Does air temperature affect the UV index? The short answer is no. There’s a common notion that the hotter it is outside, the more likely you are to develop a sunburn. But, the amount of UV radiation that reaches the earth on any given day is not affected by air temperature.
If you read our previous post, Unexpected Sunburns: Is Your Weather App Lying To You?, you’ll know that the UV index is either measured or calculated. There are a number of different variables that affect its calculation; ozone thickness, latitude, time of year, and cloud cover all impact the UV index in one way or another. However, air temperature does not¹.
In addition to UV radiation, the sun emits infrared and visible light, which heat and illuminate the earth. This is why you feel warmer while standing in the sunlight compared to standing in the shade². But be careful not to confuse the warmth you feel from infrared radiation with UV radiation. You can’t see or feel UV radiation, so the UV index can’t be estimated based on the temperature outside.
You may be wondering, “If the UV index doesn’t depend on temperature, why do dermatologists promote sun safety during the hotter summer months, and not in the cooler fall and winter months?” This is because the UV index is indeed higher in the summer than it is in the winter, but not because of a change in air temperature. When the sun sits higher in the sky, the UV index is higher, and vise versa. Since the sun sits lower during winter, the UV index tends to be lower, which lessens your risk of developing a sunburn. But, this isn’t to say that sun protection isn’t important in the winter – UV radiation reaches the earth year round.
Snow reflects up to 80% of UV rays³, making overexposure a major concern for winter athletes and mountaineers. For these individuals, the friction of the wind on their skin can cause irritation, and when paired with overexposure to UV rays, a painful windburn can result⁴.
All things considered, it’s important to take sun safety into account, no matter the air temperature, when the UV index is 3 or higher. Just because it happens to be a cooler day doesn’t mean you should rule out sun protection – your skin will thank you later!
- Environment Canada. (2013). About the UV Index. Retrieved June 9, 2016
- Health Canada. (2016). Sun Safety. Retrieved June 17, 2016
- United States Environmental Protection Agency. (2004). A Guide to the UV Index. Retrieved June 17, 2016
- Barrymore, J. (2009). Windburn Overview. Retrieved June 17, 2016