Mackerel Skies: Nature's Weather Forecast
If you looked up over Squamish this week, you may have noticed the sky covered in beautiful rows of tiny, rippled clouds—often called mackerel clouds because they resemble the scales of a fish.
For centuries, sailors and mountaineers have recognized these clouds as one of nature's oldest weather forecasts.
"Mackerel sky, not twenty-four hours dry."
As it turns out, there is real science behind the old saying.
What are mackerel clouds?
Mackerel clouds aren't actually a single cloud type. The pattern is usually made up of cirrocumulus (high altitude) or altocumulus (mid-level) clouds arranged in evenly spaced ripples across the sky.
These ripples are produced by a fascinating atmospheric phenomenon known as gravity waves.
No—not the kind that come from black holes.
Atmospheric gravity waves occur when stable layers of air are pushed upward and then begin to oscillate, much like ripples spreading across a pond after throwing in a stone. As the air rises it cools, allowing moisture to condense into cloud bands. Between those bands, the air sinks, warms, and dries, creating the clear gaps.
The result is the striking fish-scale pattern we see overhead.
Why do they form?
Gravity waves can be triggered in several ways:
- Air flowing over mountain ranges like the Coast Mountains.
- A strengthening jet stream overhead.
- Approaching warm or cold fronts.
- Changes in wind speed and direction with altitude (wind shear).
In mountainous regions like the Sea-to-Sky corridor, terrain frequently helps generate these wave patterns, which is why they're a familiar sight to local hikers and climbers.
What do they mean?
On their own, mackerel clouds don't “guarantee” rain but they are early harbingers of change.
They often indicate that moisture and instability are increasing high in the atmosphere. They're commonly one of the first visible signs that a weather system is approaching.
Often, within the next 12 to 24 hours, you'll notice thicker middle and lower clouds developing, followed by deteriorating weather as the front arrives.
It's nature's way of saying:
"The atmosphere is changing."
Why mountain travellers should care
A beautiful morning can quickly become an afternoon of low cloud, poor visibility, rain, and slippery terrain. Those changes affect route finding, increase the risk of slips and falls, and can even prevent rescue helicopters from safely reaching injured hikers.
Recognizing early weather clues gives you time to adjust your plans before conditions deteriorate.
The best mountain travelers don't just watch the trail.
They watch the sky.
Coming next: Part 2 – Sun Halos
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