Meteorological Features. Physical Basis of Radiation

The radiation of heat and the intensity of the sunlight on the surface of the Earth over the course of the year are determined by the geographical latitude, the weather and the varying conditions of the sky (clear, clouded, dull, partly clouded, etc.).

The facts given below are important with regard to our typical patterns of daylight and sunshine duration.

There are 8760h in a year. The duration of 'bright daylight' during the course of a year amounts to around 4300 h on average.

The number of hours of sunshine per year varies from one country to another. Even within the same country it may vary from one location to another. The majority of these hours of sunshine usually occur during summer.

Over most of the year, that is, during ²/з of the daylight hours, the sunlight that reaches the Earth is scattered to a greater or lesser degree owing to the local weather conditions.

The direct and indirect solar radiation (global radiation) which reaches the surface of the Earth produces a locally varying climate on the surface and in its near vicinity (see – (15)). The periods of sunshine are considered in units of tenths of hours. The data represent only the macro-climate; local variations in the micro-climate are not accounted for. Climatic data relating to a specific location (temperature, sunshine duration, sky conditions etc.) can be obtained, for example, from the Meteorological Office in Bracknell, UK.

During 'bright daylight hours', varying intensities of solar radiation are received on the surface, depending on the geographical latitude and the weather conditions, as are varying qualities of daylight – (16).

Physical basis of radiation. Solar radiation is a very inconstant source of heat. Only a small proportion of the solar energy radiated toward the Earth is transferred to the surface of the Earth as heat energy. This is because the Earth's atmosphere diminishes the solar radiation and does not permit a uniform intensity to penetrate to the surface.

This reduction essentially occurs because of various turbidity factors, such as scatter, reflection and absorption of the radiation by dust and haze (the cause of diffuse daylight), and also because of the water vapour, carbon dioxide and ozone in the air.

The total energy of solar radiation reaching the Earth is transmitted in the wavelength range 0.2-3.0pm. Distribution of the total energy on the Earth's surface is as follows: approximately 3% ultra-violet radiation in the wavelength range 0.2-0.38pm; approximately 44% visible radiation in the wavelength range 0.38-0.78pm (the maximum lies at 0.5pm in the visible light range); approximately 53% infra-red radiation in the wavelength range 0.78-3.0pm.

The chart shown in – (17) represents the solar radiation which reaches the Earth. This is the solar constant, and has a value in our region of approximately 1000W/m² on an illuminated vertical surface.

The radiation power is reduced by very thick cloud to approximately 200W/m², and in the case of only diffuse radiation (a cloudy sky with the sun completely obscured) to approx. 50-200W/m² (see – (16)).