If the relative humidity of the atmosphere increases, the rate of evaporation of sweat from the human body?

Step 1: Analyze the question We're asked what happens to the rate of evaporation of sweat when the relative humidity of the air increases.

Step 2: Explain the concept Relative humidity (RH) compares the amount of water vapour in the air with the maximum it can hold at that temperature.

• High RH means the air is already close to saturation, so it can take up very little extra water vapour.
• Low RH means the air is dry and can absorb a lot more water vapour.

Evaporation happens when water molecules leave the liquid surface. At the same time, vapour molecules in the air return to the liquid (condensation). If the air already has lots of vapour, more molecules return, so the net rate of evaporation falls.

Step 3: Apply it to sweat

• When RH rises, the air near the skin holds more water vapour.
• Sweat evaporates more slowly, so it stays on the skin.
• That's why we feel sticky and uncomfortable on humid days (e.g. in Lagos or Port Harcourt during the rainy season), and why cooling by sweating is less effective.
• On dry, harmattan days (low RH), sweat evaporates quickly and we feel cooler.

Step 4: Choose the answer Increased relative humidity → decreased rate of evaporation.

decreases**.

Tip: Remember "High humidity, slow evaporation." Factors that increase evaporation rate are: higher temperature, larger surface area, wind or air movement, and lower humidity. Humidity works in the opposite direction to the others.