1. How Dogs Pant and Humans Sweat to Release Body Heat
Dogs do not produce enough sweat to cool themselves. Their sweat glands are located only on the pads of their feet and have a small cooling effect. Veterinary resources explain that the main way dogs release excess heat is through panting (rapid breathing) and drooling [2]. The same resources note that evaporation of sweat from the skin is the primary cooling method for horses and primates. Humans are on that side. Human skin has about 2 to 4 million eccrine sweat glands, which are sweat-producing glands, spread over the whole body [1]. Researchers from Harvard University and other institutions measured the density of these glands in the skin of macaques, chimpanzees, and humans. Humans had an average density about 10 times higher than chimpanzees or macaques. The densities for chimpanzees and macaques were surprisingly similar [3]. Although they studied only three species, the researchers believe that the increase in sweat gland density happened only in the human lineage.
Why did this happen? One leading theory is the ability to run long distances. In a 2004 paper, Bramble and Lieberman argued that endurance running abilities appeared in the Homo genus about 2 million years ago. They listed increased sweat glands and reduced body hair as features that help release heat. However, the paper itself states that it is not known exactly when these skin features evolved [4].
2. How Sweat Cools Your Body When It Evaporates
Fresh sweat is at a temperature close to body temperature. The cooling happens when water on the skin turns into water vapor. During this process, the water takes heat from its surroundings. The heat carried away by the evaporation of 1 gram of sweat is about 2.4 kilojoules (2,426 joules). This is enough to lower the temperature of 580 grams of water by 1°C [1]. Even a small amount of sweat can carry away a lot of heat if it evaporates.
The body has two ways to release heat. One is to increase blood flow in the skin to transfer heat to the air. The other is to produce sweat and let it evaporate [5]. Heat flows from higher to lower temperatures. So, when the air temperature gets close to the skin's surface temperature, it becomes harder to transfer heat to the air. This makes the role of sweat evaporation even more important.
However, not all sweat evaporates. Some sweat flows off without evaporating, according to physiology reviews. In situations where sweat is not used efficiently, such as high humidity, more sweat is needed than the amount required for effective evaporation [1]. Sweat that stays on the skin or runs off, especially when wearing clothes that hinder evaporation, is called "ineffective sweating" [5]. So, what stops sweat from evaporating?
3. How Humidity Affects Sweat Evaporation
Weather stations use a device called a psychrometer, which has two thermometers. One has a gauze wrapped around its tip and kept wet. When the water in the gauze evaporates, it takes heat away, so the wet bulb (wet thermometer) shows a lower value than the dry bulb (dry thermometer) [6]. The drier the air, the more water evaporates, and the greater the difference between the two thermometer readings. If relative humidity is 100%, water does not evaporate, and the difference is zero [6]. The Ministry of the Environment explains that this wet-bulb temperature represents the "degree of coolness felt when sweat on the skin evaporates" [7]. The wet thermometer is essentially mimicking sweaty skin.
Therefore, even if the air temperature is the same, it is harder for heat to escape from the body on humid days. The Ministry of the Environment also explains that in places with high humidity, sweat evaporates less easily, so the power to release heat from the body to the air decreases [8]. Normal air temperature is the value from the dry thermometer. Whether sweat dries or not cannot be determined by air temperature alone.
4. Why Sweat Is Salty and What Happens in the Body During Heatstroke
Sweat tastes salty because the body cannot fully reclaim the salt. Deep inside the sweat glands, a fluid with a concentration almost the same as blood plasma (the liquid part of blood) is first created. As this fluid travels through tubes toward the exit, much of the sodium and chloride ions are reclaimed by the body, so the sweat is diluted when it reaches the skin. However, when a lot of sweat is produced and the flow becomes fast, reclamation cannot keep up, and the salt concentration increases. Measurements from the forearm show that as the amount of sweat increases, the sodium concentration in sweat can rise from 19 millimoles per liter to 59 millimoles per liter [1].
Heatstroke is a general term for disorders caused by an imbalance of water and salt in the body, or a failure of the systems that regulate circulation and body temperature, in hot and humid environments [9]. Guidelines from the Japanese Society of Biometeorology explain the mechanism. When skin blood flow increases due to heat, less blood reaches the brain and other organs. Dehydration also progresses due to heavy sweating. If water is not replaced, reactions occur that suppress skin blood flow and sweating, leading to insufficient temperature regulation, rising body temperature, and further dehydration in a vicious cycle [5].
Heatstroke is divided into four types based on severity: heat fainting, heat cramps, heat exhaustion, and heat stroke (the most severe form) [9]. The most severe form, heat stroke, involves body temperature (especially brain temperature) rising above 40°C, with possible loss of consciousness and cessation of sweating [5]. The same guidelines state that if water is replaced and dehydration does not progress, skin blood flow and sweating can continue to function sufficiently, making dehydration prevention a key point for prevention. Regarding heat cramps, it is noted that after heavy sweating, if only water or low-salt drinks are consumed, the salt concentration in the blood may drop, leading to cramps. This is because salt, not just water, is lost with sweat.
Safety Note: This notebook is a personal record of investigating mechanisms and is not medical advice. If you feel unwell or suspect heatstroke, follow the guidance of medical institutions or public health authorities.
5. Why You Keep Sweating at Night During a Tropical Night
The Japan Meteorological Agency calls a night when the minimum temperature is 25°C or higher a "Tropical Night" [10]. According to guidelines from the Japanese Society of Biometeorology, people sweat even during sleep. This is why water is needed before bed and upon waking. Buildings with large heat capacity, such as concrete structures, slowly release heat stored during the day as the outside temperature drops at night. The guidelines also state that because tropical nights are becoming common in urban areas, it is not recommended to set air conditioners to turn off on a timer during sleep [5]. If humidity remains high at night, sweat dries less easily for the same reasons as during the day.
Statistics from the Fire and Disaster Management Agency also show heat inside homes. From May to September 2025, 100,510 people were transported by ambulance for heatstroke, the highest number since records began in 2008. By location, residences had the most cases with 38,292 people (38.1%), followed by roads with 19,773 people (19.7%) [11]. This is not just nighttime data, and the breakdown by time of day is not listed. However, it shows that heatstroke is not only an event that happens under the sun.
6. What Wet Bulb Globe Temperature (WBGT) Measures: Humidity and Sunlight
The Wet Bulb Globe Temperature (WBGT) is an index proposed in the United States in 1954 to prevent heatstroke. It includes three factors closely related to the body's heat exchange: humidity, radiant heat from the sun and ground/buildings, and air temperature. The unit is °C, like air temperature, but it is a different measure [8]. Outdoors in sunlight, it is calculated by adding the wet-bulb temperature (weight 7), black-globe temperature (weight 2), and dry-bulb temperature (weight 1). Indoors or without sunlight, it is wet-bulb (7) and black-globe (3) [7]. The black-globe temperature is measured by placing a thermometer inside a hollow copper ball painted black, measuring the temperature inside the ball when exposed to direct sunlight. The difference in heat between sunny and shaded spots is reflected in changes in this black-globe value. The wet-bulb has the largest weight because high humidity makes sweat evaporate less easily [8]. The Ministry of the Environment states that when the WBGT exceeds 28, heatstroke patients increase significantly.
The Heatstroke Alert is issued jointly by the Ministry of the Environment and the Japan Meteorological Agency when the daily maximum WBGT is predicted to be 33 or higher at any observation site in a region for the next day or the current day. Since April 24, 2024, a higher level called the "Extreme Heatstroke Alert" has also begun. It is issued when the daily maximum WBGT is predicted to be 35 or higher for the next day at all sites within a prefecture (excluding some sites defined by law). Both alerts use WBGT, not air temperature [12].
7. Japan's Names for Hot Days: Summer Day, Midsummer Day, Scorching Day, and Extreme Hot Day
In Japan Meteorological Agency terminology, a day with a daily maximum temperature of 25°C or higher is a "Summer Day," 30°C or higher is a "Midsummer Day," and 35°C or higher is a "Scorching Day" [10]. There was no official name for days with temperatures of 40°C or higher.
So, the Japan Meteorological Agency conducted a survey from February 27 to March 29, 2026, asking people to choose from 13 candidates. There were 478,296 responses in total. "Kokusho-bi" (Extreme Hot Day) received the most votes with 202,954, followed by "Chao-mo-sho-bi" (Super Scorching Day) with 65,896. Experts also gave many opinions that the term was socially familiar and appropriate in Japanese, and it was decided on April 17. Other suggestions included "Sauna Day," "Stay-at-Home Day," and "Boiling Day." The Japan Weather Association had already been using the same term independently since 2022 [13].
The Japan Meteorological Agency gives very hot days names to help people recognize the danger and take care. However, these names are determined only by the maximum air temperature. The WBGT used for Heatstroke Alerts includes humidity and sunlight. There are two different measures, and on the body's side, whether sweat dries or not is a major factor.
8. How to Make a Wet Thermometer at Home
You can make a wet thermometer at home. First, read the temperature of a thermometer you have (an alcohol thermometer or a digital cooking thermometer is fine). Next, wrap a thin cloth (gauze or tissue paper) around the part that measures the temperature and wet it with water. Make sure it is not dripping. Point a fan on a weak setting at it and wait about 3 minutes before reading it again.
The wet one should be lower. The difference should be large on dry days and small on rainy days or in a dry room (keep the thermometer and fan away from water). It is not as accurate as a real psychrometer, but how this difference changes is the same as how easily your sweat dries.
Safety Note: Do not use thermometers containing mercury or medical thermometers. Handle glass thermometers carefully on a table so they do not break. Do not conduct this experiment outdoors on hot days or while exercising.