Link #116: A Raindrop Can Only Fall as Fast as 29 Km per Hour

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Link #116: A Raindrop Can Only Fall as Fast as 29 Km per Hour

Rain in the city, Zurich Switzerland by Thomas8047, (CC BY 2.0)

The Wall of Tears is easily one of the most beautiful places on Earth. It’s basically a mountainside that has multiple waterfalls on its sides. Nature has arranged the waterfalls in such a manner that it looks like the mountainside is crying.

The main cause for the Wall of Tears is that the mountain summit receives heavy rainfall almost throughout the year. The mountain summit receives an average of 452 inches of rain every year which makes it one of the wettest places on Earth.

Rain, in fact, is one of the most curious and well balanced natural phenomena on the planet. There are many characteristics of rainfall that will astound us. For example, have you ever wondered how fast a raindrop falls or how fast it can go? The fastest a raindrop can fall is at the speed of 29 kilometres per hour. It can’t go any faster!

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How Fast Does Rain Fall?

Rain and Wind
Image credit: Lantash, GFDL.

There is no fixed speed for falling raindrops. The speed with which they fall depends entirely on their size and mass. This means that the bigger a drop of rain is, the faster it will fall. While it isn’t possible to come up with exact speeds of raindrops, if the influence of winds is removed from the picture, we can define a range of speeds.

The reason why wind has to be removed from the equation is that it can accelerate or decelerate the speed of the raindrop. Its influence depends upon the direction in which it is blowing. Without the wind, falling raindrops can have speeds varying between 11 kilometres per hour and 29 kilometres per hour.

What Determines the Falling Speed of a Raindrop?

Rain drops
Image credit: Unsplash, CC0 Public Domain.

The speed of a raindrop depends on its mass and size. This means that its speed depends on its radius because it’s the radius will define how big and heavy it is. The size and mass of a raindrop determine its falling speed because they decide how much the Earth’s gravity will affect them.

For a raindrop to start falling, the gravitational force of Earth needs to be balanced with the friction it creates with the air while falling down. When these forces are balanced, the raindrops start falling.

Why Can’t Raindrops Fall Faster Than 29 Kilometres per Hour?

Raindrops sizes
Raindrops sizes: Own work of Pbroks13, (CC BY 3.0)

No raindrop can fall at speeds greater than 29 kilometres per hour. This speed is tied with the mass and size of the raindrop. Droplets that achieve speeds of 29 kilometres per hour have a diameter of just more than five millimetres.

The main reason why no droplet can fall faster than 29 kilometres per hour is that no droplet can attain a size greater than five millimetres in diameter. Any droplet wider than five millimetres will break apart because of its friction with air.

Why Can’t Raindrops Fall Slower Than 11 Kilometres per Hour?

Drizzle by sonicimac, (CC BY 2.0)

Conversely, you must be wondering why raindrops can’t fall slower than 11 kilometres per hour. A raindrop falling at 11 kilometres per hour is about 0.5 millimetres in width. A raindrop smaller than 0.5 millimetres in diameter is scientifically not seen as a raindrop. Instead, these little raindrops are collectively called a drizzle!

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