influence of the atmosphere
5. Ocean Currents
6. Nature of surface
7. Prevailing winds

Intensity of insolation

The intensity of insolation is controlled by the movement of the earth around the sun. As the earths orbit is not a perfect circle, the distance between the earth and the sun varies during the course of the year. On January 3^rd, when the earth is nearest to the sun, the intensity of insolation is about 7% higher than on July 4^th, when the distance between earth and sun is at its maximum. This difference is the same at all latitudes. Theoretically, the difference in the distance from the sun would make the summers of the southern hemisphere hotter and its winter colder than those of the northern hemisphere. But this effects are completely masked by the stronger continentality of the northern hemisphere which causes exactly the opposite conditions.

Length of Day and Night

the duration of insolation is indicated length of day which depends entirely on the earth rotation around it axis. Because of the angle of 67 degrees between the axis of rotation and the plane of the earths orbit around the sun. Places on the summer hemisphere enjoy longer days than those having winter; but the total annual exposure to the sun is the same for all places on the earth. The difference between summer and winter days increases with distance from the equator reaching its extreme at the poles where six months of continuous daylight are followed by six months without it. Only on the equinoxes, March 23^rd and September 22^nd are days and nights of equal length everywhere. At the equator, all days of the year are of equal length, namely 12 hours and 7 minutes. Astronomically, the duration would be 12 hours exactly, but it takes 3minutes for the upper half of the sun to disappear under the horizon at sun set and similarly, 3 minutes before the centre of the suns disc is at the horizon while the upper half of it already provides insolation, at sunrise. With increasing latitude, the difference between the shortest and the longest day of the year grows. In the low latitudes the increase is about 7 minutes per degree, while in the higher latitudes the difference is larger. Between 50⁰ and 60⁰ it amounts to about 28 minutes per degree of latitude.

Latitude

The intensity insolation tends to be higher where the suns rays strike vertically, as they do at the noon at the latitude equal to the suns declination ranging between the Tropic of Cancer and Capricorn. With diminishing angle, the same amount of solar energy spreads over a greater area of ground surface. Hence, on the average, the polar areas receive least heat per unit area. This fact helps to explain the general distribution of average temperature over the globe from a maximum at low latitudes to a minimum near either poles.

Influence of the Atmosphere

Even though the air appears transparent, it does not allow the rays of the sun to pass through it without loss. The atmospheric layers to be crossed also influence the energy of the sun through reflection and absorption. The absorption decreases the heat and this decrease is accentuated (intensified) as the mass of the atmospheric traverse (obstruction) increases. By and large, the chief elements of the atmosphere that impedes (obstructs) the solar rays are the drops of water, dust particles, water vapor, salt and smoke. These are most numerous in the lower layers of the atmosphere. Hence, insolation is intense on the desert areas where the atmosphere is the clearest and on higher altitudes where these impediments are not to be found in the atmosphere.

Nature of surface

Heat affects land and water differently, and therefore the distribution of temperature is greatly influenced by the distribution of continents and oceans. The land surface warms up and cools down more quickly than the sea surface, therefore in temperate latitude, the sea warms the coastal regions in winter, while in summer they are cooled by it. The temperature of such coastal areas is always affected by the influence of the cooled wind from the sea in summer and that of warm wind from the sea in winter.

Prevailing Winds

The effect of wind on temperature is to transport temperature prevailing in areas over which they blow, either from land or from sea. Onshore winds in the tropics, blowing from over the cooler oceans tend to modify temperatures on the coastal margins. On the other hand, onshore winds, such as Westerlies, may in winter carry mild temperatures from over the oceans on to the continental margins. Local winds may produce the rapid temperature changes.

Ocean Currents

These are huge movements of water in the oceans.Ocean currents changes the effect of wind blowing over them, hence generally influencing the temperature of the coastal lands. A cold current causes the wind blowing over it to be cold and as such dry, whereas, a warm current causes the wind blowing over it to be warm and laden with moisture. Currents that blows from the equator to the poles when heated are light and less dense, thus move on top of the ocean. These currents raise coastal temperatures of polar regions. Whereas the currents that originates from the poles are cold and dense due to added weight of ice, these sink downwards and move towards the equator were they can lower coastal temperatures in their wake.