Clues in the clouds

An understanding of what lies ahead may be written in water vapor.

By Karsten Shein
Comm-Inst, Climate Scientist

Anvil of a distant cumulonimbus approaches Geneva, Switzerland in May 2011. Approaching storms are often preceded by cirri that form as the leading edge of the anvil is sheared by strong winds aloft.

As they left the hotel, the co­pilot looked up and remark­ed to the pilot, "Red sky in the morning... Maybe we ought to ask the boss if we can leave an hour or two early."
"That's an old wives' tale," said the pilot. "The briefing I got last night has the front holding to our west until after we're done for the day. Should be smooth sailing all the way. No need to bug the boss."

But, by the time the boss showed for the noon flight, the sky was already covered with an ominous stratus deck. Although the pilots managed to climb over the worst of it, the approach was made with the deicing boots humming, and it took 3 approaches to get a fix on the runway before hitting the MDA.

Long before weather maps, sailors, farmers and just about anyone else who needed to figure out what the weather had in store for them that day had to rely on the one of the few visual clues available to them—the clouds. For­tunately, as prognostication tools went, clouds were a pretty good predictor of forthcoming weather. They still are.

Why are clouds so good at helping us understand what the atmosphere is going to do? The answer is fairly simple. Clouds are an integral part of atmospheric processes. They are a function of the energy and moisture contained in the air.

They are also easily moved by the motion of the air—and, since certain larger weather phenomena usually behave in the same way time and again, the cloud patterns associated with them are well known. This combination of factors lets clouds serve as indicators of the near-future state of the atmosphere through which you are going to fly.

To make a cloud

At any given time and place, there is some water vapor in the air. In general, only about 2–3% of the lower atmosphere is made up of water molecules. In fact, since the limit on the amount of water the air is capable of holding is a function of temperature, the most that Earth's atmosphere can hold, given its normal temperature range, is about 4%. However, given that the air exerts a pressure of almost 15 psi, that means that at 3% about 1/2 lb of that is water.

Over one square mile that amounts to 1.8 billion pounds of water (actually a little less, since a water molecule is lighter than the oxygen and nitrogen molecules that make up dry air). Again, because the actual amount the air is capable of holding is temperature-dependent, that amount will be lower in cold environments and higher in warm places. The amount is, of course, also dependent on the availability of water to put into the air.

Moisture gets into the air as energy is absorbed by surface water from solar radiation and other heat sources to the point where liquid water is evaporated or ice is sublimated into a gas. Most often, these water molecules lose that energy to the atmosphere and condense before they get more than a few millimeters off the surface.

But, when there is sufficient energy in the atmosphere already, the molecules can be entrained and carried away from the surface. Once at the mercy of atmospheric currents, the water molecules can be transported higher into the sky or moved long distances—which explains why you may find clouds over a desert.

Inevitably, though, the molecules arrive in an environment where the energy they stored away when they evaporated is transferred to the atmosphere, and the molecule condenses. If there isn't enough excess energy in the air to re-evaporate the water, it may attach itself to a dust, salt or other aerosol particle.

When enough water molecules are condensing and attaching to these particulates, the resulting droplets will grow to a size sufficient to intercept and scatter light. The size is not large at all, just about 20 micrometers diameter (20/1000 of a millimeter). A typical raindrop is about 2 mm in diameter. Then, if there are enough droplets of this size, the light scattering will become intense enough for your eye to see—and a cloud is born.

All shapes and sizes

Visible satellite image of the eastern US on the afternoon of Apr 1, 2012. The image quickly shows a pilot the presence of some stratocumuli from the Gulf states north to Illinois, and an area of cumulus congestus with some embedded cumulonimbi over eastern Tennessee and western North Carolina. Mackerel skies are present over northern Indiana, while more cumulus congestus and thunderstorms appear over Ohio and Pennsylvania. Satellite imagery is an excellent way to get a broad overview of cloud types and patterns that might affect your flight.

At one end of the cloud spectrum are those that are moisture limited. These clouds tend to be thin and translucent, a common example being cirrus. Cirri occur at high altitudes where temperatures are extremely low, meaning that there is a limited capacity for water vapor to even be present.

At the other end are the towering cumuli that appear over the tropical oceans, where neither heat nor water are in short supply. Usually, it will be the thicker clouds that also indicate that enough cloud droplets are present to support the formation of precipitation.

The differences in clouds promote some categorization that offers many hints at the forces that formed them and, therefore, what you can expect from the air around them.

Clouds have been classified since 1802 and those classifications have evolved into the ones we use today. The current scheme has 3 parts. The first is the breakdown of clouds by how they develop. This is a simple division into clouds that develop vertically versus those that develop horizontally. Cumuli are generated by vertical motion in the atmosphere—usually convection.

An individual cumulus cloud will normally cover a small area, although the sky may be full of them. A stratus cloud, on the other hand, is formed primarily by horizontal, or advective, flow. Strati may also cover a small area but, due to the horizontal flow that aided their formation, they more commonly blanket large areas of the sky with a broken or overcast coverage.

A second branch of cloud classification is by altitude. High clouds are generally those above about 20,000 ft (or about 10,000 in polar regions). These clouds, identified with the prefix "cirro," are almost always made up of ice crystals due to the extreme cold at such altitudes.


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