Facts of the Matter
Richard Brill

Sunday, July 17, 2005

Clouds help balance
heat on Earth

"I've looked at clouds from both sides now
From up and down, and still somehow
It's cloud illusions I recall
I really don't know clouds at all."

Joni Mitchell

At any given time clouds of one kind or another cover about 70 percent of the Earth, reflecting sunlight and playing an important role in the planet's heat balance.

Clouds occur only in the lowermost layer of Earth's atmosphere, called the troposphere, which extends up to around 40,000 feet.

Clouds are made of tiny droplets of water that have condensed from water vapor that was dissolved in the air.

When water evaporates, it becomes part of the air, existing there as gaseous molecules just like the nitrogen, oxygen, and other gases that comprise the atmosphere.

Water vapor is the third-most-abundant gas in the atmosphere, ranging from one-third of a percent in an arid desert to more than two percent in a tropical rain forest.

It is also a greenhouse gas, capturing outgoing radiation just like carbon dioxide.

The amount of water vapor that the air can keep in solution depends on the temperature of the air and varies widely.

When air at any temperature contains as much water vapor as it can hold it is said to be saturated.

For example at a balmy 86 degrees Fahrenheit, saturated air would contain 1.6 percent water vapor. But on a cool Hawaiian evening of 60 degrees, just a little more than one-third as much water vapor, or 0.62 percent would saturate the air.

Relative humidity is the measure most commonly used to specify the amount of water vapor in the air. It is the actual amount of water vapor compared to the saturated amount at a given temperature, expressed as a percentage.

Another measure meteorologists use to describe humidity is the dew point. Dew point is the temperature to which air with a given amount of water vapor would need to be cooled in order to become saturated.

On that balmy 86 degree Hawaiian day, with a relative humidity of 50 percent, the air contains half of the saturated amount, or about 13 grams per kilogram. That is the amount that air at 68 degrees can hold when saturated, so the dew point for that air would be 68 degrees.

This is why your cold beverage "sweats" when you take it from the refrigerator at 40 degrees. The warm air contacts the cold surface of the can and condenses as it is lowered below the dew point.

Relative humidity is also the reason why the air gets "muggy" as the sun sets and the air begins to cool. The amount of water vapor doesn't change, but the relative humidity goes up because the saturation point of the air goes down.

Clouds are made of microscopic droplets of water that have condensed from the atmosphere. They almost always indicate rising air.

As air rises it expands and cools because air pressure decreases with altitude. Unsaturated air cools about 5.5 degrees Fahrenheit for every 1,000 feet that it rises.

When the rising air cools below the dew point and condensation occurs, the cooling rate lessens since the condensing water gives off heat. Once a cloud has formed, the cooling rate depends on the amount of water vapor and the rate of ascent of the rising air.

If condensation of clouds was merely due to rising air and there were no other factors, meteorology would be a simple science.

There are other factors.

Cloud formation also depends on the temperature of the surrounding air and the presence of microscopic particles in the air.

Air can be uplifted by several different processes. One common one is thermal heating, which occurs unevenly. Some areas of the surface absorb more solar radiation and will become warmer than their surroundings in the sunlight. The warmer air rises, causing "thermals," which soaring birds and human-piloted gliders seek to keep them aloft.

Wind blowing over a mountain range lifts air. This is why the windward side of any mountain range has more clouds and receives more rain than the leeward side. That is why those clouds hang over the Koolaus.

On a larger scale, when a continental-sized mass of cold, dry air runs into a comparably sized mass of warm, moist air something's got to give.

Cold air tunnels under the warm air and lifts it. This type of large scale uplift can cause severe weather such as thunderstorms and tornadoes. These "frontal" storms may span an entire continent and persist for days as they move eastward and gradually dissipate.

The atmosphere is heated from the ground up, so it gets colder the higher you go. The average rate is 13 degrees Fahrenheit for each 1,000 feet. This "environmental lapse rate" can vary considerably during the course of a day as well as seasonally and from place to place.

Meteorologists measure environmental lapse rates with radiosondes, which are balloons equipped with instruments that radio temperature and pressure measurements back to the ground.

Whether a cloud keeps building upward depends on how the cooling rate of the rising air compares with the change in temperature with altitude.

If the rising air gets to a height where it is the same temperature as the surrounding air it will not rise any further and so the cloud formation stops.

A rising "bubble" of air can become unstable and be uplifted like a hot air balloon if its cooling rate is less than the air that it is rising into.

There is one more important thing that affects cloud formation. Air can become supersaturated, meaning that its relative humidity can be well in excess of 100 percent unless there is a surface on which to condense.

In the atmosphere, there are tiny particles of dust, smoke, or salt particles that serve as nuclei around which condensation occurs.

A condensation nucleus is microscopically small, on the order of one to two micrometers (one one-thousandth of a millimeter, or one one-millionth of a meter). By comparison, the width of a human hair is about 75 micrometers.

The droplets of water in a cloud are tiny, averaging less than 20 micrometers in diameter. Because they are so small, their rate of falling is extremely slow, usually much slower than the rate at which the air is rising.

An average-sized cloud droplet falling from a cloud base 3,000 feet above the ground would take 48 hours to reach the ground. It would never get there because it would evaporate long beforehand.

A raindrop large enough to reach the ground without evaporating contains the water of a million droplets, so for rain to fall those tiny droplets must somehow coalesce.

There are reasons why some clouds rain and some do not, and why there are different kinds of clouds, but that is another story.

Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu

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