Nothing on this page competes with a modern forecast. Numerical weather prediction is extremely good and if you can get one you should use it.
This page is for the situation this app is written for, where you cannot, and the honest position is that a careful observer can see roughly twelve to twenty four hours ahead with useful reliability, and beyond that is guesswork.
Pressure is the strongest signal
A barometer is the single most useful weather instrument a household can own, and a cheap aneroid one costs very little and needs no power. A phone with a barometric sensor does the same job while it has battery.
The trend matters, not the number.
Read it at the same times each day and write it down. A single reading tells you almost nothing; three readings over six hours tell you a great deal.
Rapid fall plus rising wind is the combination to respect. That is a deepening depression and it produces the conditions that hurt people on high ground.
Reading the cloud
The sky ahead of a warm front follows a sequence that is genuinely readable, and it is the classic British deterioration.
- Cirrus. High, thin, wispy, hooked. The first sign, often in an otherwise blue sky, and typically 12 to 24 hours ahead of the rain.
- Cirrostratus. A thin milky veil, often producing a halo around the sun or moon. The front is closer.
- Altostratus. Thicker and greyer, the sun visible as through frosted glass. Rain within hours.
- Nimbostratus. Low, grey, featureless. Continuous rain, now.
Cloud thickening and lowering means deterioration. Cloud thinning and lifting means improvement. That single sentence covers most of what the sky is telling you.
Other clouds worth knowing:
- Cumulus, small and fluffy with flat bases and gaps between, is fair weather. Watch whether they grow.
- Towering cumulus, building vertically through the afternoon, means showers, hail or thunder later.
- Cumulonimbus, huge with an anvil top, is an active storm cell.
- Lenticular cloud, a smooth lens shape parked over a hill, means strong winds aloft and severe turbulence on the lee side.
- A cap cloud sitting on a summit means it is windy and in cloud up there, whatever it is like at the bottom.
Wind direction and what it tells you
Buys Ballot's law: in the northern hemisphere, stand with your back to the wind and low pressure is on your left. That tells you where the weather is coming from.
Backing and veering are the terms that matter, and they are easy to get the wrong way round.
- Backing is an anticlockwise change, for example south west to south to south east. In Britain it commonly precedes deterioration.
- Veering is a clockwise change, for example south to south west to west. It often follows the passage of a cold front, with clearing and showers behind.
Watch the direction over an hour or two, using smoke, a flag, or cloud shadows moving across a hillside.
Cloud moving in a different direction from the surface wind means the system is changing and is worth noticing. Cloud crossing from the left of the surface wind broadly supports the backing, deteriorating picture.
The sayings that hold up
The good ones all encode the same physics: what the air is doing to the west, and how much moisture is up there.
Showers thunder and hail
Different beast from frontal rain, and more dangerous on the hill.
- They build during the day with surface heating, peak in the afternoon and evening, and die away overnight.
- Towering cumulus by late morning means expect them by afternoon.
- They are local and heavy, and they can produce hail, sudden temperature drops, and very poor visibility for short periods.
Fog and visibility
- Radiation fog forms on clear still nights as the ground cools, sits in valleys and hollows, and usually burns off with morning sun. Clear sky and no wind at dusk in autumn is the signature.
- Hill fog is simply cloud at your level, and it can arrive as the cloud base lowers without any change on the ground below.
- Sea fog rolls in with an onshore wind and is far more persistent, and it can arrive on a warm sunny day with no warning from the sky.
- Fog in a valley you are descending into means you will lose visibility exactly as the ground gets complicated.
Visibility is what makes navigation difficult, more than rain or cold. Plan the route for the visibility you might get, not the one you have.
What the wind does to you
The most underestimated factor, and the one that turns an unpleasant day into a dangerous one.
- Wind removes heat far faster than still air, and at low temperatures the effect on exposed skin is substantial. A modest temperature with strong wind is more dangerous than a colder still day.
- Wind speed rises considerably with altitude. A breeze in the valley is a serious wind on a ridge, routinely two or three times as strong.
- It exhausts people. Walking into a strong wind is far harder work, and the noise makes communication and decision-making worse.
- It makes wet catastrophic. Wet clothing plus wind is the classic route into hypothermia, and it happens at temperatures well above freezing. See hypothermia.
Judge the day by the wind on the tops, not the wind at the car.
Keeping your own record
The thing that turns this page from theory into something you can rely on.
- Write down the pressure, the wind direction and the cloud, at the same time each day, in a notebook by the door.
- Then write what actually happened.
- After a few months you will know your own place, because local topography dominates. Which wind direction brings rain to your valley, which brings the cold, and which hill goes into cloud first are all local facts that no general rule gives you.
- Note what your household's water, fuel and travel plans depend on, and which weather breaks them.
Local knowledge beats general rules. A farmer who has watched one hill for forty years forecasts that hill better than any book, and the only way to acquire that is to start the notebook now.