How do rain clouds defy gravity?
In: Physics
Contrary to what many people believe, clouds are not vapour, they are not gas, they are liquid. They are formed from vapour that has condensed into microscopic liquid water droplets that are attracted to one another to form different types of cloud. In really high clouds they even freeze and form solid ice crystals.
The reason they stay in the air involves a bit of math. As soon as the droplets form they start falling due to gravity. Every body that falls into a fluid (like Earth’s atmosphere) will find resistance (due to air drag) and achieve a maximum velocity called terminal velocity, V.
The terminal velocity V of a sphere with a radius R (a droplet in this case) is close to 100,000 times R squared. So if you take a droplet of radius R = 0.005 mm (an usual size for the droplets found in clouds) you get that V = 100,000xR^2 = 2.5 mm/s.
Now you have to think that in our atmosphere we have hotter air below and colder air above. This creates convection air currents, wind currents that go up. So at the same time we have droplets falling at about V = 2.5 mm/s, they are also being pushed up again by this ascending currents, so the clouds keep bouncing and floating up and down these currents most of time.
Eventually though, more vapour arrives at the cloud coming from the evaporation of oceans and lakes and rivers, and the droplets get bigger and bigger. Whey they get big enough that the terminal velocity exceeds the ascending winds velocity they fall as rain.
Latest Answers