How is dry ice seeding used to increase visibility at airports?

When the weather has been cold enough, using dry ice has proved to be effective in clearing fog around airports. Unfortunately, conditions are right for this method only about five percent of the time in the United States.

What is a cloud chamber?

Cloud chambers were originally designed to study radioactivity. Scottish physicist Charles Thomson Rees Wilson (1869-1959) invented the chamber in 1912, winning the Nobel Prize in physics in 1927 for his invention. The procedure involved saturating an enclosed chamber in water vapor until it was supersaturated. Ionized particles would then be passed through the chamber, serving as nuclei around which droplets would form. This had the advantage of making the particles visible to physicists, and the behavior of the particles could be studied.

Who was the first person to classify clouds?

The French naturalist Jean Lamarck (1744-1829) was the first to propose a system for classifying clouds in 1802. His work, however, did not receive wide recognition. A year later, the Englishman Luke Howard (1772-1864) developed a cloud classification system that has been generally accepted and is still in use today.

In Howard's system, he distinguished clouds according to their general appearance ("heap clouds" versus "layer clouds") and their height above ground. Latin names and prefixes are used to describe these characteristics. The shape names are cirrus (curly or fibrous), stratus (layered), and cumulus (lumpy or piled). The prefixes denoting height are cirro (high clouds with bases above 20,000 feet [6,000 meters]) and alto (mid-level clouds from 6,000 to 20,000 feet [1,800 to 6,000 meters]). There is no prefix for low clouds. Nimbo and nimbus is also added as a name or prefix to indicate that the cloud produces precipitation.

What are the four major cloud groups and their types?

Clouds are categorized in the following manner:

1. High clouds are composed almost entirely of ice crystals. The bases of these clouds start at 16,500 feet (5,000 meters) and reach up to 45,000 feet (13,650 meters).

A. Cirrus clouds (from the Latin for "lock of hair") are thin, feathery, crystal clouds that appear in patches or narrow bands.

B. Cirrostratus clouds are thin, white clouds that resemble veils or sheets. These clouds can be striated or fibrous in appearance. Because of the ice content, they are associated with the halos that surround the Sun or Moon.

C. Cirrocumulus clouds are thin clouds that appear as small, white flakes or cottony patches; they may contain super-cooled water.

2. Middle clouds are composed primarily of water. The height of the cloud bases range from 6,500 to 23,000 feet (2,000 to 7,000 meters).

A. Altostratus clouds appear as bluish or grayish veils or layers of clouds that can gradually merge into altocumulus clouds. The Sun may be dimly visible through them, but flat, thick sheets of these clouds can obscure the


B. Altocumulus clouds are white or gray and occur in layers or patches of solid clouds with rounded shapes.

3. Low clouds are composed almost entirely of water and may at times be supercooled; at subfreezing temperatures, snow and ice crystals may be present as well. The bases of these clouds start near Earth's surface and climb to 6,500 feet (2,000 meters) in the middle latitudes.

A. Stratus clouds are gray, uniform, sheet-like clouds with a relatively low base, but they can also be patchy, shapeless, low gray clouds. Sometimes thin enough for the Sun to shine through, these clouds bring drizzle and snow.

B. Stratocumulus clouds are globular, rounded masses that form at the top of the layer.

C. Nimbostratus clouds are gray or dark, relatively shapeless, massive clouds that contain rain, snow, and ice pellets.

4. Clouds with vertical development contain super-cooled water above the freezing level and grow to great heights. The cloud bases range from 1,000 feet (300 meters) to 10,000 feet (3,300 meters).

The tops of cumulonimbus clouds look like giant cotton balls in the sky.

The tops of cumulonimbus clouds look like giant cotton balls in the sky.

A. Cumulus clouds are detached, fair-weather clouds with relatively flat bases and dome-shaped tops. These usually do not have extensive vertical development and do not produce precipitation.

B. Cumulonimbus clouds are unstable, large, vertical clouds with dense boiling tops that bring showers, hail, thunder, and lightning.

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