Modern science has furnished mankind with miraculous technological advances and wondrous discoveries about the human body--the Apollo Program, The Internet, self-driving cars, Rogaine--now, we finally have an answer to the age-old question: Why do hot drinks keep you cool on a hot day? Shouldn't cold drinks do that instead? Who wants hot tea or hot coffee on a scorching day?
It's not just an old wives tale. According to Ollie Jay, researcher at University of Ottawa's School of Human Kinetics and the University of Sydney's Faculty of Health Sciences, consuming beverages hotter than the normal human body temperature (98.6 degrees Fahrenheit) produces an increase in perspiration, which cools you down. Even though you're adding heat to the body in the form of a hot beverage, the corresponding increase sweating offsets the heat input.
While the exact mechanism behind this process of drinking hot drinks remains unclear, Dr. Jay's theory is that it is "thermosensors that line the throat and mouth that elicit the additional sweating response," not an actual increase in the body core temperature. This theory was put to the test at Dr. Jay's Thermal Ergonomics Laboratory where he monitored test-subjects wearing skin temperature sensors while riding stationary bicycles as they drank either hot or ice water.
The results were definitive; a hot liquid will cool you down on a hot, dry day. Counterintuitive as it may seem, you may want to consider ordering a hot cup of coffee, steaming mug of coffee, or bubbling bowl of soup next time on a sweltering summer day.
When Hot Drinks Won't Keep You Cool
But there are a couple of exceptions to this counterintuitive cooling calculus, Dr. Jay discovered. Most importantly, your body sweat must be able to evaporate in order for the body to effectively dissipate heat. This means your skin surface has to be exposed to open air so your body heat storage system can jumpstart into action with the sweat response.
Dr. Jay also found higher levels of ambient moisture (i.e. relative humidity) greatly diminish the cooling effect of a hot beverage and its increase in sweating. So, it's probably for the best to avoid that glass of warm milk on humid day on the coast. Instead, stick to colder drinks in warm weather and skip the warm drinks themselves. Or maybe grab an ice cream, because ice cream solves everything.