The Core Reason: Impaired Evaporation
High humidity makes hot weather feel hotter because it significantly hinders your body's primary cooling mechanism: the evaporation of sweat. When sweat evaporates from your skin, it carries away heat, cooling you down. In humid conditions, the air is already saturated with water vapor, reducing the rate at which additional moisture (your sweat) can evaporate into the atmosphere, trapping heat on your body.
How Sweat Cools the Body
Our bodies maintain a stable internal temperature through a process called thermoregulation. When we get hot, sweat glands release water onto the skin's surface. As this liquid sweat turns into gaseous water vapor, it absorbs a considerable amount of energy (latent heat of vaporization) from the body, thus cooling the skin and the blood flowing beneath it. This evaporative cooling is crucial for preventing overheating.
A Practical Comparison: Dry Heat vs. Humid Heat
Consider a 35°C (95°F) day in a desert climate versus a tropical climate. In the desert, with very low humidity, sweat evaporates quickly, providing effective cooling, making the dry heat tolerable. In contrast, the same temperature in a tropical region with high humidity feels much more oppressive and dangerous because sweat lingers on the skin, unable to evaporate efficiently, leaving you feeling sticky and overheated.
The Heat Index and Health Implications
The 'Heat Index' or 'feels like' temperature is a measure that combines air temperature and relative humidity to estimate how hot it actually feels to the human body. High humidity can raise the perceived temperature by several degrees, increasing the risk of heat-related illnesses like heat exhaustion and heatstroke. Understanding this effect is vital for public health advisories and personal safety during hot weather.