UV SOLAR RADIATION – a basic review

Solar radiation includes ultraviolet (UV) radiation, visible radiation (light), and infrared (IR) radiation. The radiation is often characterised by its wavelength, usually expressed in nano-meters (1nm=10^-9m). When describing biological effects ultraviolet radiation is often subdivided into three spectral bands: UV-C radiation (100-280nm), UV-B radiation (280-315nm) and UV-A radiation (315-400nm). UV radiation can be measured as an irradiance – the power incident upon a surface unit area – in units of W/m², or as a radiant exposure, or dose – the energy incident upon a surface unit area during a specified period of time – in units of J/m². The most important factors affecting the UV radiation reaching the Earth’s surface are described below.

Atmospheric ozone
UV radiation is absorbed and scattered in the atmosphere. UV-C radiation is completely absorbed in the upper atmosphere by oxygen and ozone molecules. Most of the UV-B radiation is absorbed in the stratosphere by ozone molecules and only a few percent reach the surface of the Earth. Therefore, at the surface of the Earth the solar UV radiation is composed of a large amount of UV-A radiation and only a very small amount of UV-B radiation. UV-B radiation is known to be biologically damaging, whereas UV-A radiation is much less damaging but is known for its ability to tan the human skin. As ozone is the main absorber of UV-B radiation the UV-B intensity at the Earth’s surface depends strongly on the total amount of ozone in the atmosphere, and thus on the thickness of the ozone layer. A factor, which describes the relation between the sensitivity of the UV-B intensity to changes in total ozone, is the so-called Radiation Amplification Factor (RAF). For small changes in the ozone layer thickness the RAF represents the percent change in UV-B intensity for a 1-percent change in the total column ozone. For CIE-weighted irradiance, i.e., for the erythemally effective UV radiation, and for varying solar elevations and ozone, the RAF’s are in the range of 1.1-1.3.

Solar elevation
Solar elevation is the angle between the horizon and the direction to the sun. The solar zenith angle (SZA) is often used in place of the solar elevation: it is the angle between the zenith and the direction to the sun. For high solar elevations the UV radiation is more intense because the rays from the sun have a shorter path through the atmosphere and therefore pass through a smaller amount of absorbers. As the UV irradiance depends strongly on the solar elevation it changes with latitude, season and time, being highest in the tropics, in summer and at noon.
 

Altitude
The UV irradiance increases with altitude because the amount of absorbers in the overlying atmosphere decreases with altitude. Measurements show that the UV irradiance increases by 6-8% per 1000 m increase in altitude.
 

Atmospheric scattering 
At the surface of the Earth solar radiation is composed of direct and scattered (diffuse) components. Solar radiation is scattered on air molecules and on particles such as aerosols and water droplets. The direct component consists of the rays from the sun that has passed directly through the atmosphere without being scattered or absorbed. The diffuse component consists of rays that have been scattered at least once before reaching the ground. Scattering depends strongly on wavelength. The sky looks blue because blue radiation is scattered more than the other  visible wavelengths. UV radiation is scattered even more and at the surface of the Earth the UV-B is roughly composed of a 1:1 mixture of direct and diffuse radiation.
 

Clouds and haze
The UV irradiance is higher when the sky is cloudless. Clouds generally reduce the UV irradiance but the attenuation by clouds depends on both the thickness and the type of cloud (optical depth of clouds). Thin or scattered clouds have only a little effect on UV at the ground. In certain conditions and for short times a small amount of cloud may even enhance the UV irradiance compared to fully clear skies. In hazy conditions UV radiation is absorbed and scattered by water droplets and aerosols and this leads to decrease in  the UV irradiance.
 

Ground reflection
Part of the UV radiation that reaches the ground is absorbed by the Earth’s surface and part of it is reflected back to space. The amount of reflected radiation depends on the properties of the surface. Most natural surfaces such as grass, soil and water reflect less than about 10% of the incident UV radiation. Fresh snow, on the other hand, may reflect up to about 80% of the incident UV radiation. During spring and with a cloud-free sky the reflection of snow may increase the UV irradiance on inclined surfaces to summer values. This is important at higher altitudes and at higher latitudes. Sand may reflect about 25% of the UV radiation and can increase the UV exposure at the beach. Up to 95% of the UV radiation penetrates into the water and up to  50% penetrates to a depth of about 3 m (in clear ocean water).