Smartphone users across the world rely on various weather apps for the daily forecast. But, counting on your favourite app for the daily UV index could cause you to develop an unexpected sunburn.

Before delving into why your weather app is probably wrong when it comes to the UV index, it’s important to know how the UV index is calculated in the first place. According to National Oceanic and Atmospheric Administration (NOAA) and Environment Canada, first, the “clear sky” UV index is mathematically calculated based on ozone thickness, latitude, and time of year. This value would be the “actual” UV index if there were no clouds or precipitation expected for the day. To make the UV index more accurate, the “clear sky” UV index is adjusted based on the daily cloud and precipitation forecasts for the particular city. This gives the “actual” UV index.¹⁻²

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The last step of the calculation (adjusting for daily clouds, precipitation, and estimated cloud transmission ratio) is where the problem arises. Weather stations usually measure cloud and precipitation forecasts from the city’s closest airport, and in some cases, this airport isn’t so close. Take Newmarket, ON as an example – its weather data is measured from an airport 27km away. While it may be overcast at the airport, there could be minimal cloud cover in your exact location, meaning that the UV index where you’re standing is actually higher than you think. The problem? The higher the UV index, the more likely you are to develop a sunburn.

Your environmental situation also affects your UV exposure. Reflective surfaces can increase your exposure, but this isn’t factored into the daily UV index. Snow reflects up to 80% of UV radiation, so if you’re skiing or snowboarding, you may be exposing yourself to nearly twice as much UV radiation as you think.³

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All in all, the UV index in your exact location could be higher than what is reported on your weather app, since it is based on data measured from locations that may be far from you, and it does not consider reflection of UV radiation. For these reasons, it is not safe to rely on this number to help us decide which sun protection methods to use.

Sun Index provides real-time, location-specific UV index readings, and gives personalized sun safety tips to help you stay safe outdoors and avoid unexpected sunburns.

  1. Environment Canada. (2013). About the UV Index. Retrieved June 9, 2016
  2. National Weather Service Climate Prediction Center. (n.d.). UV Index: How It is Computed. Retrieved June 9, 2016
  3. World Health Organization. (n.d.). Ultraviolet radiation and health. Retrieved June 9, 2016