Heat
As soon as a person is exposed to heat the body begins working to cool itself off which disrupts homeostasis; the heart rate increases (usually 30 beats faster for every 1 degree Celsius rise in core temperature) as it tries to push blood to the surface and release heat, the body also begins to sweat and loses essential water and salt which can lead to dizziness, cramps, nausea, and fatigue. The human body tries to maintain an internal temperature of 98.6 F, but when in extremely hot climates (or exposed to infection) the body temperature can rise to dangerous levels (105-107 degrees F). This is known as hyperthermia and if the body is not relieved of the heat source then internal organs can deteriorate, the person can collapse, and even die.
Short Term Adaptation
Sweating is a short term adaptation to heat and it helps cool the body in order to maintain a normal core temperature. The human body has around 2.6 million sweat glands distributed around the skin except on the lips, nipples, and external genital organs. These sweat glands are located in the dermis and are always producing sweat; this production increases when the body temperature rises through exercise or environmental heat. Sweating is most effective in hot, dry climates because evaporation is quicker which optimizes the cooling effect. However, a body unaccustomed to high heat can only produce one liter of sweat an hour and needs to be able to rehydrate to continue producing.
Facultative Adaptation
Sweating is a great short term adaptation for cooling the body and adapting to heat, but if someone moves to an area of high heat then the body can adapt even further to save the essential salt it would normally lose through the sweating process. Gradually the body will lower the concentration of salt in sweat so it's able to create more without potentially harming itself. This way, after only a couple of weeks, the body can produce the maximum of 2 to 3 liters of sweat in order to stay cool. Though it may take weeks for a body to adapt to heat, it only takes a couple days in a cool environment for everything to return to normal.
Developmental Adaptation
A developmental adaptation to heat is having a low body mass. Usually the taller and leaner individuals are found in places of high heat. Carl Bergmann suggested that individuals with more surface area will radiate off more heat, thus cooling themselves, and Joel Allen noticed that the length of all the appendages affects heat loss as well. Individuals found nearer to the equator had tall, slender bodies with longer limbs. This body type optimizes radiation which is the primary process for heat loss.
Cultural Adaptation
A main cultural adaptation to deal with heat is the Air Conditioner. A/C units allow humans to control the temperature of their homes or cars which is a great relief in areas with high heat. The invention of the air conditioner is one of the most effective ways we have culturally adapted to heat.
Studying human variation with regard to heat is important because then we are able to find ways to survive in warmer climates. With this information we see that it is possible to survive in high heat if we keep our body hydrated and provide it with the necessary protection to keep functioning. Knowing our breaking point with regard to heat opens the door to new possibilities for technology that can help us survive any climate.
I would not use race to understand the variation of adaptations to heat because none of them are based around race. Race is a modern idea that divides and categorizes people based on their appearance, but it has no genetic influence. Two people of the same race may have completely different tolerances to heat so it's not an accurate representation of variation. Looking at the environmental influences to understand human variation is better because we can see factual evidence for how humans deal with high heat. When looking at environmental influence we can see the cause in the environment and the effect in adaptation which gives us a clearer picture of human variation across the globe. Race is too narrow of a window to see the world through.
