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, calledphotons.
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 longerwavelength 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, theelectromagnetic 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 thestratosphere. 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 aerosolsabsorb 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 totalirradiance. 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 bygoing to the experiments page! Here you can compare UV radiation measured by the NSF network in Antarctica with satellite ozone data.
Sun burning is not a pleasant experience. Fortunately, we have ways to prevent it. Avoiding the sun between the hours of 11 am and 3 pm during day light saving time can reduce your UV exposure by more than 50%. If you have to be outside during these hours, seek shade, wear T-shirts on the beach, hats with a broad brim, and sunglasses, and apply sunscreen of a Sun Protection Factor of at least 15 to all parts of your body that are not protected otherwise.
How do sunscreens work?
A sunscreen works in one of two ways; it either scatters oncoming UV rays away from the skin (physical sunscreen) or it absorbs the UV rays before they can reach the skin (chemical sunscreen). Both methods are effective, though chemical sunscreens tend to only block UV-B, which is mostly responsible for sunburns. Sunscreens that block out both UV-A and UV-B rays, with an SPF of at least 15 (SPF 30 is better) are preferable. They should especially be applied between the hours of 11 and 3, when the amount of UV rays hitting the earth is generally the greatest.
In order to allow the sunscreen to achieve its maximum protection, the sunscreen should be applied about 20 minutes before you head out into the sun. If your intended activity includes swimming or other water-related events, a waterproof sunscreen is best. It should be noted, however, that no sunscreen is completely water or sweat proof. Therefore, more sunscreen needs to be applied after any activity in which these factors are involved.It should not be assumed,however, that applying sunscreen allows you to safely stay out longer in the sun. Sunburn is the body's natural warning sign - eliminating the warning sign does not mean that you have eliminated all of the danger.
What is SPF?
SPF stands for Sun Protection Factor. It is the rating that the U.S. Food and Drug Administration (FDA) puts on sunscreens that describes how much they extend the time you can stay in the sun without getting sunburned. For example, if you can normally stay outside for about 15 minutes before you start to burn, then applying a sunscreen with an SPF of 20 allows you to stay out for 20 times 15 minutes before burning - or rather, 300 minutes. (Of course, that's assuming you apply the right amount and none of it comes off during your day.)
How to protect your eyes
The protection of the eyes from UV rays is actually very simple. Sunglasses with at least 99% UV-A and UV-Bblockage do an effective screening job, especially if they wrap around the head. This prevents UV rays from entering from the side. Check the label of your sun glasses when you buy new ones! Does it say that it blocks UV? Older glasses often did not absorb enough UV. If this is the case they may be doing more harm than good. The dark tint of the sunglasses causes the pupils to widen in order to let in more light. If sunglasses are only dark in the visible but not in the UV, actually more UV could penetrate the eyes.
I want a tan, not asunburn!
First keep in mind that there is no scientific evidence that a tan does any good for your skin. If you still can't withstand the appeal of a bronze-colored appearance, sunbathe responsibly and always avoid over-exposure. Spend small amounts of time in the sun over a period of several days, to give your skin time to start its own built-in protection against UV rays, for example the production of a substance called "melanin". Never risk a sunburn. In addition to itching and skin damage that sunburn creates, sunburned skin will soon shed, and with it goes much of the bronze tan.
