![](http://upload.wikimedia.org/wikipedia/commons/thumb/b/b5/Mackerelskybig.jpg/450px-Mackerelskybig.jpg)
A mackerel sky over Edmonton, Alberta. Altocumulus clouds in this form indicate moisture and instability at high levels, the two main ingredients needed for precipitation. If the lower atmosphere is stable and no moist air moves in, the weather will most likely remain dry. However, moisture at lower levels combined with instability can lead to spectacular thunderstorms.
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