Student's Guide - Ultraviolet Radiation
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Ultraviolet Radiation

 

What Is It?

Simply put, ultraviolet radiation (also known as UV radiation or ultraviolet rays) is a form of energy traveling through space.

Some of the most frequently recognized types of energy are heat and light. These, along with others, can be classified as a phenomenon known as electromagnetic radiation. Other types of electromagnetic radiation are gamma rays, X-rays, visible light, infrared rays, and radio waves. The progression of electromagnetic radiation through space can be visualized in different ways. Some experiments suggest that these rays travel in the form of waves. A physicist can actually measure the length of those waves (simply called their wavelength ). It turns out that a smaller wavelength means more energy. At other times, it is more plausible to describe electromagnetic radiation as being contained and traveling in little packets, called photons.

The distinguishing factor among the different types of electromagnetic radiation is their energy content. Ultraviolet radiation is more energetic than visible radiation and therefore has a shorter wavelength. To be more specific: Ultraviolet rays have a wavelength between approximately 100 nanometers and 400 nanometers whereas visible radiation includes wavelengths between 400 and 780 nanometers.

 

Where does it come from?

The sun is a major source of ultraviolet rays. Though the sun emits all of the different kinds of electromagnetic radiation, 99% of its rays are in the form of visible light, ultraviolet rays, and infrared rays (also known as heat). Man-made lamps can also emit UV radiation, and are often used for experimental purposes.

 

What does it do?

Light enables us to see, and heat keeps us from being cold. However, ultraviolet rays often carry the unfortunate circumstance of containing too much energy. For example, infrared rays create heat in much the same way as rubbing your hands together does. The energy contained in the infrared rays causes the molecules of the substance it hits to vibrate back and forth. However, the energy contained in ultraviolet rays is higher, so instead of just causing the molecules to shake, it actually can knock electrons away from the atoms, or causes molecules to split. This results in a change in the chemical structure of the molecule. This change is especially detrimental to living organisms, as it can cause cell damage and deformities by actually mutating its genetic code.

 

What stops it?

Ultraviolet rays can be subdivided into three different wavelength bands - UV-A, UV-B, and UV-C. This is simply a convenient way of classifying the rays based on the amount of energy they contain and their effects on biological matter. UV-C is most energetic and most harmful; UV-A is least energetic and least harmful. 

Luckily,UV-C rays do not reach the earth's surface because of the ozone layer. When UV-C rays meet the ozone molecules at high layers of the atmosphere, the energy inherent in them is enough to break apart the bond of the molecule and absorb the energy. Therefore, no UV-C rays from the sun ever come into contact with life on earth, though man-produced UV-C rays can be a hazard in certain professions, such as welders.

UV-B rays have a lower energy level and a longer wavelength than UV-C. As their energy is often not sufficient to split an ozone molecule, some of them extend down to the earth's surface. UV-A rays do not have enough energy to break apart the bonds of the ozone, so UV-A radiation passes the earth's atmosphere almost unfiltered. As both UV-B and UV-A rays can be detrimental to our health, it is important that we protect ourselves. This can be done through a variety of ways. The most obvious is to reduce the amount of time one spends in the sun, particularly between the hours of 11 am and 3 pm, when the sun is at its highest in the sky. However, especially during the summer holidays, this does not always work out. More ways to protect ourselves can be found here.

 

Variability of UV

UV levels are not constant over the course of a day, or even over the course of a year. An obvious factor is the position of the sun in the sky. At noon, for example, the electromagnetic waves emitted from the sun travel a much shorter path through the earth's atmosphere then they would at, say, 5 pm, and thus noon-time intensity is stronger.  A second important parameter determining UV at the ground is the amount of ozone present in the stratosphere. Low ozone correlates with much UV. However, there are many other features of the environment that contribute to UV radiation variability. Most important are clouds. On cloudy days, UV levels are usually lower than during clear skies as clouds can deflect rays up into space. Clouds can, however, also lead to increased UV levels. This happens, for example, when the sun is not obscured by clouds but clouds in the vicinity of the sun reflect additional radiation to the ground. So a general rule is not to feel save from UV radiation just because it's cloudy!

The amounts of UV one is exposed to also varies with altitude. As a rule of thumb, UV levels increase about 4% for every 1,000 foot gain in altitude. This increase has nothing to do with being closer to the sun - any elevation you might gain would be miniscule in comparison to the distance from the earth to the sun, and so would have an insignificant outcome on UV levels. Instead, the increase is the result of a thinner atmosphere with a smaller number of molecules being present to absorb or scatter UV. Examples of such molecules are tropospheric ozone (commonly associated with smog) and aerosols, molecules that remain suspended in the air. Aerosols can be a multitude of substances - dust, soot, sulfates, etc. These aerosols absorb and scatter UV rays, and so cut down on the ultimate UV irradiance.

Other factors that have an influence on UV levels are the physical features of the land - sand, snow, and water all tend to reflect UV rays. This phenomenon is called albedo. Some of the ultraviolet rays reflected off the ground encounter scattering by air molecules, aerosols or clouds back down to the earth, thus increasing the total irradiance. When there is snow on the ground the amount of time it takes for sunburn to occur is therefore significantly reduced.

Also, the closer one is to the equator, the more ultraviolet rays one is exposed to. This can be explained by the fact that the sun is usually higher at the sky at low latitudes. In addition, the ozone layer is thinner at the equator as it is over, for example the United States or Europe, and this also contributes to more UV.

Since the 1980s, polar regions are affected by the ozone hole. Under the ozone hole, biologically relevant UV levels are 2-3 times as high as they were before. Learn, based on real data, how UV levels are affected by the ozone hole by going to the experiments page! Here you can compare UV radiation measured by the NSF network in Antarctica with satellite ozone data.

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