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Cumulus clouds look all light and fluffy but did you know they weigh about 500 tons? That’s the equivalent of about 100 elephants.
In this fascinating extract from their new book, British meteorologists Simon King and Clare Nasir look at the thrilling science behind a subject that affects us all.
WHAT ARE CLOUDS MADE OF?
Ask a class of five-year-olds what clouds are made of and at least one will shout out ‘fluff!’ And they certainly do look like that! But, in fact, the white stuff that floats in the air is a product of millions upon millions of tiny cloud droplets that eventually battle for space as more ping into existence.
Ultimately, they become one assuming the guise of a white fluffy cloud.
Putting fluff to one side, the basic ingredients for a cloud are water vapour and heat energy. Heat transports air loaded with water vapour into a cooler environment through convection, and then condensation transforms water vapour into tiny water or ice droplets.
The lightness of cloud droplets mean they remain suspended and become the body of a cloud as the feed of water vapour continues.
AT WHAT POINT DO CLOUDS RAIN?
An average cumulus cloud weighs about half a million kilograms or 500 tons – that’s the equivalent of about 100 elephants.
It’s hard to imagine how such a hefty beast can remain suspended in the air. However, a cloud is made up not only of cloud droplets but also a lot of air (a mix of gases, including water vapour).
And it’s the strength of these local currents within the cloud, that keep it suspended despite gravity
But as cloud droplets collide with each other, producing larger cloud droplets. This process is called coalescence. As further collisions happen, cloud droplets grow into rain droplets, the rain will eventually become too heavy for the forces within the cloud to hold it, and gravity wins.
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WHY DOESN'T A CLOUD POUR OUT ALL ITS RAIN IN ONE GO?
Rain droplets are so tiny that the maximum speed at which they fall is around 10 metres an hour. To put this in perspective, falling from, say, 2 000 metres it would take 200 hours for a typical cloud droplet to land on the ground.
Also rain droplets don’t all form and fall together at one time, that’s why a shower can vary in intensity. Until the air dries out, the process of cloud droplets appearing from water vapour and condensation nuclei and then growing to raindrop size is a continual one.
Raindrops grow at different rates and fall only when they are heavy enough to fight against the upward forces of air within the cloud.
WHAT DOES RAIN SMELL LIKE?
The scent of rain has been around since the first of the continents rose up and separated from the oceans, as storm clouds parted allowing sunlight to energise the air, warm the seas and dry the newly exposed lands.
But it wasn’t until 1964, when the smell of rain finally received its name – Petrichor. Petrichor describes the scent of the air just before, during and after it rains, more especially after a dry spell. It is a modern-day description of an ancient and life-affirming smell.
“Petra” translates to stone and “Ichor” to “blood from the veins of heavenly bodies”, with the literal translation of Petrichor being “heavenly fluids from a stone”.
The first scientific study on Petrichor was carried out by two Australian scientists in the 1960s, who analysed every part of the process that makes up the scent. Before the rain, increased moisture loosens earthy compounds, allowing gases and particles from the dry ground to diffuse into the atmosphere.
As it rains, the action of raindrops hitting the dry ground releases gases from the pores of stones and rocks – essentially, particles from the ground fly up into the air. The scent becomes more intense.
After it rains, the smell of rain lingers in the moisture-laden air. The process is part mechanical, the action of raindrops, and part chemical, as liberated bubbles of gas and particles bounce and burst into the air.
WHAT CAUSES THUNDER?
Once cumulonimbus clouds develop, collisions between water droplets and ice contained in them create friction which leads to static charges within the cloud. As this electrical charge intensifies we get lightning strikes.
When lightning passes through the air it creates a channel, with the temperature rising to almost 27 500°C in a split second – that’s nearly five times hotter than the surface of the Sun.
This creation of a channel, coupled with the heating, suddenly compresses the surrounding air and leads to massive shockwaves. It is these extreme vibrations that travel to our ear and give us the sound.
If the lightning strike is nearby, it might sound like a loud whip and crash. If the lightning is much further away, it gets distorted in the air and when it reaches us, it will be more like a longer grumble.
COULD WE HARNESS THE POWER OF LIGHTNING?
The numbers are impressive: a single bolt of lightning contains around a billion joules of energy, which is enough to power a home for a month. So, theoretically, it would be a brilliant idea to collect that power and use it as a renewable energy source.
However, there is a very good reason why this wouldn’t work. For starters, while the amount of energy available in one strike is huge, it is delivered in a split second. We don’t have the engineering techniques to be able to handle this surge of energy in a short space of time, store it and then release it as an energy source over a longer period.
The other major problem is that lightning is extremely sporadic and it is almost impossible to specifically pin down where it might hit. Most of the lightning strikes occur in areas of the tropics, where population density is sparse. Even if it was possible to harness the power from all the lightning striking Earth, scientists have calculated that every year we would only be able to power around 8% of USA households.
WHERE DOES LIGHTNING STRIKE MORE THAN ONCE?
Lightning occurs most frequently on land around the tropics as this is where there is more heat energy and therefore convection to produce the towering cumulonimbus cloud necessary to generate thunderstorms.
The most lightning strikes across the world happen over Central Africa, Central America and the Asia Pacific. Within these regions, it is over mountains where they are most frequent. Mountainous terrain enhances the upward motion of air, so the convection is boosted further by local topography. If you add a lake into the mix as well, the additional available moisture can easily be transferred above, populating the sky with thunderstorms and dramatic lightning displays. In Venezuela, where the Catatumbo River meets Lake Maracaibo, there are on average 250 lightning strikes per square kilometre every year, with around 28 lighting flashes each minute.
Another electrically charged place is the mountain village of Kifuka in the Democratic Republic of the Congo, where there are 158 lightning flashes per square kilometre each year.
DO PINE CONES OPENING UP MEAN GOOD WEATHER IS ON ITS WAY?
There is some sound science behind this. The opening and closing of the pine cones is related to how humid the air is. Each leaf of the cone is made up of scales, in dry weather these scales shrivel up and the cone becomes stiff as it opens up. Thereby exposing its seeds to the air where the wind can pick them up and spread them out.
High-humidity air has the opposite effect on the pine cone where the scales absorb moisture, gain elasticity and are able to close up to protect their seeds in their natural position. Therefore, when pine cones open; dry weather on the way, when closed; wet weather is on its way.
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This is an extract from What Does Rain Smell Like? 100 Fascinating Questions on the Wild Ways of the Weather by Simon King & Clare Nasir, published by 535.
