This morning it was raining so hard the drops were painful. What causes this? Is it drop size and the height from which they fall? Or is there another mechanism that increases the speed at which drops fall? The rain seemed to be coming down especially hard, and drops hitting the ground bounced high into the air.
鈥� Two things affect the energy of a raindrop and hence how painful it feels when it hits your head: its size, and the speed at which it falls. In practice, raindrops cannot grow bigger than about 5 millimetres in diameter, as air resistance will cause drops above this size to disintegrate into smaller ones.
Now, the larger the raindrop, the higher its maximum speed, or terminal velocity. When it reaches this speed, air resistance balances the downward force of gravity, so the drop no longer accelerates.
Advertisement
A 5-mm-diameter drop rapidly reaches its terminal velocity of around 9 metres per second, depending on the prevailing air temperature and humidity, so the height from which a raindrop falls has no bearing on its pain-inducing ability.
The school in which I teach is housed in a tall building with narrow stairwells where there is minimal horizontal air movement 鈥� ideal for investigating the behaviour of 鈥渞aindrops鈥� we make.
We find that a 4-mm-diameter drop falling two floors into a puddle produces a splash or 鈥渂ounce鈥� of about 50 mm. The bounce for one falling four floors is about 150 mm, and for one falling six floors, just over 200 mm. This last result is no surprise as drops probably reach terminal velocity after falling about five floors.
Our experiments show that a drop 4 mm in diameter falling six floors on to skin feels like being hit by grains of rice from a few metres away. Drops 5 mm in diameter have slightly greater terminal velocity, but they are also almost twice as heavy. This means they have about twice as much energy and are mildly painful when falling on to skin from six floors up.
Thanks to bald colleagues for their assistance.
David Muir, Science department, Portobello High School