Low clouds occur from ground level up to 6,500 feet (2,000 meters). Cumulonimbus , or storm clouds, generally have flat bases and rounded domes . Cumulus clouds vary in size from the small puffball-like clouds to huge dome-clouds that often develop into thunderclouds. Stratus clouds are usually dark shapeless clouds that appear as streaks across the sky or as gray layers hanging above the Earth. Stratocumulus clouds are large, round and lumpy and usually cover the entire sky.

Mid-level clouds occur from 6,500 to 23,000 feet (2,000 to 7,000 meters). Nimbostratus clouds are thick, dark, and shapeless and usually bring rain or snow. Altostratus clouds usually cover the sky with a partially transparent layer that you may see the sun through (as a bright spot). Altocumulus clouds appear as large rounded puffy clouds.

High clouds occur from 16,500 to 45,000 feet (5,000 to 13,700 meters). Cirrostratus clouds often look like tangled webs or thin whitish sheets. When they cover the sky, you may see a large ring or around the sun or moon which is caused by the bending of light rays as they pass through the ice particles in the clouds. Cirrocumulus clouds look like very small round balls or flakes. Cirrus clouds are white and look like bands of feathers.

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Some days it is perfectly still. Other days it will turn your umbrella inside out, blow shingles off your roof, and push your car all over the road. The power of the wind is amazing. A few years ago we visited Jockey's Ridge park in the Outer Banks of North Carolina. Jockey's Ridge is a gigantic sand dune system that is continually reshaped by the wind. There is actually a miniature golf course that lies buried under the sand as a result of the relentless wind. What causes the wind to blow and behave in this manner?

All wind is caused by differences in the temperature of the atmosphere and by the rotation of the Earth. Have you ever noticed at the beach on a nice day that the wind usually blows on-shore during the day and off-shore during the evening? This is a perfect example of the effect of temperature differences.

The sun heats land and water differently. Oceans and lakes absorb much of the heat energy from the sun and the air above them tends to be relatively cool. The land, however, absorbs much less heat than water, and as a result the air above it is heated more. Hence the air over land tends to be warmer than the air over the water.

The warmer air over the land expands and becomes lighter. This creates a low pressure situation. The air over water is cooler and denser (heavier), than the land air and this creates a high pressure situation. The high pressure air over the water pushes into the low pressure area on the land. The air moving in from the water toward the land pushes the lighter land air up and out of its way. This movement is what causes a sea breeze.

When night comes the opposite occurs because the air over the land cools more rapidly than the air over the sea. This causes the breeze to blow out to sea during the night.

All winds near the Earth's surface are caused by differences in atmospheric pressure brought about by uneven heating. Air moves from regions where pressure is high toward regions where pressure is low. Generally, the larger the difference between the low and high pressure areas, the harder the wind blows. This is why there is often very strong wind when a cold or warm front moves into an area.

Differences in temperature and pressure are also responsible for large scale winds called planetary winds. The huge difference between the heated air in the tropics and the cold air in the polar regions is a primary contributor to planetary winds.

In the area of the equator, there is a low pressure area called the doldrums (see diagram below). This is an area of relatively calm weather. Here the air rises upward from the surface of the earth and toward the poles.

The high pressure air that moves into the equator area creates winds that are known as the Northeast Trade winds . The trade winds do not blow directly toward the equator. They are deflected by the Earth's rotation.

Beyond the regions of the trade winds, there are two other calm areas (at about 35 degrees north and 30 degrees south latitude) known as the Horse Latitudes. The air which moves upward at the equator settles down to the Earth in these areas. Since the air settles nearly straight down in this region, there is little wind and a high pressure area.

Some of the air which settles into the horse latitudes goes back immediately into the trade winds. The rest moves toward the poles to form the Prevailing Westerlies . The polar regions tend to send strong winds toward the equator. These winds are called the Polar Easterlies.

Cyclonic storms frequently are formed where the Prevailing Westerlies meet the Polar Easterlies and where the Northeast Trade Winds collide.

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