Why Does Water Feel Cold When It’s Hot?
Most of us know that the colder water is, the more intense the sensation it provides will be. But why does this happen when the water is hot?
If you’ve heard that very hot water sometimes feels cold, it’s because a decrease in temperature can also cause a decrease in tissue sensitivity. For example, when you submerge your hand in hot water and then it suddenly decreases to very hot water it may feel cool because you are decreasing the sensitivity of your skin. A chemical reaction takes place that also responds to specific chemicals. This is why very cold water can sometimes feel warm.
The human body is equipped with a thermoreceptor called the hypothalamus, which is in charge of regulating our temperature. When the temperature of the water decreases, it will send signals to the hypothalamus that your body isn’t overheating and then it will lower your body’s temperature. When you get used to cold water, this effect becomes stronger because that is what your body has grown accustomed to.
One way to test this effect is to put a thermometer into the water and then cool the water down, so the thermometer will read lower than when the water was at room temperature.
However, there is also a difference in behavior of hot and cold air. The colder a body of air is, the lower its pressure will be. This means that in order for us to perceive heat, we need at least 100 degrees Celsius (or 212 degrees Fahrenheit) of water or air to be near our body.
The difference between cold and hot water is due to the brain’s assessment of pressure changes. A hot body of water will raise the air temperature around it. This will cause the pressure around the air to also decrease with a concomitant increase in relative humidity due to the evaporation of some of the water. When this happens, we experience less heat from something that is hotter, but we are able to feel it more when it is cold because the pressure around us has not changed significantly so we are able to feel more intense sensations from it.
This happens only with water and air. When we are in contact with another material, like ice, for example, the cold sensation is caused by the pressure of our body against the ice and not to temperature alone.