Here at ForecastWatch, we analyze forecasts for temperatures, among other elements, for accuracy. All these factors (wind, precipitation, humidity, cloud cover, etc.) play a role in how temperatures feel to people. When a weather forecast is generated, the “temperature” indicated is how fast molecules are moving in the air around you. But there is often more beyond the temperature of the air that affects how hot or cold you feel.
A recent article on FiveThirtyEight.com referenced different ways that measure how temperatures may feel to different people. During summer in the United States, the heat index is usually used, which combines a measure of the temperature and humidity to indicate how easily perspiration can evaporate from the human body – and thus how efficiently your body can cool down. However, there are other measurements that can be used. Canada uses an index called Humidex, which also uses temperature and relative humidity. Some private companies use propriety formulas, such as AccuWeather’s RealFeel temperature. That index, according to AccuWeather’s website, uses humidity, cloud cover, winds, sun intensity, and angle of the sun. Another index that uses those factors is the wet bulb globe temperature, for which the National Weather Service even provides forecasts. According to the FiveThirtyEight article, more than 116 different indices are in circulation to measure how temperatures may actually feel on your body.
To make it even more complicated, each person can have a different comfort level even with the same value of a specific index. These can vary for several reasons, one of which is physiological acclimation. The human body is sensitive – anything that is not normal to it will prove tough until the body begins to acclimatize. This is seen, for example, when people from cooler climates are exposed to hot conditions or mountain climbers are exposed to colder temperatures and lower oxygen levels.
When one’s body has not been acclimated to a certain level of heat, the body can act in strange ways. Heart rates increase to pump more blood to extremities. The body is at an increased risk for heat-related illnesses if not acclimated as the heart works harder and internal body temperature increases. This happens at lower temperatures in normally cooler areas. For example, temperatures in the 80s Fahrenheit in Ireland and the United Kingdom cause a higher risk of heat-related illnesses than the same temperatures in most of the United States where people are acclimated to those summer temperatures. It also helps explain why temperatures of any abnormal heat wave in a region, such as the 2020 Pacific Northwest heat wave, can kill so many but the same temperatures in places further south such as California and Arizona do not. They simply feel completely different to people who are not acclimated. With heat acclimatization normally taking at least a week or two for most people, sudden intense heat waves, and even continuing temperature increases over time, can lead to widespread heat illnesses. Even the same person can feel completely different from season to season in the same location. This is an important reason why increased heat wave frequency due to climate change is such a concern.
Acclimation is not the only difference between the feeling or risk of heat- or cold-related illnesses. Behavioral adaptations, gender, amount of melanin/dopamine, and other factors also affect heat sensitivity. Thus, any heat index or wind chill will not be able to account for how different temperatures feel to different people. ForecastWatch evaluates the accuracy of temperature forecasts, but how can the accuracy of how hot or cold individuals feel be evaluated, measured, or forecast due to all these factors?
ForecastWatch evaluates accuracy of temperature, precipitation and wind forecasts. Our data offers the ability to request customizable collections – by location, time period, and forecast parameters, such as wind and temperature – and have them conveniently delivered as easy-to-use CSV files. Contact us for more information.