A mackerel sky over Edmonton, Alberta is an indicator of moisture (the cloud) and instability (the cumulus form) at high levels. 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 temperature instability can lead to spectacular thunderstorms should the rising moist air reach this layer.

Source: english wikipedia (original upload 11 May 2004 by en:User:Denni, self made)

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Ø2005 D.Windrim (124809 bytes) (mackerel sky large format)