Bad weather almost always shows up in the sky hours before it arrives. You just need to know what to look at, and in what order.
UNDERSTAND
A weather front is warm, moist air sliding up over cold air, along a very gentle slope. It reaches the highest layers of the sky first, hundreds of kilometers ahead of the rain: that's why the first signs are very high clouds made of ice. A halo is sunlight bent by those ice crystals, always at the same angle, 22 degrees. Then the slope brings the moisture lower and lower, and the clouds thicken until it rains. The whole thing usually takes about a day, hence the twelve to twenty-four hours.
An afternoon thunderstorm is a matter of heat. The sun warms the ground, the ground warms the air, and the air rises. If the air higher up is cold enough, the bubble of warm air keeps rising and the cloud grows taller. The anvil forms when it hits the ceiling of the layer where clouds form, and spreads out.
In the northern hemisphere, wind turns counterclockwise around a low, drifting slightly toward its center. Hence the back-to-the-wind rule, stated in the 19th century by the Dutch meteorologist Buys Ballot, and hence the way the wind shifts as a low goes by.
Dew forms when the ground cools a lot during the night. That takes a clear sky and no wind, the two conditions of settled weather.
None of these signs is a certainty. Several signs pointing the same way, though, are rarely wrong.
ACT
Look where the weather comes from
In Western Europe, and in most of the mid-latitudes, weather usually comes from the west. Look that way, and look high: the first signs are the highest clouds.
Rain within twelve to twenty-four hours
The clouds of an incoming front always arrive in the same order. When you see it unfold, rain is on its way:
- very high white wisps, stretched into commas, "mares' tails";
- a milky white veil covering the sky, often with a halo around the sun or the moon: a big ring of light;
- a thicker gray veil: the sun is just a pale spot, as if behind frosted glass, and things stop casting shadows;
- low, dark gray, even clouds: the rain, steady, for hours.
The thicker and lower the clouds get, the closer the rain.
The afternoon thunderstorm
In the morning, small flat white clouds, fair-weather cumulus: nothing to worry about. If they grow taller before noon and start looking like ever-higher cauliflowers, a storm is likely in the afternoon. A top that spreads into an anvil: the storm is here. In the mountains, be off the ridges before early afternoon. To know how far it is, count the seconds between flash and thunder and divide by three: that's kilometers (see Measuring with your hand).
A shifting wind
Back to the wind, in the northern hemisphere, the low pressure is on your left, slightly ahead. If the wind picks up while turning counterclockwise, from southwest to south for example, a low is coming. If it turns clockwise, toward northwest then north, it's moving away: often one last round of showers, then the sky clears and the air gets cooler.
Small signs, morning and evening
- Dew or frost at dawn: the night was clear and calm. The day will most often stay dry.
- No dew after a clear night: wind or clouds have come in. The weather is changing.
- Red sky at night: the sky is clear to the west, where the weather comes from. A good sign for tomorrow.
- Red sky in the morning: the sky is clear to the east, but the clouds are coming from the west. Be wary.
- Contrails that linger and spread: the air is moist up high, often ahead of a front. Contrails that vanish quickly: dry air, stable weather.
- Smoke from a fire rising straight up: calm, stable air.
ADAPT
Southern hemisphere. The rotation rules flip: back to the wind, the low is on your right, and the wind shifts the other way.
In the mountains. Storms are more frequent there, build faster, and lightning strikes high points. At the first rumble, get off ridges and summits, stay away from metal objects, don't shelter under a lone tree, and spread out from each other. Caught in the open, crouch with your feet together, on your pack if you can: it's only the lesser evil, the real fix is to be down before.
By the water. A swell building with no wind and visibility dropping also signal a change. Sailors' tricks are in Sailors' tricks.
A storm is really coming. Getting ready and sheltering from the wind: Tempête et vent violent.