Rain Water

Lessening raindrops are often depicted in popular culture as "teardrop-shaped" — round at the bottom and narrowing towards the top — but this is untrue. Only drops of water dripping from some sources are tear-shaped at the moment of creation. Small raindrops are nearly globular. Larger ones become increasingly flattened on the bottom, like hamburger buns; very large ones are shaped like parachutes. The shape of raindrops was studied by Philipp Lenard in 1898. He found that small raindrops (less than about 2 mm diameter) are approximately spherical.

As they get larger (to about 5 mm diameter) they become more doughnut shaped. Beyond about 5 mm they become unsteady and fragment. On average, raindrops are 1 to 2 mm in breadth. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 — some of them were as large as 10 mm. The large size is explained by concentration on large smoke particles or by collisions between drops in small regions with particularly high pleased of liquor water.

Raindrops impact at their terminal velocity, which is greater for larger drops. At sea level and without wind, 0.5 mm trickle impacts at about 2 m/s, while large 5 mm drops impact at around 9 m/s. The sound of raindrops hitting water is caused by bubbles of air oscillating underwater.

Generally, rain has a pH slightly under 6. This is because special carbon dioxide dissolves in the dewdrop to form minute quantities of carbonic acid, which then partially dissociates, lowering the pH. In some desert areas, airborne dust contains enough calcium carbonate to counter the natural acidity of rainfall, and rainfall can be neutral or even alkaline. Rain below pH 5.6 is considered acid rain.