When someone says, "It is really hot today" what does he mean?
Our bodies perceive warmth not so much as a condition of the air temperature, but as a perception of the effect it has on our skin. Evaporation of water from our skin, or sweat, is what makes us feel cool or warm. And this evaporation is a function of both the air temperature as well as humidity.
Humidity is the amount of water vapour in the air. The higher the humidity the lower the ability of the skin to evaporate sweat leading to cooling the body down thus ending in the perception of being warmer than it is. At the same air temperature, the body will feel cooler or warmer depending on whether the humidity is lower or higher. The more the water vapour in the atmosphere, the warmer it will feel. Some people experience difficulty breathing in areas with high humidity. Some cases may possibly be related to respiratory conditions such as asthma, while others may be the product of anxiety. Some will hyperventilate causing numbness, faintness, loss of concentration, etc.
With increased value of Heat Index, there is a possibility that the body's cooling mechanism shuts down leading to dehydration, heat exhaustion, heat stroke and finally death.
It is therefore, extremely important to understand how a combination of humidity and temperature affects our perception of warmth and heat. This is calculated by what is termed the Heat Index or Humidex. It calculates a value based on a combination of temperature and humidity. If you know the Heat Index, you will be better able to cope with the environment you find yourself in.
Take a look at the chart below to better understand this concept.