SECTION 4: THE AERIAL VAPOUR CYCLE
YOU KNOW WHAT it’s like… well, if you’re a woman you will. You’ve just spent hours blow-drying your hair to celebstandard sleekness. Then you step outside and your silky tresses morph into a candyfloss frizz. Why? It’s because the air is damp and human hair stretches when it’s moist. The air around us is full of varying degrees of water vapour, and it’s the behaviour of this invisible vapour that is responsible for much of our weather.
It’s not for nothing that Earth is nicknamed the Blue Planet, since so much of its surface is covered with water. When it’s swooshing around in the oceans, seas, lakes and rivers, water exists in the form of a liquid. But when water is warmed by the sun, it turns into water vapour (a gas that accounts for about a third of all atmospheric gases) and rises into the air. So, the world’s water doesn’t disappear, it is perpetually changing form and keeps circulating around the planet’s weather system in a perfect example of natural recycling – and the engine that powers it all is the sun.
More than two-thirds of the Earth’s water is found in the oceans and seas, but lakes and rivers also contain large amounts. The sun acts on these wet surfaces in the same way as it does on dry land: it heats the surface, triggering a chain reaction. In the case of land, the warmed surface reflects heat, warming the air above it and causing it to rise; in the case of seas and oceans, the warmed surface waters evaporate and rise as water vapour.
Other, smaller sources of atmospheric water vapour are the moisture ‘breathed out’ by vegetation (in a process known as ‘transpiration’) and given off during the burning of fossil fuels, and in the gases that the Earth belches out during volcanic eruptions.
For obvious reasons, the Earth’s wettest air is above the seas and oceans, but nowhere is the air completely dry – even above the middle of the Sahara there is some moisture in the atmosphere. It is dry simply because the hotter the heat of the sun, the faster the rate of evaporation will be.
So there’s all this water vapour floating around with the other gases in the atmospheric soup that makes up what we call ‘air’. What happens next? As the sun’s rays heat the surface of the Earth, it in turn heats the air in contact with it. Bubbles of warmed air start to rise and as they do they also expand. It uses up energy to do this and so the bubble starts to cool down as it continues to rise. If the cooling air is full of moisture as it rises it eventually reaches a point where it is completely saturated with water vapour. The bubble rises till further cooling leads to the water vapour condensing into tiny water droplets and a cloud starts to form. What aids this process are the minute particles of dust, salt and smoke that occur naturally in the atmosphere. These provide tiny nuclei around which the droplets can condense – without this kind of miniature space debris, there would probably be many fewer clouds.
It makes sense that the air above the oceans and seas would be the wettest – why, then, aren’t clouds and rain confined to these places? Answer – the air itself does not stay in one spot. Moisture-laden air masses are constantly on the move, with jet streams – the fastest-moving winds of the lot – blowing clouds and weather systems about. Meteorologists have a posh word for this process: ‘transportation’.
Inside the clouds the water droplets collide and join together to form larger drops. If they get large enough and heavy enough, they fall to Earth as rain. In fact, much of the rain that we get starts life as snow. It’s pretty chilly up there at the top of clouds, and the tiny water droplets they contain in their upper reaches freeze into ice particles. The ice particles merge to form snowflakes, which flutter down towards the planet’s surface. If the flakes pass through warmer air on the way down, they melt – and turn into raindrops. But if it stays cold, they may remain frozen and descend as snow, sleet or hail. Rain, drizzle, snow, sleet and hail are all different forms of ‘precipitation’.
The water that has fallen from the sky eventually makes its way back to the sea. If the ground is absorbent enough, the water may soak into the ground and, when it hits impermeable rock, start to build up, spread sideways and eventually come to the surface, running downhill into streams and rivers which carry it back to the seas and oceans – for the whole process to start again. Or, if the ground is too hard or precipitation has been too heavy for the surface to absorb it, the water may run directly into the streams and rivers.
DO IT YOURSELF!
Apart from the effect it has on your hair, air humidity can also be measured by looking at trees. The leaves on oak or maple trees curl up in high humidity, giving you an early warning of rain ahead, while pine cones stay closed in high humidity and open in low humidity.
NB A temperature change of around 11°C will allow air to contain approximately twice as much water vapour. So, for example, air warmed to 21°C (70°F) may be twice as saturated with water vapour as air at only 10°C (50°F), which may have seen some water vapour condense to water droplets.
Around 10 per cent of the water that falls to Earth does not make its way back into the atmosphere from the sea, but from vegetation. Plants soak up the water in the ground through their roots. The moisture travels up through their stems and evaporates out into the air from pores in their leaves – creating a suction effect that draw up more water from the soil. Clever, don’t you think?
It’s behind the frizzy-hair effect – ‘humidity’ is the name for the amount of moisture in the air. ‘Relative humidity’ is the ratio (expressed as a percentage) of water vapour present in the air compared to the maximum that could be held by air at a particular temperature.
• At the top end, a relative humidity of 100 per cent is as bad as it gets – think equatorial rainforest. The air is saturated and cannot get any damper. Moisture on surfaces cannot evaporate so your skin will feel clammy and you’ll have no energy.
• If relative humidity is low, the air is drier. In the inland areas of some deserts, for example, the figure can be less than 3 per cent. In such places, perspiration can evaporate more quickly so you’ll feel more comfortable. Rainforest or desert – the temperature may be similar but it’s the humidity that makes the difference.
WATCH YOUR LANGUAGE!
The hydrologic cycle The swanky name for the water cycle, which is made up of evaporation, condensation, transportation, transpiration and precipitation – and back to evaporation again. So now you know.
Another way that meteorologists measure the level of humidity in the air is by means of the dewpoint. Remember how warm air can accommodate more water vapour than cool air? The dewpoint, then, is the tipping point between saturation and condensation – the temperature at which an air mass gets too cool to hold its vapour content and this condenses into droplets.
• At any one time, the atmosphere contains enough moisture to produce about 2.5 centimetres (1 inch) of rain across the Earth’s surface.
• In one year, the Earth receives a total of about 5,000 million million tonnes of rain, sleet, snow and hail.
• Most raindrops measure about 1 millimetres (0.05 inch) in diameter.
• The average raindrop contains a million times more water than the average cloud droplet.
• Race you! Raindrops can fall at up to 9 metres per second, in contrast to leisurely snowflakes, which drift down at a mere metre per second, at most.