For the past 21 years, parents, educators and students have been encouraged to look up at the sky during Sky Awareness Week. The celebration takes place this year from April 22-28.

Sky Awareness Week encourages people across the nation and around the world to notice the myriad cloud types, ranging from fair weather cumulus puffs to high-flying cirrus streamers. It encourages everyone to learn how to read the sky, first by learning cloud types and their weather and then by forecasting from them.

Cumulus clouds are low individual billowy globs that have flat bases and look a little like cauliflower. They're constantly changing outlines, and they are fun to watch because they can take the shapes of almost anything, including animals and faces. Cumulus clouds usually signal fair weather. If they build into the middle or high part of the atmosphere, they get the name cumulonimbus.

A cumulonimbus cloud is tall, deep and dark and can bring lightning, heavy rain and even severe weather such as hail, damaging winds or tornadoes. They are a sign of rapidly rising and sinking air currents.

Stratus clouds are layered and cover most of the sky. They are much wider than they are tall. When stratus clouds are very thick, they become dark nimbostratus clouds, which can produce rain, drizzle or snow.

Cirrus clouds are high and thin and made entirely of ice crystals. Forming above 20,000 feet in the atmosphere, they often look like wisps of white hair. Cirrus clouds are a sign of warm, moist air rising up over cold air, and they are sometimes an early signal that thickening clouds could bring light rain or snow within one or two days.

In addition to suggesting people make their own weather forecasts, the program also encourages everyone to appreciate the sky's natural beauty and to protect the sky as a natural resource.

First Published: April 19, 2012, 4:45 p.m.
Updated: April 30, 2012, 4:52 p.m.