Little Drops of Water
Musings are thoughts, the thoughtful kind. For the purpose of these articles, a-musings are thoughts that might amuse, entertain and even enlighten.
Do you wake up feeling the same every morning? I don’t. Most of the time I wake up happy, I have to admit; in fact I think I am a pretty happy person. Today I woke up feeling very technical, which was a good thing because my friend Doretta, the Acting Editor of The Star, has been bugging me for some time about why raindrops come in various sizes, and you know how one thing leads to another. Well, here goes …
Take clouds, for instance, where rain comes from. Put simply, clouds are made up of water vapour and lots of small particles called condensation nuclei like dust or smoke or even salt left over after seawater evaporates (you surely haven’t forgotten The Water Cycle from your school days now, or have you?) Well, when condensation occurs, the water vapour wraps itself around these tiny particles to form tiny droplets between 0.0001 and 0.005 centimeters in diameter depending on the size of the particles.
But why does condensation occur, you might well ask. Well, let me explain. Air normally contains invisible water vapour. Warm air can contain more water vapour than cold air. Now imagine a ‘packet’ of air that is heated by some hot surface below. Hot air rises, as I hope you will agree. Well, as the packet rises, it cools, which means that it can no longer support the same amount of water vapour and condensation occurs. Please, Dear Reader, forgive the oversimplification, but I promise you, believe me, you don’t want to know the whole truth behind this process.
As some of you know, I am an avid pilot, and I can tell you that this process of condensation can be a matter of life and death in colder climates. Imagine yourself flying along either above or in the clouds and you are preparing the plane landing. You check the automatic weather report for your destination, which tells you the visibility, height of clouds, wind direction and so on at the airport. It will also tell you the temperature and the dew point, the dew point being the temperature at which the air will be cool enough to cause condensation and become saturated causing cloud or fog. If the spread between temperature and dew point is less than a degree, then watch out for trouble.
We often see pretty, little, bubbly clouds drifting slowly or scurrying along propelled by winds blowing from the east towards the Caribbean, and we never think of other winds that are essentially updrafts and downdrafts rushing up and down in the air above us. Take those towering thunderclouds that we call cumulonimbus clouds, CBs; they have inside them ‘chimneys’ of air shooting up and down, side by side, at tremendous speeds strong enough to tear a giant airliner apart, which is why pilots avoid CBs like the plague! Sometimes we talk of cloudbursts, which are vast amounts of wind and water that explode out of CBs and smash into the ground below shooting off in all directions; aviation history is full of crashes caused by flying too close to thunderstorms.
Anyway, back to our teeny, tiny droplets of condensed water vapour too light to tumble out of the sky. Something has to happen to them or they will never get big enough to fall. Actually, it’s quite simple: if one droplet bumps into another droplet, the bigger droplet gobbles up the smaller one. This process is called coalescence. Once a droplet is too heavy for the cloud to hold it, it starts falling, gathering even more small droplets on its way down. Droplets reaching the size of 0.5mm in diameter are considered to be raindrops – not that anyone is riding along to measure them. If a raindrop grows bigger than 4mm, it usually splits into two separate droplets and the process begins all over again. A raindrop continues to fall until it reaches the ground; where else is it going to go? Well sometimes, an updraft might carry it back up into a cloud where it continues to cannibalise other droplets and put on weight.
The force with which rain falls depends on the velocity of downdrafts from clouds and on the size of the drop. In falling, frictional drag counters the downward force of gravity. When gravity and frictional drag are balanced, the result is the terminal velocity, which depends on the size, shape and mass of the raindrop and the density of the air. Raindrops are at least 0.5mm in diameter. Large raindrops, about the size of a housefly, have a terminal fall speed of about 10 meters per second or about 20 mph that can cause compaction and erosion of the soil on impact. Smaller raindrops fall at about 2 mph.
When rain reaches the ground, its biggest drops have had a bumpy ride and have coalesced several times over; the smaller ones have had a smoother journey. So Doretta, raindrops come in different sizes for two primary reasons: a difference in initial particle or condensation nuclei size and different rates of coalescence. Now let me get back to sleep. Editor’s note: many thanks Michael. Interesting and humorous – just like you!