Sooner or later every walker ends up somewhere the forecast does not reach: a third day out with no signal, a hut whose bulletin is two days old, a valley where the printed forecast is for a town on the other side of the range. The sky still tells us most of what we need, provided we know which signs carry information and which are folklore. What follows is the short list we actually use: the cloud sequence that announces a warm front, the cumulus behaviour that announces an afternoon storm, the pressure trend read off an ordinary altimeter, the wind shift, and a few honest words about the signs that do not work.
The warm-front sequence: high, then lower, then rain
The most reliable sky reading there is comes from the way a warm front approaches. Warm air rides up over cold air along a slope so gentle that the leading edge of the cloud arrives 12 to 24 hours before the rain, and it arrives from the top down. First come cirrus: thin, high, fibrous streaks at 6,000 m and above, often with hooked ends, on an otherwise blue day. Then cirrostratus, a milky veil that thins the sun and puts a halo around it or the moon; the halo is worth more than any other single sign, because it means an ice-crystal sheet is overhead and a front is coming. Then altostratus, a grey layer through which the sun shows as a blurred disc like a coin through frosted glass, and finally nimbostratus, low, dark and featureless, with rain or snow that goes on for hours.
The timing is not exact but the order is, and the useful rule is: cirrus thickening from one direction into a halo means bad weather likely within a day; the sun going from a disc to no disc means within hours. On a morning with cirrus building in the west, we plan a shorter day and do not commit to a high camp.
Cumulus: the daily clock of instability
Cumulus are the other diagnostic cloud, and they tell us about the afternoon rather than tomorrow. Small, flat-based, cotton-wool cumulus that appear mid-morning and stay roughly the same size are fair-weather clouds; the atmosphere is stable and they will fade by evening. Cumulus that grow taller than they are wide by late morning, with hard cauliflower tops and darkening bases, mean the air is unstable and afternoon showers or storms are likely; when a top flattens into an anvil, a thunderstorm is mature and the first lightning is minutes away. The earlier the towers appear, the worse the afternoon. Towers by 10:00 is a turn-back-early day.
Two other cloud shapes carry specific messages. Lenticular clouds – smooth, lens-shaped, stationary caps and stacks over or downwind of summits – are the signature of strong wind aloft, often 60 km/h and more, even if the valley is calm; a ridge day under lenticulars will be a battle. And a cap cloud that forms on a summit and thickens through the morning means moist air being forced up the mountain, and often precedes the general lowering of cloud that brings rain.
Pressure: the altimeter is a barometer
Falling pressure means approaching low pressure and, usually, worse weather; rising pressure means improving. Without a barometer we read the trend with a barometric altimeter, which is simply a barometer with a different scale. Set it to the known altitude of a hut or a col in the evening. At dawn, in the same place, if it reads higher than the true altitude, the pressure has fallen overnight; if lower, pressure has risen. Near sea level 1 hectopascal corresponds to about 8 m of altitude, at 2,000 m to about 10 m, so an altimeter reading 40 m high in the morning means a fall of roughly 4 hPa. Wristwatch barometers and many phone sensors give the number directly and plot it, which is easier still.
What counts as significant: a fall of 3 hPa or more in three hours is a real change and a front or a deepening low is at work; a fall of 6 hPa in three hours is fast and the weather will deteriorate quickly, probably with wind; a steady, slow rise over a day says the high is building and the good spell will hold. The trend matters more than the absolute reading, and readings taken on the move are useless because our own change of altitude swamps the signal.
Wind: direction shifts and the valley breeze
In the northern hemisphere, standing with the wind at our back, low pressure lies to the left. A wind that backs – shifts anticlockwise, say from west to south – typically means a depression is approaching; one that veers, clockwise from south to west, usually means the front has passed and things will improve, often with a colder, clearer, gustier airmass behind it. Reverse the rule south of the equator.
Mountains add their own wind clock. On a fair day the valley breeze blows upslope and up-valley from mid-morning as the slopes heat, and reverses to a cool downslope, down-valley flow after sunset. This rhythm is a sign of settled weather. When it breaks down – a strong wind at noon blowing down-valley, or gusts from a direction the terrain does not explain – a pressure-gradient wind from a weather system has overwhelmed the local one, and change is coming.
Dawn signs and the ones that do not work
A heavy dew or a hard frost at dawn means the sky was clear all night and the air is dry and calm: high pressure, and a good morning at least. Mist in the valley bottoms with clear summits is the same story and usually burns off by 10:00. Poor visibility with a warm, close feel and no dew means moist air, and often cloud lowering later. A moon with a halo says the upper air is wet even if the surface feels fine.
Then there is the folklore. “Red sky at night, shepherd’s delight” has a physical basis in mid-latitudes where weather comes from the west: a red sunset means the western sky is clear of cloud, so the next system is still some way off. It is a weak signal, right perhaps two times in three. Animal behaviour, aching joints and pine cones are not reliable enough to plan a day around, and the claim that we can smell rain coming is mostly the smell of the first drops on dry ground, which is too late to be a forecast.
Putting it together on the hill
None of these signs stands alone; the skill is in stacking them. Cirrus from the west at breakfast, a halo by lunchtime, the altimeter 30 m high in the afternoon and the wind backing from west to south add up to a front tomorrow with high confidence. Fair-weather cumulus, a valley breeze on time, a steady altimeter and frost on the tent add up to a settled day we can spend high. The habit worth building is the two-minute sky check at each rest stop: cloud type and trend, altimeter against the last known height, wind direction against the terrain. Written in the margin of the map with the time, three or four of these across a day are a forecast of our own, and on the third day out they are the only one we have.