Summarize:

These are considered to be clouds that are between 19,500 feet (5,943 meters) and 42,500 feet (12,954 meters). This encompasses cirrus, cirrostratus and cirrocumulus clouds. These clouds tend to be frozen (filled with ice crystals) and have blurry outlines; they are generally thin and wispy to the human eye.   Contrails from jet airplanes are also found at this level in the Earth's atmosphere. High clouds produce beautiful red, orange and yellow colors in the sky at sunrise and sunset. Rings of light around the moon or the sun are caused by cirrus clouds. A halo around the sun or moon can suggest rain or snow on the way, especially if accompanied by thickening lower clouds. Cirrus clouds often partially obscure the sun. Cirrus clouds are distinguishable by their thin, white and wispy appearance. They generally reach an altitude above 20,000 feet (6,000 meters). The thinness tends to be due to the frigid winds in the upper atmosphere. Cirrus clouds are made up of ice crystals formed by supercooled water droplets.  Cirrus clouds that are sparse and stay that way usually show signs of fair weather. When cirrus clouds change into cirrostratus clouds, wind and precipitation are probably on their way within the next 24-36 hours. Cirrus indicates moisture above; if cirrus changes to altostratus and then stratus, a storm approaches soon. You'll see cirrus clouds as a warm front advances. The direction in which cirrus clouds move will usually tell you the direction of air movement, namely the weather coming your way. Cirrus clouds are sometimes referred to as horse's or mare's tails because they curl out like a horse's tail. These clouds are often found in rows and they look like little puffed-up ripples. Some people think these clouds look like fish scales. The broken or rippled nature of these clouds is caused by air turbulence; it's bad news for anyone flying through them, but great news on the ground because these clouds usually mean that the weather is going to stay the same with no major sudden changes. These clouds tend to lack shape and have a hazy appearance. They usually spread across much of the sky. They suggest moisture is present in a large part of the sky, indicating that rain might be due soon. Thicker cirrostratus clouds can come from the top of an advancing storm system. Cirrostratus clouds are made up of ice crystals as well, but unlike cirrus clouds they can cover the entire sky and be up to several-thousand feet thick. Cirrostratus clouds are sheetlike and nearly transparent. Although these are machine made by jet airplanes, contrails can help you to know the weather in the heights above. Contrails are condensation, formed when the jet's warm exhaust mixes with the very cool air surrounding the jet in the upper atmosphere.  If the contrail disappears quickly, or you see a jet with no contrail, this means that the upper atmosphere you're observing is dry. On a lovely day, you can usually take this as a sign that the weather is going to remain fine for a while. On the other hand, if the contrail is obvious for a time, is long and spreads, the upper atmosphere you're observing is wet. Grab your coat and umbrella because the weather will probably change soon, if it hasn't already.
Look for high altitude clouds (also known simply as "high clouds"). Know your cirrus cloud. Spot a cirrocumulus cloud. Look for cirrostratus clouds. Learn how to tell the difference between cirrus clouds and cirrostratus clouds. Watch for contrails.