Al Stahler: Cloud-watching
Lay out a large square of cloth, and put some sand in the center. Pull up on the corners, and you’ve got sand in a sort of sack. Open a corner of the sack, and the sand pours out.
We might say that the sand “wants” to fall downward … as if the sand had a mind. But the sand is simply following the laws of physics, responding to gravity, which pulls it downward. Unable to get all the way down to Earth’s core, the sand piles up, on the ground, in a cone-shaped pile.
Orbiting Earth, in a space station, we can ignore gravity. Take that sack of sand … open one corner … and the sand just floats there.
That the sand on Earth forms a pile, instead of just floating in mid-air, tells us that something is pushing or pulling on it. Ah yes … gravity.
Still on Earth, suppose we pour out the sand … while a wind is blowing. The sand again forms a pile, but now, it’s off-center. Something besides gravity must be pushing or pulling on the sand. Oh yeah … wind.
All the things around us … and we ourselves … are shaped by the basic forces of the universe. We can sometimes figure out how those forces are doing it, by looking at the shapes of things.
We’ve all made clouds, simply by exhaling on a cold winter’s day … or by blowing into a freezer. The water vapor – gaseous water – in our breath, suddenly cooled, condenses into tiny cloud droplets. How tiny? Think of the droplets in fog … how the slightest motion sends them swirling in all directions – up, down, and sideways. Fog is simply a cloud, touching the ground; fog droplets are cloud droplets. It takes around a million cloud droplets to make a raindrop.
Warm air rises; as it rises, it cools (which is why higher elevations are cooler than those below). Just as water vapor condenses when we blow into a freezer, water vapor condenses when warm air rises.
Ask a child to draw a cloud and s/he will likely draw a cumulus – a cauliflower of cotton-balls. Cumulus clouds accumulate (the words “cumulus” and “accumulate” are related) when bubbles of air rise, cool, and water vapor condenses.
When we sweat, it is not the moisture that cools us off. We’re cooled when the moisture on our skin evaporates. Evaporation is like slow-motion boiling. Like boiling, evaporation requires energy; when sweat evaporates, it sucks energy – heat – from our body.
Condensation – turning gas to liquid – is the opposite of evaporation. Evaporation sucks heat away; condensation dumps heat into its environment. When cloud droplets condense, they warm the air around. And warm air rises.
Keeping an eye on a young cumulus cloud, we can watch it grow. From one cotton-ball, another emerges as air is warmed and rises further … and then another.
This self-feeding growth, in our dry-summer climate, can be dangerous. It can create clouds rising miles into the sky, with violent updrafts and downdrafts – up-and-down winds that – by means not well-understood – strip electrons off atoms … collect those electrons in different regions of the cloud … and send those electrons – as lightning – through the sky, and into the Earth.
Building towering cumulonimbus requires a substantial amount of water vapor to condense and release heat, over-and-over. So far, the air around us has been too dry for electrical storms. I’ll describe cumulonimbus in the future.
Notice that the bases of all the cumulus clouds in the sky at one time are pretty much at the same height – the height where rising air becomes cold enough for water to condense. If condensation warms the air around efficiently, cauliflower cumulus result. But sometimes, the cumulus grow only a little ways, then lose too much heat off their tops to rise any further. The result is a flat layer of small cumulus clouds: Stratocumulus.
Blow gently on a spoonful of soup, and ripples roll across the surface. Suppose a layer of air were floating overhead, a layer of air almost – but not quite – cold enough for water vapor to condense.
Now imagine a wind blowing across that layer of air. Like breath on soup, the wind would create ripples on the top of the layer. As the ripples roll, they cause the air in the layer to rise and fall. That little bit of rise might cool the air enough for cloud droplets to condense … forming ripples of cloud in an otherwise blue sky.
Water droplets – like water vapor – have a greenhouse effect: They absorb heat energy – infrared radiation – coming off the warm Earth, then re-radiate that heat in all directions … including back down to the ground. So, while a cloud on a hot summer’s day is a welcome sun-shade, a cloud on a hot summer’s night is an unwelcome source of heat: Clouds at night keep the night warm.
Clouds are complicated … and they strongly affect the climate. But it’s hard to keep track of the gazillions of clouds over the whole planet.
This Friday night, the moon will be new, and invisible. In the nights after that, the young crescent moon will appear in the west, after sunset. When the night grows dark, the dark side of the moon – outside the crescent – will be lit by a dim, ghostly light. Sunlight, reflecting off clouds over the Pacific, and over Asia, hits the moon, and reflects again, back down to the Earth … to our eyes. Such “earthshine” is easy to see, and worth looking for. And earthshine is useful … in determining how much of the Earth is covered with cloud.
Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at email@example.com
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