Wednesday, July 29, 2015

What's My UV Exposure?

Here is where the home form goes.

Your home data should appear here, but something has kept the script from updating it.

Your Current Latitude (please use decimal degrees, no minutes or seconds):
North or South:
Choose your nearest Home Elevation:

Please enter your current position data above.

Many places talk about the UV index as a way to indicate your exposure day to day, for example as part of the local weather report; however, it is difficult for most people to use this information effectively. That is why earth42 has developed an exposure calculator here, one that will compare your current location to that of your home, ie, the place you are accustomed to, as well as to a reference location on the Earth's equator at sea-level.
Here are a couple of assumptions:
  1. the earth's atmosphere is our major protection from harmful radiation, and this protection is essentially uniform throughout the planet. I know there are local variations in the temperature, ozone content, etc., but for our purposes we can idealize a bit and use a "rule of thumb", the Standard Atmosphere Model.
  2. The major difference from one place to another in the level of protection comes from its latitude. The farther from the equator, the greater the angle of sunlight, and thus the greater amount of atmosphere it passes through. This is what gives rise to the differences in climate as one travels from equator to pole. Additionally, as the Earth orbits the Sun, its tilted axis of rotation modifies the effective latitude of a given location. This is what gives rise to seasonal changes.
  3. Elevation above sea-level also affects ones exposure. The higher you are in the atmosphere, the less there is of it to protect you. This can undo much of the protection from higher latitudes, and measurements of air density by altitude have been made that we can use here.
 Combining these factors, we can calculate your exposure in a way that will give you a more intuitive sense of what to do about it.