6 Cloud Types That Tell You What Weather Is Coming

The Sky Is the Cheapest Forecast

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Before phones carried weather apps, hikers and sailors read the sky. Clouds still tell you a lot, especially in the mountains where conditions change faster than forecasts can follow. Hiking organizations and weather agencies teach the same basic signs, and the six below are the ones that matter most on the trail. Cloud reading is not an exact science, and sources sometimes disagree on details, so treat it as a supplement to an official forecast, not a replacement. Washington Trails Association advises doing your research on the weather before you hike, ideally using multiple reliable sources. A key habit is to watch how clouds change over time, because lowering and thickening clouds often signal a shift. Learn the shapes and you will notice trouble earlier.

Cumulus: Fair Weather Puffs

Small, fluffy cumulus clouds with flat bases are the clouds children draw. On a warm day, they usually mean fair weather. They form as warm air rises and cools, and they often appear in the late morning. Keep an eye on their growth. If cumulus clouds begin to build vertically through the day, getting taller and darker, they can develop into cumulonimbus, the storm clouds. That change is common on summer afternoons in the mountains. Small, scattered cumulus that stay small are no concern. The rule of thumb is to ask whether the clouds are getting bigger. If they are, plan to be off high ground by early afternoon. A pleasant morning sky can hide the beginnings of an afternoon storm.

Cirrus: Wispy Warnings

Cirrus clouds are thin, wispy streaks high in the sky, made of ice crystals. WTA says they typically appear in fair weather and can clear up, but they often signal warming temperatures, rising snow levels and possible rain or snow showers within 24 to 48 hours. Another source notes that if a cirrus sky darkens and thickens into a sheet called cirrostratus, it is a sign of rain or snow, depending on temperature. A halo around the sun or moon often accompanies cirrostratus. When you see cirrus thickening over a day, take it as a prompt to check the forecast and plan a shorter route. Cirrus alone is not a reason to cancel a trip, but it is a heads-up that conditions may change.

Mackerel Sky: Rippled Warnings

A mackerel sky is covered with rippled, scale-like patterns in high or mid-level clouds, called that because they resemble the scales of a mackerel fish. These are usually altocumulus or cirrocumulus clouds. WTA adds that if these clouds appear on a warm, sunny morning, rain showers or thunderstorms often develop in the afternoon. Another source says altocumulus usually occur after a storm, which shows that sources can differ, so pay attention to the context and the trend. The old saying goes, mackerel sky and mares' tails make tall ships carry low sails. Treat a mackerel sky as a prompt to check the weather and plan an early start so you are off exposed terrain by midday.

Cumulonimbus: The Storm Cloud

Cumulonimbus is the cloud hikers need to respect. These towering clouds can produce heavy rain, snow, hail, thunder, lightning, gusty winds and even tornadoes, according to hiking sources. A flat, anvil-shaped top is a strong indicator of a thunderstorm. NOAA says they grow on hot days when warm, wet air rises very high into the sky. When you see one building, get off ridges, summits and open ground, and move to lower terrain away from isolated tall trees. Lightning can strike from a storm miles away, so do not wait for rain. The safest approach is to plan around the forecast and start early. If thunder is audible, the storm is close enough to be a danger.

Lenticular: The Lens Over the Peak

Lenticular clouds are smooth, lens-shaped formations that hover near mountain peaks. WTA notes they are spotted over or downwind of a large peak. A hiking source says they may indicate approaching rain or snow within 48 hours, especially when clouds are lowering and thickening, and that they might also point to strong winds downwind of peaks. NOAA notes that they do not predict weather by themselves, so the sources are not in perfect agreement. The practical lesson is that lenticular clouds show powerful winds aloft, which can be dangerous on summits and ridges. If you see them, expect gusts and consider choosing a sheltered route. They are also beautiful, and photographers love them at sunrise and sunset.

Watch for Lowering and Thickening

One of the most reliable cloud lessons is not a single cloud type but a change. A hiking source says that one of the key things to remember while watching clouds is to look for the lowering and thickening of clouds, as this often signals a change in weather. A high, thin layer that becomes a gray, lower ceiling usually means moisture is arriving. Combine this sign with other clues: a falling barometer on a watch, rising wind, sudden temperature drops and a change in wind direction. If conditions deteriorate, turn around early instead of pressing on. Having a plan to descend is part of good mountain judgment. Clouds will not tell you everything, but paying attention gives you time to make better decisions.

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