Behind the Forecast: What the UV index really means

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Science Behind the Forecast: UV Rays (7/5)

LOUISVILLE, Ky. (WAVE) - Sunburn is almost synonymous with summer. The reason sunburn is more likely in the summer: Ultraviolet (UV) rays.

There are three types of UV rays: UV-A, UV-B, and UV-C. UV-C rays are almost completely absorbed by the ozone layer as they enter the Earth's atmosphere. UV-B and UV-A rays can easily make it through this layer. Both can easily damage our skin, just in slightly different ways. UV-A rays do not cause sunburns but they can lead to long-term damage and potentially cancer. UV-B rays can cause sunburn by damaging DNA. While we are all vulnerable, those with darker skin are equipped with much more melanin which helps to block the radiation and minimize DNA damage.

The UV Index explains how much UV radiation will reach the earth's surface on a particular day. The typical UV Index forecast is given for when the sun is highest in the sky and the time of highest fluctuation in UV radiation.

The UV Index runs on a scale from 1 to 11+ with 1 meaning low danger and 11 meaning extreme danger. The higher the UV Index, the larger the amount of skin and eye damaging UV radiation. Therefore, the higher the UV Index, the less time it takes before skin or eye damage happens.


There are a few factors that affect UV radiation.

First the season: The peak daily ultraviolet radiation level changes throughout the year with the highest being at the Summer solstice (21st June) and the lowest at the Winter Solstice (21st December).

One of the biggest factors: the time of day. Direct and diffused sunlight affects the amount of UV radiation seen. If radiation has to travel a larger distance through the atmosphere then more of it is absorbed by the atmosphere. This means there's less direct UV radiation reaching the ground when the sun is lower in the sky. However, since UV wavelengths are scattered more than visible light, the diffused UV radiation is greater when the sun is closer to the horizon. UV radiation is greatest during solar noon, which isn't necessarily the hottest time of day. During the first week of July, solar noon in Louisville is around 1:47 PM, while the hottest part of the day may be several hours later.

Of course clouds have a massive effect on solar radiation. The water droplets in clouds can reflect and scatter radiation. It's all dependent on the size, shape and thickness of the cloud. The larger the cloud, the less UV radiation transmitted. Keep in mind, those fluffy cumulus clouds can actually reflect quite a bit of radiation and in turn increase the amount of radiation that reaches the ground.

More UV radiation is seen at higher elevations because there's less atmosphere for it to travel through. This means there's less of it being scattered at higher heights.

Air pollution actually decreases the amount of UV radiation reaching the ground because UV radiation coupled with heat actually works with pollution to create smog.

The body fights DNA damage by killing off affected cells in a process called apoptosis. Apoptosis reduces the potential for mutated cells to reproduce wildly potentially forming a tumor. Increased blood flow in the affected areas makes the skin feel hot and the creation of certain proteins causes itchiness and pain. Dead skin cells flake off leading to the peeling typically seen days after a sunburn.

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