Stratus opacus undulatus (St op un)
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Stratus opacus undulatus (St op un)

Here’s a scene from the Virginia countryside from February 2019. It was a cool, overcast morning. One of those gloomy weekend mornings that make you want to sleep in. Very low wind, temperatures in the 40s.

Summary: This is an example of a dark and undulating featureless layer cloud, classified as stratus opacus undulatus.

Cloud Type. Let’s breakdown the cloud’s altitude level and appearance to determine the cloud type. First, it’s pretty darn close to the ground. The cloud is relatively featureless and blanketing the entire sky. The five clouds whose bases are low to the ground are nimbostratus, cumulonimbus, cumulus, stratus, and stratocumulus.

It’s certainly not stormy, so cumulonimbus doesn’t make sense. Cumulus clouds are your classic low, puffy clouds, so you can eliminate them. Nimbostratus clouds bring rainy weather, but it’s not raining at all.

So we’re left between stratocumulus and stratus. When deciding between a stratus and stratocumulus cloud, remember that stratus clouds are generally featureless layer clouds. On the contrary, stratocumulus clouds can contain plenty of features. Both clouds don’t share any of the same cloud species, which can help you decide between the two.

Stratus clouds are best defined as featureless low-altitude layer clouds, and this is a good example of that. The type of cloud present is identified as a stratus cloud.

Cloud Species. When determining the species of a stratus cloud, know that stratus clouds can be associated with only two of the fifteen cloud species: fractus (ragged, broken up) and nebulosus (full of vapor, lacking detail). This  example can’t be specifically categorized as one or the other. Remember, when classifying a cloud, you don’t need to classify a species if it isn’t appropriate.

In this case, since this stratus cloud doesn’t fit the bill of either the fractus or nebulosus species, it’s acceptable to not add a species to its classification.

Cloud Varieties. Stratus clouds can be associated with three cloud varieties: opacus (opaque, masks the sun), translucidus (see-through, sun’s position visible), and undulatus (wavelike, undulating). In this case, the cloud is definitely dark and blocking out the sun. Additionally, you can see undulations and rolls in its base as it approaches the horizon. Both the opacus and undulatus cloud varieties would be appropriate in this instance.

Supplementary Features. Of the eleven cloud features, stratus clouds are associated with two: fluctus (Kelvin-Helmholtz waves, curls) and praecipitatio (precipitation reaching the surface). Fluctus clouds are rare… none seen here. And there isn’t any precipitation falling from these clouds. No need to add any cloud features in our classification.

Cloud Accessories & Other Clouds. Stratus clouds don’t have any associated accessory clouds, but they do have three other clouds associated with the cloud type: cataractagenitus (formed from a waterfall), homogenitus (formed from human activity), and silvagenitus (formed from a forest). It’s safe to say that none of these classifications apply here.

In this lesson, we determined the cloud classification starting from the cloud type and working our way down through cloud species, varieties, features, and accessories taxonomy. We’ve arrived at our official classification: Stratus opacus undulatus (St op un).

A graphical illustration of a stratus cloud

Stratus (St)

Low, featureless layer

A graphical illustration of a stratus opacus cloud

Opacus (op)

Opaque, masks the sun

A graphical illustration of a stratus undulatus cloud

Undulatus (un)

Wavelike, undulating

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