Student's Guide - Glossary
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Action spectrum - A description of the wavelength dependence of effects introduced by electromagnetic radiation on biological matter. For example, a sunburn action spectrum describes the degree to which varying wavelengths of ultraviolet rays inflict sunburn. It is usually displayed as a function on a graph.

Aerosols - Small particles suspended in the air. Examples of aerosols are dust, smog, fog, and smoke.

Albedo - The ratio of radiation reflected from the ground to incoming radiation.

Calibration - A procedure to convert an instrument's raw data (i.e. voltage) into the physical unit (i.e. irradiance). For example, instruments for measuring sun light are calibrated with special lamps. As the irradiance that these lamps produce is known, radiometers are calibrated by comparing the signal provoked from the sun light with the signal caused by the lamps'radiation.

Chlorofluorocarbons - Also known as CFCs, are composed of chlorine, fluorine, and carbon. In the 1970s and 1980s they were extremely popular as refrigerator coolants and circuit board cleaners, but have since been phased out due to fact that they contribute to the ozone hole.

Dobson Unit - A unit used to measure the thickness of the ozone layer. It can be visualized as a tube extending up into the stratosphere, capturing ozone (and other gases) as it goes. If all of the ozone in that tube was compressed into the bottom at a standard temperature and pressure, it would form a stack x millimeters high. This number (x) multiplied by 100 is the number of Dobson Units.

Electromagnetic radiation - Any of a number of types of energy (i.e. radio waves, micro-waves, ultraviolet rays, infrared or heat rays, light, X-rays, and gamma rays) that travel at the speed of light.

Erythema - The medical term for sunburn.

Extrapolation - Projecting conclusions from a model or experiment beyond that of the observed time frame or conditions.

Heterogeneous reactions - Chemical reactions between dissimilar substances (i.e. a liquid and a solid, or a gas and a liquid).

Irradiance - The amount of radiative power impinging a surface, divided by the area of this surface.  Irradiance is often measured in the units watts per square meter.

Latitude - The geographical distance from the equator. Regions close to the equator have low latitude. The Arctic and Antarctic are at high latitudes. The United States are in between, at "mid-latitudes".

Melanoma - Most serious form of skin cancer. Melanoma is a malignant tumor that originates in melanocytes, the cells which produce the pigment melanin that colors our skin, and is heavily concentrated in most moles.

Metastasize - Used most commonly in reference to cancer, it describes the occurrence of the cancer extending beyond its area of origin into other regions and organs of the body. When this happens, the cancer becomes difficult to stop and often results in death.

Nanometer - One billionth of a meter. Unit to measure the wavelength of UV radiation and light.

Non-melanoma skin cancer - Cancer of the skin's basal cells, which are at the bottom of the epidermis (outer skin layer) and squamous cells, which comprise most of the upper layers of skin.

Non-reactive or inert - Name given to a substance that does not react easily with other substances, and so has a long life expectancy (or in other words, it remains in its original form for a long time).

Ozone - A molecule composed of three atoms of oxygen and mostly found in the stratosphere. Ozone absorbs the most dangerous ultraviolet rays before they can hit the earth, thus protecting life on this planet from its adverse effects.

Photon - Discrete packets of energy that make up electromagnetic radiation.

Polar Stratospheric Clouds (PSCs) - Clouds that form during the winter months in Antarctica and sometimes in the Arctic. Extremely cold temperatures are necessary for PSCs to exist, and it is on the surface of these clouds that reactions take place that contribute to the seasonal ozone loss.

Polar vortex - A condition that exists during the winter time in the Antarctic, in which the air moves in a ringing, circular motion, allowing little air to come in or out. This is one of the factors resulting in the low stratospheric temperature during the polar night, which are a prerequisite for the ozone hole. When the vortex dissipates during ozone-deprived air can spread over other parts of the globe, leading to excess UV radiation.

Reactive - A name given to substance that, when introduced to other substances or molecules, react easily with them and cause the break-up of the original molecule.

Rickets - A childhood disease in which a lack of vitamin D results in soft and weakened bones.

Smog - A combination of the words "smoke" and "fog"; it has come to mean a polluting substance that remains suspended in the air. Smog can be made up of a variety of matter, such as aerosols, ozone, car exhaust, and sulfurous compounds, and can also aggravate medical conditions such as asthma or other breathing difficulties.

Spectroradiometer - An instrument used to measure spectral irradiance.

Spectrum - A range of components, arranged in order. For example, the spectrum of electromagnetic radiation is the array of wavelengths composing it.

Stratosphere - Area of the earth's atmosphere just above the troposphere and under the mesosphere. It takes up an altitude from about 15 km to 50 km above the earth's surface (The boundaries of these regions are approximate - they vary according to season and latitude.) The majority of the ozone found in our atmosphere is contained in the stratosphere, and it is also the location of the ozone hole. In the stratosphere, the temperature gradually increases until the stratopause, which is the boundary between the stratosphere and mesosphere, is reached.

Sun Protection Factor (SPF) - A number quantifying the effectiveness of sunscreens. 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. The SPF is measured in the laboratory under standardized conditions, so caution has to be applied when using it to estimate the time one can actually stay in the sun. As many sunscreens only block UV-B radiation, they are not as effective as the SPF suggests in protecting the body from harmful effects caused by UV-A radiation, like photoaging and possibly melanoma.

Troposphere - Area of the earth's atmosphere extending from the surface of the earth up to the borders of the stratosphere, which is about 15 km above the earth. This is the area in which we live, and all of the weather takes place. The troposphere becomes increasingly colder the higher up you go, until the tropopause (the boundary between the troposphere and the stratosphere) is reached.

Ultraviolet radiation (or UV radiation) - Electromagnetic radiation with wavelengths between 100 and 400 nanometers. These rays are emitted from the sun and are not visible. They inflict increasingly more damage upon a recipient as the wavelength decreases. Based on its effects, UV radiation is subdivided into three wavelength ranges named UV-A, UV-B and UV-C:

  • UV-A covers the wavelength range 320-400 nm. UV-A is not absorbed by the ozone layer and is the least harmful UV radiation.

  • UV-B covers the wavelength range 280-320 nm. UV-B is more energetic than UV-A, and is partially absorbed by the ozone layer. UV-B rays that are not filtered out cause sunburn and other harmful effects to humans.

  • UV-C covers the wavelength range 100-280 nm. UV-C is the most dangerous form of UV radiation, but is completely absorbed by the ozone layer. Artificial UV-C (for example emitted by electric discharges) is a threat for certain occupational group, like welders.

UV Index - UV Index is a way of expressing the amount of sun-burning UV radiation. It was invented to inform the public about the intensity of UV radiation, and is now published in newspapers and on TV. The definition of the UV Index is the same throughout the world, so it's a great way to learn about the UV hazards at your travel destinations. The Index is a simple number. 1-3 means low exposure; 4-6 means medium; 7-9 means high; and more than 10 means extreme exposure. 

Mostly wavelengths in the UV-B contribute to the UV Index. The contribution from the UV-A is only about 10%.  Therefore, the UV Index is highly affected by the thickness of the ozone layer, in contrast to UV-A radiation.  

The time you can stay outside in the sun at a given UV Index depends also on on your skin type. For example, if you have fair skin (skin type I) and the UV Index is seven, it takes less than 20 minutes until your skin starts to redden. If you have a dark skin color the same UV level may need more than 40 minutes to cause an effect. Check out the noontime UV Index at San Diego measured by the NSF network! During summer it is typically 10 - this means extreme. 

Visible light - Electromagnetic radiation with wavelengths between 380 and 780 nanometers. It is the only type of radiation that the human eye can see.

Wavelength - A concept used to describe the energy of electromagnetic radiation, in which the rays are visualized as traveling in a wave-like pattern. The length of these peaks varies and is the identifying factor in the type of ray (e.g. x-ray, ultraviolet, visible). A shorter wavelength means more energy.

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