What are the factors affecting evaporation?

Why does water dry faster on some days?

Wet clothes dry quickly on a hot, sunny day and very slowly in the rainy season. A wet floor dries faster when the fan is on. Sweat dries fast on a windy day. These everyday clues tell us that some conditions make water evaporate faster or slower. Let us test them one at a time.

1. Exposed area

Take the same amount of water in a small bottle cap and on a plate, and keep them side by side. The water on the plate dries first.

On the plate the water is spread out, so a larger surface is open to the air. This open surface is called the exposed area. Evaporation happens at the surface, so a larger exposed area lets more water escape every minute. That is why we spread out wet clothes instead of leaving them in a heap.

2. Sunlight

Pour equal amounts of water into two identical bottle caps. Keep one in sunlight and the other in shade, and look at them every 15 minutes. The water in the sunlight dries first. Warmer water evaporates faster, which is why clothes dry quickly on a hot, sunny day.

3. Moving air

Clothes dry faster on a windy day, and a wet floor dries faster under a fan. Water evaporating from a wet surface makes the air just above it moist. Moving air carries that moist air away and brings in drier air, so evaporation keeps going quickly.

4. Humidity

On a rainy day the air already holds a lot of water vapour: it is humid. Water evaporates slowly into air that is already full of vapour, so clothes take a long time to dry. To dry clothes faster on such a day, spread them out well and hang them where air moves, for example under a fan.

How do you run a fair test?

In each test, change only one condition and keep everything else the same. Otherwise you cannot tell which change made the difference.

Kept the same	Changed	Measured
Amount of water, place, starting time	Exposed area (bottle cap or plate)	Time for the water to dry completely
Amount of water, identical caps	Sunlight or shade	Time for the water to dry completely

Try your own test too, for example a fan against still air.

Why does evaporation cool things?

Water needs heat to evaporate, and it takes that heat from the surface it is on. So evaporation has a cooling effect.

• Matka (earthen pot). Water seeps slowly through the tiny pores of the clay and evaporates from the outside of the pot. This takes heat from the pot and the water inside, so the water stays cool. A steel pot has no pores, so its water does not cool this way. A surahi works the same way.
• Sweat and a fan. The moving air helps sweat evaporate, and the evaporation takes heat from our skin. That is why we feel cool under a fan.
• Water on a hot roof. Sprinkling water on a floor or roof in summer cools it as the water evaporates.
• Sanitiser. Your hands feel cool when you rub sanitiser on them, because it evaporates quickly.

Make a pot-in-pot cooler

You need two clay pots of different sizes and some sand. Put a layer of sand in the bottom of the large pot, place the small pot in the middle, and fill the gap between them with more sand. Pour water into the sand and cover the small pot with a lid or a wet jute sack. In about 4–5 hours the inside cools down. Keep fruits or vegetables in it, add water to keep the sand moist, and check every day how long they stay fresh.