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.

The Star (St. Lucia) - - LOCAL - By Michael Walker

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!

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