Humidity and drying in daily life

Why is a humid day uncomfortable?

On a dry, hot day at $35\ ^\circ\text{C}$, sweat evaporates quickly and we feel less hot. On a humid day at the same temperature, the same sweat stays on the skin and we feel sticky and hotter. The reason is the humidity.

Humidity is the amount of water vapour in the air. The air can hold only a certain amount of water vapour at a given temperature. When the air is close to that limit, it is very humid, and very little more water can evaporate into it.

Relative humidity

Weather reports give the relative humidity as a percentage. It compares the water vapour that is in the air with the most that the air can hold at that temperature.

• $100\ \%$ means that the air is full. No more water can evaporate, and the water starts to condense.
• $50\ \%$ means that the air holds half of what it can.
• A low percentage means dry air, and fast evaporation.

Warm air can hold more water vapour than cool air. This is why the same amount of vapour can make a cold evening feel damp (the percentage is high) and a warm afternoon feel dry (the percentage is low). It is also why dew forms at night. The air cools and can hold less vapour, so the extra vapour condenses on the grass and on cold things, such as a glass of cold water.

You should find that air at $31\ ^\circ\text{C}$ with $14\ \text{g}$ of vapour in each cubic metre is quite dry, yet it forms dew if it cools below about $17\ ^\circ\text{C}$.

Humidity and drying

• In the dry season, clothes dry quickly.
• In the rainy season, the humidity is high, and clothes dry slowly, and may even smell.
• In a bathroom after a hot shower, the air is full of vapour, so a wet towel dries slowly.
• An evaporative cooler (a cooler that blows air over wet pads) works well in dry weather, because the water evaporates easily and cools the air. It works poorly in a humid place, because little water evaporates.
• Washing hung in a closed room dries slowly, because the vapour has nowhere to go. Hung in the open or near a window, it dries faster, because the wind carries the vapour away.

Evaporation and the water cycle

The Sun warms the surface of the oceans, rivers and lakes, and water evaporates. The vapour rises. In the cold air above, it condenses into tiny drops of water, and forms clouds. When the drops become large, they fall as rain. This is the water cycle. Evaporation and condensation are the two changes of state that drive it.

Plants add to it too. They take water from the soil, and a large part evaporates from the leaves in a process called transpiration. A big tree can send hundreds of litres of water into the air in one day.

A good example of the cycle in a small place: heat water in a closed jar with a cold lid. The water evaporates, and the vapour condenses on the cold lid as drops, which fall back like rain.

Practise with new numbers

Try it in Python

The program follows the air outside after sunset, as it cools while its vapour stays the same.

Putting it together: a summary of the chapter

• Matter is made of very small particles that move, have spaces between them, and attract each other.
• A solid, a liquid and a gas differ in how close their particles are and how strong the force between them is.
• When matter changes state, the temperature stays the same, while the latent heat goes in or comes out.
• Evaporation takes place at the surface at any temperature. It is faster when the surface is large, the temperature high, the humidity low and the wind strong. It cools.

Common mistakes

• Thinking that humid air is air with a lot of liquid water. It is air with a lot of water vapour, which is invisible.
• Thinking that a high temperature always means fast drying. A hot, humid day dries clothes slowly.
• Confusing humidity and temperature. They are different, and both change evaporation.
• Thinking that clouds are water vapour. Clouds are tiny drops of liquid water, or ice crystals. The vapour itself is invisible.

Remember these

• Humidity is the water vapour in the air. High humidity slows evaporation.
• On a humid day, sweat and wet clothes dry slowly.
• Warm air can hold more water vapour than cool air.
• Evaporation and condensation drive the water cycle.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why do we feel hotter on a humid day than on a dry day at the same temperature?

2. Air at $25\ ^\circ\text{C}$ can hold at most $23\ \text{g}$ of water vapour in each cubic metre. It holds $11.5\ \text{g/m}^3$. What is the relative humidity?

3. Why does a desert cooler work well in Rajasthan in May but poorly in Mumbai in July?

4. Why does dew form on grass on a clear night?