The warmest time of day depends to some extent on precise geographic location, but in most places it’s somewhere between 3:00 and 6:00 p.m., and the daily high temperature is usually recorded between 5:30 and 6:30 p.m. This is not usually the time of day when the sun is most intense, but intensity and high temperatures don’t always go hand in hand. In most places it takes a few hours for the sun’s rays to be absorbed into the environment, a phenomenon known as “thermal response.” People who are planning to spend a lot of time outdoors during the warmest time of day should typically drink a lot of water to stay hydrated, and avoiding strenuous activities is usually recommended as well.
Understanding Thermal Response
Nearly all parts of the Earth have the greatest amount of sun exposure during the early afternoon hours, but it takes time to actually heat the surface. The delay between maximum sun exposure and the warmest time of the day is called thermal response. How long it takes depends on latitude and the time of year, which means that different places will have different warmest times, often by as much as a few hours. Locations on or near the equator often experience peak warmth in the early afternoon, for instance, while those closer to either pole tend to get warmest much later.
As a general rule, it takes about three to four hours after the moment of highest solar intensity to achieve maximum warmth. In most places the sun is most intense somewhere between 10:00 a.m. and 2:00 p.m. People usually run the greatest risk of sunburn and skin damage during this window since the concentration of ultraviolet rays tends to be highest, but heatstroke and exhaustion are most common later in the day, when the sun’s energy has warmed the environment.
Warmth and ultraviolet (UV) concentration don’t usually go together, in other words. People who are sensitive to the sun or prone to sunburn often find it preferable to be outdoors later in the day to avoid intensive solar exposure. The environment is usually warm in the late afternoon because the sun’s rays have warmed it previously, and in most cases the majority of harmful rays have dissipated by the time maximum temperatures are achieved.
Importance of Hydration
Just because the sun is setting doesn’t mean that heat-related dangers have gone away, though, and for many people the warmest time of day is actually the most dangerous, at least in terms of exhaustion and dehydration. Staying hydrated during the heat of the day is vital to preventing heat stroke and other heat related problems. Water and juices are usually the best choices; alcoholic and caffeinated beverages are known as “diuretics,” which means that they can actually dehydrate the body, making things worse in most cases.
Understanding Sweat Rate
People who plan to perform a vigorous activity on hot days may find that it’s useful to know their sweat rate, and making the calculation is fairly simple. A person starts by weighing himself naked before a workout. After he's done, he should towel off and weigh himself again. The difference will be what he has lost while exercising. This weight difference should replaced with water, but not excessively; too much water comes with its own set of problems. Usually drinking only what was lost or slightly more is the best course.
Risks and Precautions
It is a good idea to avoid strenuous activity during the warmest time of the day, especially running, even for people in good health. High intensity cardiovascular workouts require a great deal from the body. If the body can’t sweat due to humidity or the evaporating sweat doesn’t cool a person off fast enough, he or she could be in danger of heat stroke or heat exhaustion.
Heat-related illnesses can have a variety of symptoms. Anyone who feels suddenly lightheaded, sick to his or her stomach, headachy or even confused during the warmest time of the day typically needs to cool off and get some water. Once these symptoms appear, it’s usually best to take a break, drink some water, and call it a day no matter what time it is.