The forecast on your phone is for the town at the bottom. It is not wrong, it is just answering a different question. A summit two thousand feet above that town can be ten degrees colder, in cloud, and carrying three times the wind, and none of that shows up in an icon of a sun behind a cloud.
Most places with real hills have a mountain-specific forecast that gives summit conditions directly: wind speed and gusts at height, the chance of the tops being in cloud, the freezing level, and how visibility changes through the day. Learning to read one properly is the cheapest safety improvement available, because it happens at home, on a sofa, with time to change your mind.
Wind is usually the number that decides the day
Rain makes a day unpleasant. Wind makes it dangerous. It rises with altitude, accelerates as it squeezes over cols and around ridge ends, and it is the thing most likely to put you on the ground somewhere you cannot afford to fall.
Read the gust figure, not the average. Averages are what the day feels like; gusts are what knock you off a step. A forecast averaging 25 mph with gusts to 55 is a very different proposition from a steady 30, and the second number is the one that matters on a narrow ridge.
| Around 20 mph | Noticeable. Hood up, hat on, conversation still works. |
|---|---|
| Around 30 mph | Hard work. You are being shoved around and you will tire faster than planned. |
| Around 40 mph | Walking a straight line is difficult, and a gust can put you down. |
| Above 50 mph | Exposed ridges, edges and steep ground are not a reasonable place to be. |
These are impressions rather than rules, and they shift with how heavy you are, what you are carrying, and whether the ground drops away on one side. A gust that is merely annoying on a broad plateau is a different matter on a crest with a hundred feet of air beside you. If the wind number looks marginal, the honest move is a lower, sheltered route rather than a hopeful one.
The temperature is not the temperature
Air cools with height. As a rough working figure, expect somewhere around 3 to 5°F for every thousand feet you climb, or roughly 6°C per thousand meters. It varies with humidity, and on some winter mornings an inversion flips it so the valley is the coldest place, but as a planning number it is close enough. A pleasant 50°F at the car is around 40 at three thousand feet before the wind gets involved.
Then apply the wind chill figure the forecast gives you, which is the number that describes what exposed skin experiences. Cold air moving fast strips heat far quicker than still air at the same temperature. When wind chill goes well below freezing, cover everything: goggles or at least sunglasses, a buff over the face, and gloves you can keep on while eating.
Freezing level and cloud base
The freezing level is the height at which the air reaches 0°C. Below it, precipitation falls as rain; above it, as snow, and wet rock glazes over. The worst version is a freezing level sitting just above summit height, which gives you rain falling onto frozen ground and paths turning to sheet ice while you are on them. If the freezing level is forecast to drop during the day, ground you walked up easily may not be ground you can walk down.
Cloud base tells you whether you will see anything. A forecast of a high chance of hill fog above three thousand feet means you are navigating by map, compass and counted paces from that height upward, not by looking at where you are going. If you are not confident doing that, the honest response is to pick a route that stays below the cloud.
Rain, and what it does to the ground
Timing matters more than total. Two inches of rain overnight and a clear afternoon is a good day. The same rain arriving at two o'clock, when you are at the far end of a horseshoe, is a bad one. Look at when the front comes through and plan the exposed section for the other side of it.
Prolonged or heavy rain also changes water levels fast, especially on already-saturated ground or when it falls on melting snow. A stream you stepped across in the morning can be a serious obstacle by late afternoon. If your route depends on a crossing to get home, that dependency is worth removing before you start, not discovering after dark. Rivers in spate are not something to talk yourself into; wait, walk to a bridge, or take the long way round.
Thunderstorms
If storms are forecast, summits, ridges and isolated high points are exactly the wrong place to be, and the answer is to be off them before storms are due rather than to watch and react. In many mountain areas convective storms build through the afternoon, which is the whole argument for a pre-dawn start. If you hear thunder, you are already close enough to be struck: get down and away from high ground, edges and lone trees, and keep going down.
Turning a forecast into a decision
A forecast you read and then ignore is decoration. The value comes from converting it into specific commitments before you leave, while you are warm, unhurried, and not invested in a summit that is now only an hour away.
- Read at least two forecasts. When they disagree, plan around the worse one.
- Check again on the morning, not just the night before. Fronts move.
- Write down the condition that sends you back: a wind figure, a cloud base, a temperature, or simply summit cloud plus a route you have not done before.
- Set a turnaround time and treat it as binding. Sunset minus the time it takes to get down, minus an hour, is a reasonable starting point.
- Have a lower alternative already chosen, so that backing off means doing something else rather than driving home.
The last part is the one people skip, and it is the one that makes the rest work. Nobody enjoys turning around, and it is much easier to do when there is a valley walk with a good view already in your pocket. A plan with only one acceptable outcome is a plan that pushes you into weather you already decided to avoid.