Should you trust the UV index forecast?

The UV index is estimated from the EPA. It is an estimation of how strong the UV is at the zipcode level.

Over the past few decades, science has come to understand how ultraviolet rays from the sun affects human health. Today it is medically accepted that UV is a leading cause of sunburn, skin cancer, lupus flares and a whole host of other symptoms. Public education efforts have also disseminated this information to us, to the point where we are aware of UV and use sunscreen to protect ourselves against it.

UV index prediction
‍The UV Index forecast on iPhone and Android

The UV Index is a convenient way to understand the strength of UV as related to our health. Developed by doctors in 1987, it accounts for the way high-energy UV (UVB) affects us differently from low-energy UV (UVA). It’s also on a simple scale of 0 to 12, with accepted guidance for different levels. You’re probably already familiar with the UV Index forecast. Whether it’s from your local news channel or the weather app on your phone, information about the UV Index forecast is available at your fingertips.

The forecast is useful as a guide to certain choices, such as whether to apply sunscreen, or wear that pair of sunglasses when going out. However, using it to make other decisions, such as when to go out and how much time to spend outside, is not a good option. Here are some reasons why.

Many forecasts don’t give you a number

A number of forecasts, especially on television, will just provide generic wording such as “Very High”, “High”, or “Moderate”. This might sway the decision to apply sunscreen, but it does not provide much guidance about safe amounts of time to spend in the sun. For example, at a UV Index of 10, most Caucasians would sunburn in under 20 minutes, without using sunscreen. But at a UV Index of 5, this safe limit doubles to 40 minutes. Imagine having to go out for a jog, and having to make a decision on how long you want to stay outside — based on a reading of “High” or “Very High”. Not very helpful, is it?

The UV Index is measured laying flat on the ground

If you had to be experiencing the forecasted amount of UV, you would have to be lying flat on your back on the ground under the open sky. It’s safe to say that most of us don’t have the luxury of spending our time in this position. When outside, most of us are upright — whether it’s walking, driving or biking. The UV Index that we personally experience can be either lower, or higher than the forecast depending on the time of the day and whether we are facing the sun or away from it.

The forecast is only for direct sunlight under open skies

The UV Index is measured under open skies, with no light-reflecting structures or shade around. This is not the usual kind of environment that we spend our time outdoors in. We go to parks, shade from trees lowers the UV. We walk in cities, where reflected light from surrounding buildings raises the UV. Even at the beach, light reflects from the sand and water to increase the UV, beyond what is forecast. We wear hats, that throws shade over our faces and parts of our body. There are innumerable situations in which the UV Index forecast fails to accurately reflect our personal situation.

It’s a forecast — It’s not guaranteed to be accurate

There are many situations that can affect the UV index within a short span of time. The primary ones are weather and atmospheric particles. When a cloud comes over the sun, the UV Index drops rapidly. The amount by which it drops depends on the thickness of the cloud cover, which is continuously changing — and consequently, so is the UV Index. The concentration of ozone, and other particles in the atmosphere also contribute to changes in the UV Index. Smog, for example, drops the UV significantly. Even though a UV Index forecast for your zip code is available, in reality the strength of UV varies even over a few miles.

‍The EPA UV Index app is accurate only 18% of the time

The Environmental Protection Agency (EPA) UV Index mobile app is the basis for forecasts provided by a number of weather apps. We performed an experiment where we compared the forecast from the EPA UV Index app to state-of-the-art laboratory equipment for measuring the UV Index. For reference, we also used a Shade UV sensor, which is capable of measuring the real-time UV Index. We took over 200 readings in various weather situations, at different times of the day. Results showed that the UV Index forecast was only accurate 18% of the time, while the Shade UV sensor was accurate more than 90% of the time.

‍The UV Index forecast is the maximum for the day

Most forecasts only talk about the maximum for the day

When the UV Index forecast comes in the form of a number, it is usually a single digit which represents the UV Index at one time of the day. This time corresponds to when the sun is at its zenith, which is local noon, or 1 pm — depending on whether daylight saving time is in effect. If you were trying to make a decision on whether it was safe to go out at 10am and spend an hour in the garden, the forecast would be of no use, because it does not tell you what the UV is between 10 and 11 a.m.

The forecast gives you only one digit

For most of us that might be fine, especially if an abundance of decimal places tends to overwhelm us. But the case is different for those with photosensitivity. For such people, very low levels of accumulated UV have the potential to cause debilitating symptoms, and hence their days need more careful planning. Having extra precision in knowing the UV Index can mean the difference between spending the 10 minutes extra to go to the store or not. Conversely, not knowing the UV Index more accurately can mean spending 5 minutes too long in the sun and suffering the consequences of it the next day.