Cloud-watching is a favorite pastime of prairie people, probably because you can see them coming a long way off (clouds, that is, not people). “Nasty looking clouds over there,” we say, or “Looks like snow clouds blowing in,” or “I see a puppy dog. What do you see?”
Whether you’re using clouds to forecast the weather or exercise your imagination on a lazy summer afternoon, you may occasionally wonder exactly what it is you’re looking at. Allow me to elucidate…
A cloud is made of small water or ice particles suspended in the atmosphere. There’s always a certain amount of water vapor in the air, but it’s usually invisible. It becomes visible, and the cloud forms, when a mass of air finds itself with more water vapor than it can hold at its temperature. Usually that happens either because the air has been suddenly cooled by being forced upward or because it’s been engulfed by a larger mass of cooler air, which causes it to radiate away some of its heat.
Whatever causes the cooling, the result is the same: the water vapour condenses into drops that, taken together, reflect enough light to be visible Ñ although you’d have a hard time seeing them individually, because even the biggest cloud droplets and ice crystals are only about a 100th of a millimetre across and the smallest are only about one 10,000th of a millimetre in diameter. At this size, they weigh so little that air resistance keeps them all-but-suspended. If condensation continues, droplets clump together and ice crystals get bigger and bigger, until eventually they weigh so much they’re able to overcome air resistance, and fall as rain or snow. (A typical raindrop is made up of as a million cloud droplets.)
Cloud droplets and ice crystals first form on small particles of dust or other airborne materials (the idea behind “seeding” clouds to make it rain) that are even smaller than the droplets, all the way down to a millionth of a millimetre in diameter.
The number of particles influences how clouds and precipitation form. Air over the ocean contains fewer solid particles for cloud droplets to condense on than air over land, and polluted air over major cities contains the most particles of all. On the one hand, the higher number of particles over land encourages cloud formation; on the other hand, that means more of the water vapour goes to forming cloud particles than to forming raindrops. That’s why rain usually falls shortly after a cloud forms over the ocean, but takes much longer to develop over land.
Clouds are classified by their shape and altitude. The main categories are stratus, cumulus and cirrus. Stratus, or layered, clouds result when the upward motion of the air that causes them to form is more-or-less uniform over an area. They typically form at about 1,500 metres. Cumulus clouds, the cottony, billowing kind, develop when the air is more turbulent, with updrafts and downdrafts located cheek by jowl. (I’ve always gotten a kick out of that expression…) They normally form at about 600 metres, but big thunderclouds can extend clear up into the stratosphere. Cirrus, wispy clouds composed almost entirely of ice crystals, form very high, at about 6,000 metres.
Of course, those altitudes are very general; clouds can and do form at just about any altitude, from the ground (where they’re called fog) on up, which is why clouds are further classified with prefixes that refer to their altitude: altostratus, for example, which form higher than ordinary stratus, or cirrocumulus, which form way up at the cirrus level.
When precipitation is falling from these clouds, the term “nimbo” is added, which gives you nimbostratus, the gray, leaden-sky clouds often produced by large-scale storms with lots of snow or rain, and cumulonimbus , associated with summer thunderstorms.
Cirrus clouds don’t drop snow or rain on us, but they do sometimes form a fairly uniform veil across the sky, resulting in sun-dogs and rings around the moon. As well, if they become thicker over time, they can signal the arrival of quite a lot of precipitation, in the form of a squall line or, in appropriate regions, even a hurricane.
They can also signal the arrival of a jet airplane, because the contrail that streams behind a high-flying plane is really just a cirrus cloud, formed when moisture in the exhaust condenses and freezes as it’s pumped into the cold high-altitude air.
Stratus clouds commonly occur when warm, moist air moves over cold ground surfaces during winter, while cumulus clouds usually form as a result of heat rising unevenly over a sun-warmed surface. That’s why gray, leaden skies make us think of winter and blue skies dotted with fluffy white clouds remind us of summer. If those fluffy clouds keep growing, though, they stop looking friendly and start looking mean. Because of their height, they cast a substantial shadow, which contrasts with the brilliant white of the sunlit part of the cloud to make their undersides look ominously black. The bigger the cloud the darker the shadow, and sometimes the more violent the storm, which is why we associate black clouds with bad weather, even though strictly speaking even the meanest-looking cloud is made up of exactly the same kind of water droplets as the friendliest, fluffiest one.
Yeah, that one. The one that looks like a puppy dog.
What do you see?
