Published: Monday, August 23, 2010, 5:17 p.m.
Clouds block out the sun. They turn our world gray.
They make us wet, miserable, even depressed. They bring lightning, storms and floods.
It's easy to forget that they also give us life. They water the plants we eat. They fill the lakes, rivers and reservoirs. They come in wild shapes and many sizes. They give us green grass, multicolored rainbows, purple-and-orange sunsets and puffy white cotton balls in bright blue skies.
Perhaps nothing so essential to our existence is so unappreciated, even reviled, as the cloud.
In the Pacific Northwest, we know clouds as well as anyone. Most of us who live here do so with a tacit acceptance, if not an embrace, of their frequent companionship.
“I guess I've gotten used to it,” said John Hughes, 43, of Everett.
“I was born and raised here, so I don't mind,” said Kathi James, 46, of Marysville. “Drip-dry and wash-and-wear, it doesn't bother me.”
Like it or not, several factors conspire to put us right smack in the path of a lot of clouds.
Earth's rotation causes winds to move in different directions depending on the distance from the equator and the poles, said Andy Haner, a meteorologist with the National Weather Service in Seattle.
“The winds are westerly in the mid-latitudes; the weather moves from west to east,” said Cliff Mass, a professor of atmospheric sciences at the University of Washington. Near the equator, winds tend to move from east to west, he said.
Those mid-latitudes range from 30 to 60 degrees in both hemispheres, Mass said. Everett is at 47 degrees north. Here, and in the rest of the Pacific Northwest, “we are right squarely under what we call the westerlies,” Haner said.
Those winds are at their strongest in a band a few hundred miles wide, five to eight miles above the Earth's surface, called the jet stream. Mass explains the phenomenon in his book “The Weather of the Pacific Northwest,” which was published last year.
The jet stream moves around like a snake, Mass writes, and is often aimed at the Northwest. The air in this stream blows 100 to 200 mph, and weather systems tend to follow it. In the summertime, the stream weakens and moves north, with high pressure building toward the south.
The westerly winds carry clouds created by the evaporation and condensation of ocean water, and in Western Washington, we're right next to the ocean. Dry Eastern Washington is at the same latitude, but the Cascade Range catches many of the clouds and causes them to dump their moisture and break up.
The rain fills the rivers, the rivers run into the ocean and the cycle continues.
If we pay attention to that cycle, clouds offer us an opportunity.
They hint at what's coming.
Types of clouds
First, it's helpful to know the basic types of clouds.
They went unnamed until 1802, according to John Day, author of “The Book of Clouds.” That's when Luke Howard, a 30-year-old British pharmacist with a keen interest in meteorology, took it upon himself to devise a classification system that is still in use to day.
Howard belonged to a group called the Askesian Society, a London debating club with an interest in science. At a meeting of the group in December 1802, he read his paper titled “On the Modifications of Clouds.”
Howard discerned four kinds of clouds and gave them Latin names. He named high, wispy clouds “cirrus,” Latin for curl of hair. He named widely extended horizontal sheets of clouds “stratus,” meaning layer. He named clouds that form in stacks “cumulus,” Latin for heap. He called clouds that bring precipitation “nimbus,” Latin for rain.
The name “alto,” Latin for high, was later given to midlevel clouds. Perhaps “alto” was used to describe these clouds because high clouds already had a name.
There are many combinations of the various cloud types.
“Hundreds,” Mass said.
We don't need to know them all, however, to discern weather patterns.
When cirrus clouds appear in a clear sky, a front is often on the way, Mass says in his book. These high clouds then can thicken and spread out into a layer called cirrostratus, the kind that often produces a ring around the sun or moon. That means ice crystals are present.
These clouds then can thicken and lower into a layer called altostratus, through which the sun or the moon can still be seen, but barely, Mass writes. The terms “watery sun” and “watery moon” are used when these clouds are around.
With the higher cirrus clouds, rain or snow is still a good six to 18 hours away, according to Mass. With altostratus, it's more like three to six, he writes. If you're out on a hike or a bike ride when altostratus takes over, “it is time to head home.”
The next stage is a further thickening and lowering into nimbostratus, the low, gray clouds that produce most of the precipitation in Western Washington. These clouds come in two categories: those with bumpy, well-delineated bottoms, and masses of dark gray fuzz that make the sky look like a giant dust bunny. The latter type brings most of the rain.
After nimbostratus clouds rain and move on, they're often followed by stratocumulus clouds, which can be thick but are more uneven. These can be very dark in one spot and light enough to let blue sky and sunbreaks through in another, Mass writes. They typically aren't rain clouds but can bring a shower or two.
Mass calls stratocumulus the signature clouds of Western Washington. These are the clouds that produce most of our gray days.
“If you saw a picture of it, you'd say, ‘Yes,'” he said.
Another signal of oncoming inclement weather is the presence of “flying saucers” or “UFOs,” Mass says. No, aliens are not seeding the skies. Lenticular clouds — smooth, streamlined clouds that hover over mountains like spaceships — are created when wind speeds increase and the atmosphere reaches near saturation, according to Mass.
The moist air condenses as it sweeps up the side of the mountain, forms a cloud, then dries out as it descends, leaving the cloud behind. Sometimes several layers are formed and stack on top of each other — sort of a cairn of clouds. Mount Rainier is famous for its “cap” clouds, but they are seen over the Olympics and other Cascade mountains as well.
These clouds have been mistaken for UFOs, according to Mass. In his book, he suggests that the pilot of a small plane in 1947 actually triggered the UFO craze by mistaking some lenticular clouds near Mount Adams for flying saucers.
These scenarios aren't the only ones in which rain can come, Mass said. “You can get all kinds of hybrids and strange things happening,” he said.
Thunderheads form when unstable, moist air and strong updrafts create what are called cumulonimbus clouds, which can build into towers of more than 40,000 feet. Sometimes they develop a flat “anvil” top, caused when the cloud bumps up against the stratosphere, about nine miles high. These clouds bring thunder, lightning, hail and big-drop rain.
A milder version of the same phenomenon creates the white puffy cumulus clouds we see on an otherwise sunny day — there are still updrafts and convection, Mass said, but with less moisture in the air.
We have few thunderstorms in Western Washington because, in a nutshell, the air here is usually too cool in the summer to contain enough moisture to produce cumulonimbus clouds, Mass writes. In the summer, high pressure offshore pushes cool ocean air over us, keeping summers mild and relatively dry.
Fog, which occurs most frequently here in the early fall, usually means clear weather awaits once the fog burns off.
“Dense fog is a good omen because it is often associated with high pressure and a lack of clouds aloft,” Mass writes in his book.
Fog is actually a stratus cloud that has formed at ground level, according to “The Book of Clouds.”
Eyes on the sky
Clouds give us many other interesting phenomena. The colors we see in sunrises and sunsets come from a scattering of different wavelengths of sunlight, which is caused by water vapor and dust particles in the air, Day writes in “The Book of Clouds.”
Billow clouds, which resemble a field of snow moguls or a washboard, are created “when wind flows at highly varying speeds in layers just above and below the clouds,” Day writes. Billow clouds are usually produced by altocumulus, which can appear bumpy to begin with, or cirrocumulus clouds.
Contrails are created by water vapor in the exhaust left behind by high-flying jets. The vapor condenses in the cold air, and the contrails can either evaporate quickly or spread out into cirrus-like clouds.
The interesting shapes we sometimes see in the sky — the flying bear or ski-jumping salmon — are produced by cumulus clouds.
The list of unusual effects created by clouds is long. One thing is certain: clouds bring variety to the sky.
“I like the fact that it's not all the same,” said Josh Nylander, 30, of Mount Vernon. “We have a diverse picture. Every day it's different. You never know what it's going to look like.”
Maria Smith, 21, of Seattle said she appreciates the frequent cover of clouds in the Northwest.
“It's a nice dense layer of insulation,” she said. “And they're pretty, kind of the curtains of our sky.”
Bill Sheets: 425-339-3439, firstname.lastname@example.org.