Sunlight
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Sunlight in the broad sense is the total spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered by the atmosphere, and the solar radiation is obvious as daylight when the Sun is above the horizon. This is usually during the hours known as day. Near the poles in summer, sunlight also occurs during the hours known as night and in the winter at the poles sunlight may not occur at any time. When the direct radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and heat. Radiant heat directly produced by the radiation of the sun is different from the increase in atmospheric temperature due to the radiative heating of the atmosphere by the sun's radiation.
The World Meteorological Organization defines sunshine as direct irradiance from the Sun measured on the ground of at least 120 W·m−2.
The sun's nuclear energy source was discovered by Hans Bethe.
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Cultural aspects
Many people find direct sunlight to be too bright for comfort, especially when reading from white paper upon which the sun is directly shining. Indeed, looking directly at the sun can cause permanent vision damage. To compensate for the brightness of sunlight, many people wear sunglasses. Cars, many helmets and caps are equipped with visors to block the sun from direct vision when the sun is at a low angle.
In colder countries many people prefer sunnier days and often avoid the shade. In hotter countries the converse is true; during the midday hours many people prefer to stay inside to remain cool. If they do go outside, they seek shade which may be provided by trees, parasols, and so on.
Sunshine is often blocked from entering buildings through the use of window blinds, awnings, shutters or curtains.
Sunbathing
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Sunbathing is a popular leisure activity in which a person sits or lies in direct sunshine. People often sunbathe in comfortable places where there is ample sunlight. Some common places for sunbathing include the beach, open air swimming pools, the park, the garden, and pavement (sidewalk) cafés. Sunbathers typically wear limited amounts of clothing or swimsuits. Additionally, in nudist areas some sunbathers prefer to go nude.
For many people with pale or brownish skin, an additional or primary purpose for sunbathing is to darken one's skin color (get a sun tan) as this is considered in some cultures to be beautiful, associated with outdoor activity, vacations or holidays, and health. Indeed, the body produces vitamin D from sunlight, and excessive seclusion from the sun can lead to deficiency. An additional reason that some people prefer nude sunbathing is that an "all-over" or "even" tan can be obtained.
Skin tanning is achieved by an increase in the dark pigment inside skin cells called melanocytes and it is actually an automatic response mechanism of the body to excessive exposure to ultraviolet radiation from the sun or from artificial sunlamps. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources. The skin of Black and other dark-skinned people may represent an evolutionary advantage developed a long time ago in races living in tropical areas, such as Africa.
Adverse effects on health
On this last point it is important to note that excessive sunlight exposure has been linked to all types of skin cancer, which are caused by the ultraviolet part of radiation contained in sunlight and sunlamps. Sunburns are mild to severe inflammation effects to the skin and can be avoided by using a proper sunscreen cream or lotion or by gradually building up melanocytes over days and weeks of increasing exposure. Another detrimental effect of UV exposure is accelerated skin aging (also called skin photodamage), which produces a rather ugly and difficult to treat cosmetic effect. The decrease in the atmosphere's ozone layer in the last decades is increasing the incidence of such health hazards and extra precautions should be taken by people who are exposed daily to strong sunlight.