Stratus clouds are one of the three main types of clouds. Remember that there are many types of clouds that fall into three main categories: cumulus, stratus, and cirrus. Using hand-motions, we would stretch out our hands as far out as we could to mimic a stratus cloud. There are stratus-type clouds at all three basic altitude levels. These are: stratus clouds (low level), altostratus clouds (mid level), and cirrostratus clouds (high level). When stratus-type clouds are present, your skies will most likely be overcast or the cloud cover is 90% or more. Note, there is a difference between overcast and obscured skies.

How can you tell which one you are looking at? NASA scientist Dr. Lin Chambers came up with tips for students, teachers, and anyone in the public to use.

The main tip is to look for clues near the Sun. Caution: NEVER look directly at the Sun!

• If it rained recently or is about to rain, you are most likely dealing with a low level stratus cloud. While it is possible for rain to fall from mid-level clouds, it is quite rare.
• If it is raining during your observation, you have nimbostratus (or cumulonimbus - but the difference should be obvious! The latter is a thunderstorm). The terms nimbo/nimbus are from a Latin word for rain.
• If a stratus cloud is so thick you can't even figure out where the sun is, most likely it is a low level stratus. The visual opacity of such a cloud is opaque.
• If you can see the sun but it looks diffused (like looking through a glass bottle), most likely you have altostratus. The visual opacity would be translucent.
• High-level cirrostratus will generally be thin enough that the sun is still quite distinct. If the cirrostratus is not between you and the sun, you may be able to distinguish cirrostratus as being so thin that parts of the cloud appear bluish (that is, you are seeing through to blue sky). The visual opacity is transparent.

Sometimes, you might see stratocumulus clouds, which are flat with a puffiness to them. Stratocumulus clouds form when the stratus layer is breaking up. It indicates that the weather patterns have changed! You see stratocumulus clouds near warm, cold, and occluded fronts.

Find photographs of clouds and their varieties by visiting the World Meteorological Organization’s International Cloud Atlas.