Green Comet

Cloud of the Day – Altostratus

Altostratus is just like stratus, only the condensation level is higher than 6,500 feet, putting it in the middle etage (6,500 – 20,000 feet.) It forms in the same kind of conditions as stratus, moist stable air. As a result, altostratus is often just as smooth and gray as its lower cousin. Depending on how thick the cloud is, it can range from light gray with the Sun or Moon showing clearly, to dark, hiding them altogether. When it’s that thick it’s officially called thick altostratus.

While stratus forms like fog, when stable air near the ground is moist enough and cool enough, altostratus normally forms when moist, stable air is lifted up to its condensation level. The lifting usually happens when the air mass rises smoothly up the slope of a warm front.

If altostratus gets thick enough, rain can fall out of it. At times the rain disappears before it reaches the ground, and is then called virga (from the Latin for twig or branch.) If the rain strengthens and persists, the cloud is renamed nimbostratus. Since it’s associated with frontal weather, it can be accompanied by other cloud types, including embedded thunder clouds.

While similar to stratus, altostratus is more complex.

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