Clouds: Both Sides Now

This is a sample lesson page from the Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any questions about this program can be directed to: Steve Seman

Prioritize...

At the completion of this section, you should be able to identify and describe the eleven major clouds types.They are: 3 high-level clouds (cirrus, cirrostratus, and cirrocumulus), 2 mid-level clouds (altostratus and altocumulus), 3 low-level clouds (stratus, stratocumulus, and nimbostratus), and 3 vertically developed clouds (fair-weather cumulus, cumulus-congestus, and cumulonimbus).

Read...

As apprentice forecasters, you will routinely look at clouds from the vantage point of weather satellites. But before you interpret clouds on satellite images, you need to learn how to classify specific clouds by observing them from the ground (hence my idea for the title of this section).

From the perspective of an observer standing on the Earth's surface, clouds can be classified by their physical appearance. Accordingly, there are essentially three basic cloud types:

  • Cirrus, which is synonymous with a "streak cloud" (detached filaments of clouds that literally streak across the blue sky).
  • Stratus, which, derived from Latin, translates to a "layered cloud."
  • Cumulus, which means "heap cloud."

As you learned in a previous lesson, meteorologists further classify clouds according to the height of their bases above the earth's surface.

Four Major Cloud Classifications
Image General Description
A wispy high cloud. High clouds (cirrus, cirrocumulus, cirrostratus) observed over the middle latitudes typically reside at altitudes near and above 20,000 feet. At such rarefied altitudes, high clouds are composed of ice crystals.
Middle level clouds that look like cotton balls. Middle clouds (altostratus, altocumulus) reside at an average altitude of ~10,000 feet. Keep in mind that middle clouds can form a couple of thousand feet above or below the 10,000- foot marker. Middle clouds are composed of water droplets and/or ice crystals.
A foggy, rainy day at a lake. Low clouds (stratus, stratocumulus, nimbostratus) can form anywhere from the ground to an altitude of approximately 6,000 feet. For the record, fog is simply a low cloud in contact with the earth's surface.
A developing thunderstorm cloud. Clouds of vertical development (fair-weather cumulus, cumulus-congestus, cumulonimbus) cannot be classified as high, middle or low because they typically occupy more than one of the above three altitude markers. For example, the base of a tall cumulonimbus cloud often forms below 6,000 feet and then builds upward to an altitude far above 20,000 feet.

Just by knowing the three basic cloud types (cirrus, stratus, cumulus) and the four classifications (high, middle, low, and clouds of vertical development), along with their corresponding prefixes and suffixes, we can name lots of different types of clouds.

  • High clouds can either be "plain" cirrus, or we can add the prefix "cirro" to a suffix that describes their appearance (cirrostratus for high-altitude, layered clouds; cirrocumulus for high-altitude, "heap" clouds).
  • Middle clouds carry the prefix "alto" and also a suffix that describes their appearance (altostratus for mid-level, layered clouds; altocumulus for mid-level, "heap" clouds).
  • Clouds of vertical development always include the word "cumulus" or the prefix "cumulo," but can have various suffixes or other descriptive modifiers (like "fair-weather cumulus").
  • The names of low clouds have more variation. Low clouds can be referred to as plain "stratus" (if they're smooth and layered) or "stratocumulus" if they have both layered and heap-like characteristics, for example. If low, layered clouds are precipitating, they're called nimbostratus. The prefix "nimbo" comes from "nimbus," which means that this low cloud produces precipitation (note that nimbus can also be used as a suffix, as in cumulonimbus when a cumulus cloud is producing precipitation). 

Learning to identify and describe the major cloud types is an important skill for any apprentice forecaster (see the Key Skill and Quiz Yourself sections below). Once you've spent ample time with those tools and are accustomed to looking at clouds from the bottom side, you're ready to look at clouds from the top side and tackle the principles of interpreting clouds on satellite imagery.

Key Skill...

Learning to identify the major cloud types can be a bit daunting. However, with some practice, you'll get the hang of it. To get started, spend some quality time right now going through the following interactive cloud atlas. It has everything you ever wanted to know about the names and descriptions of the eleven major cloud types that you should be familiar with in this course. Move your mouse over each red pin to see an example photo and description of that particular cloud type.

Quiz Yourself...

Feeling confident in your cloud identification skills? Take this quiz to see how you do.

Explore Further...

If you want to explore cloud identification further (or just look at some pretty cloud pictures), check out these online cloud atlases. I should point out that these sites delve into the details of cloud naming, which you are not required to know (thank goodness!). Also, while I have explored these sites and found them to be accurate, you may find slight discrepancies in descriptions, etc. In such cases, please defer to descriptions listed in the course text rather than on these sites.

Cloud Atlas hosted by Penn State: This atlas was created from images in the Karlsruhe Wolkenatlas (used with permission from Bernhard Mühr).        

Clouds-Online: This is a fairly exhaustive site on cloud classification.