💧 The water cycle

The water cycle is the endless journey water takes as it evaporates from oceans, lakes and rivers into the air, forms clouds, falls back down as rain or snow, and eventually flows back to the ocean to start the whole journey over again. The very same water has been cycling through this loop for billions of years, so the water falling as rain today has almost certainly fallen as rain many times before. Every part of the water cycle, evaporation, cloud formation, and rainfall, actually works because of the sun's heat driving it all along.

Evaporation happens when water at the surface of a liquid absorbs enough heat energy that individual water molecules break free and drift off into the air as invisible water vapour. Warmer water evaporates faster because its molecules already have more energy and need less of a push to escape, which is exactly why a puddle dries up faster on a hot day than a cold one. Wind speeds up evaporation too, since it constantly sweeps away the vapour right above the water, letting more molecules escape more easily.

Humidity is how much invisible water vapour is floating in the air around you, and high humidity makes hot weather feel even hotter, since damp air makes it harder for sweat to evaporate off your skin and cool you down. Different places naturally hold very different amounts of humidity, tropical rainforests tend to feel thick and sticky with moisture, while deserts usually feel dry because there is so little water vapour in their air. Meteorologists measure humidity carefully because it strongly affects how comfortable, or uncomfortable, a hot day actually feels.

Dew forms overnight when grass and other surfaces cool down faster than the air around them, and once they get cold enough, water vapour in the nearby air condenses directly onto them as tiny liquid droplets. Grass cools especially fast overnight because its thin blades lose heat quickly into the clear night sky, which is why dew often coats grass even when nearby pavement stays dry. Dew is closely related to frost, if the surface gets cold enough to drop below freezing, the same process makes ice crystals instead of liquid droplets.