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OpenUV Knowledge Base

Collection of useful tips about Sun Safety and Ultraviolet Radiation Index for Developers


Cloud Effect on UV Radiation Index

On average clouds absorb UV radiation, reducing ground-level UV intensity:

  • Clear skies allow virtually 100% of UV to pass through
  • Scattered clouds transmit 89%,
  • Broken clouds transmit 73%,
  • Overcast skies transmit 31% *.

Some weather services like Forecast.io provide you with current clouds coverage in percent. For example, you could use that information in your Android app to correct OpenUV UV Index value like that:

public double calculateCloudsFactor(double rawUV, String weather, double cloudsCover)
{
  if (weather.equals(RAIN) || weather.equals(SLEET) || weather.equals(SNOW))
    return rawUV * 0.31;
  else if (cloudsCover < 0.2)
    return rawUV;
  else if (cloudsCover >= 0.2 && cloudsCover < 0.7)
    return rawUV * 0.89;
  else if (cloudsCover >= 0.7 && cloudsCover < 0.9)
    return rawUV * 0.73;
  else if (cloudsCover >= 0.9)
    return rawUV * 0.31;
  else return rawUV;
}