The Speed of Rain

On average, rain falls at just about 20 km/h. But what really defines rain, and are there different types?

What is rain? It is liquid water coming from the sky, so a form of hydrometeor (hydro = water, meteor = well, something that falls from the sky). But as anybody who has gotten into a torrential downpour in the summer and into misty drizzle in winter will surely have realized, not all rain is created equal.

In fact, there are distinctions between the types of rain that exist. Most notably, between “drizzle” and “rain”, two categories which are entirely distinct forms of precipitation in weather stations’ reports (using the SYNOP key). Drizzle has a diameter of less than half a millimeter, and will not exceed rainfall rates of 1.3 liters per square meter each hour (l/m² = mm of rainfall). Rain, on the other hand, has drop sizes of at least half a millimeter but is then further subdivided into light rain (under 2.5 mm per hour), medium rain (2.5 to 7.6 mm/h), strong rain (7.6 to 15 mm/h), and torrential rainfall, which exceeds 15 liters per square meter in just an hour and will often lead to flash flooding.

Rain comes in different drop sizes as well, though. The size a rain drop reaches depends on various conditions in the cloud it originates from, but is mainly influenced by the strength of the updraft, as a stronger updraft has more force and can therefore carry larger, heavier rain drops for longer before they drop (no pun intended). Once the weight of the rain drop is so great that it falls, air resistance will change its shape yet again. The larger the rain drop, the flatter it becomes, as the air resistance from the bottom forms an indentation. A drop 3 mm in diameter therefore has the shape of a jelly bean, while a drop of just 1 mm is almost perfectly spherical. The upper limit for how large a rain drop can get is typically at around 4.5 millimeters, at which point it will look almost like a tiny parachute when photographed mid-fall. Any larger, and the air resistance will tear apart the rain drop into smaller droplets.

Oh, and to provide the answer to the title of this article: an average rain drop falls at roughly 23 kilometers per hour, so would take roughly eight minutes to fall from a height of three kilometers all the way to the ground. For comparison, a snow flake will fall at about 5 km/h, taking 36 minutes, while a hail stone the size of a golf ball may reach a whopping 103 km/h and cover the same distance in less than two minutes.

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