Raindrops are not teardrop-shaped; big ones look more like hamburger buns
Children draw raindrops as teardrops, but real ones never take that shape. Small drops are round spheres. As they grow, air pressure flattens their undersides until the largest resemble hamburger buns or tiny parachutes, and beyond a certain size they simply break apart. The biggest ever recorded, over Brazil and the Marshall Islands, reached about 10 millimetres.
Rain starts with saturated air. Warm air can carry more water vapour than cold air, so cooling a parcel of air to its dew point is the usual route to cloud. That can happen as air rises and expands, touches a colder surface, radiates heat away or has water evaporated into it. Vapour then condenses on tiny particles of dust, ice or salt. In clouds above freezing, droplets jostled by turbulence collide and merge, the so-called warm rain process, until they are heavy enough to fall. In colder clouds, ice crystals grow and drop, often melting into rain on the way down.
Drops range from 0.1 to 9 millimetres across. How many drops of each size fall follows an exponential curve, the Marshall-Palmer law, named for the pair who characterised it. Bigger drops fall faster: at sea level in still air, fine drizzle arrives at around 2 metres per second, while 5-millimetre drops hit at roughly 9. Heavy storms tend to be short and gentle rain long-lasting. Sometimes rain never reaches the ground at all, evaporating on the way in streaks called virga, seen even on Mars and Venus.
Rain leaves traces. Drops landing on fresh ash can make dimples that fossilise, and because the largest possible drop depends on air density, such fossil imprints have been used to estimate the density of the atmosphere 2.7 billion years ago. The pattering sound of rain on water, meanwhile, comes from air bubbles vibrating beneath the surface.
Weather fronts are the main source of rain, with thunderclouds organising into narrow rainbands. Mountains wring moisture from winds on their windward slopes and leave rain shadows behind. Cities matter too: the urban heat island effect boosts rainfall downwind. Global warming is shifting patterns, making eastern North America wetter and the tropics drier. Rain supplies irrigation, hydroelectric power and much of Earth's fresh water.
Source: Rain
