Altocumulus mackerel sky

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An altocumulus mackerel sky or mackerel sky is an indicator of moisture (the cloud) and instability (the cumulus form) at intermediate levels (2400-6100 m, 8000-20,000 ft). 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 surface temperature instability can lead to rainshowers or thunderstorms should the rising moist air reach this layer. There is an old saying, "Mackerel sky, mackerel sky. Never long wet and never long dry."

Mackerel sky over Lincolnshire, UK.

Mackerel sky over Edmonton, Alberta.

Mackerel sky over Erlangen, Germany.

Mackerel sky over Metung, Victoria, Australia

v • d • e Clouds
High Clouds (Family A):	Cirrus (Ci) • Cirrus uncinus • Cirrus Kelvin-Helmholtz colombia • Cirrostratus (Cs) • Cirrocumulus (Cc) • Pileus • Contrail
Middle Clouds (Family B):	Altostratus (As) • Altostratus undulatus • Altocumulus (Ac) • Altocumulus undulatus • Altocumulus mackerel sky • Altocumulus castellanus cloud • Altocumulus lenticularis
Low Clouds (Family C):	Stratus (St) • Nimbostratus (Ns) • Cumulus humilis (Cu) • Cumulus mediocris (Cu) • Stratocumulus (Sc)
Vertical Clouds (Family D):	Cumulonimbus (Cb) • Cumulonimbus incus • Cumulonimbus calvus • Cumulonimbus with mammatus • Cumulus congestus • Pyrocumulus  • Pyrocumulonimbus