Pressure is the number that matters
A barometric altimeter or a watch with pressure trend is the most useful weather tool a hiker can carry. Falling pressure means air is rising and weather is deteriorating. A drop of 3 to 4 hPa in 3 hours is a clear warning.
Rising pressure usually means improving conditions, though a rapid rise after a front can bring strong wind. Note the trend at the trailhead and again at lunch, and write the number on the map so the two readings can be compared.
Clouds that mean business
Fair-weather cumulus are small, flat-bottomed and spread out. When they start building vertically before 11 a.m., the afternoon is likely to bring thunderstorms. A cloud taller than it is wide is already a thunderhead in progress.
A dark, flat cloud base moving in from the west usually means rain within 1 to 3 hours. High wispy cirrus that thickens into a veil can signal a front 12 to 24 hours out. If the summit is already in cloud, assume wind and rain.
- Towering cumulus before noon: storms later.
- Anvil-shaped top: lightning is likely.
- Dark flat base to the west: rain soon.
- Cirrus thickening: front in 12 to 24 hours.
- Cloud sitting on the ridge: wind and poor visibility.
Wind shifts and temperature drops
A sudden shift from south to west often marks a cold front. The temperature can drop 5 to 8 degrees in 20 minutes, and the wind picks up right after. If you feel that shift on an exposed ridge, put on your shell and start down.
Calm air that suddenly goes still before a storm is another signal. So is a rush of cool air on a hot afternoon. Both mean the atmosphere is moving, and you do not want to be the highest thing on the ridge when it finishes.
The 30/30 lightning rule
If you can hear thunder, you are close enough to be struck. Count the seconds between a flash and the thunder; every 5 seconds is roughly 1.6 kilometers. Under 30 seconds means the storm is within about 10 kilometers.
Leave ridges, summits and open water, and avoid lone trees. Descend 100 meters or more if you can, and wait 30 minutes after the last thunder before going back up. Most lightning injuries happen at the start and end of a storm, not the middle.
Animal and plant clues
Birds going quiet and insects disappearing can signal a pressure change. Cows and horses often move to low ground before a storm. These are not forecasts, but they are worth noticing alongside the sky.
Some hikers report a sharp, metallic ozone smell before a nearby strike. It is not a reliable warning, and you should never wait for it. If a storm is close enough to smell, it is close enough to leave the ridge.
Decide early, decide once
Pick a weather turnaround at the trailhead: a wind speed, a cloud type, or a time. When the trigger arrives, turn around without a debate. Arguing about the summit on an exposed ridge costs more time than the summit is worth.
Write the forecast and the trigger on your map. A decision made in the parking lot is calmer and better than one made at 2,500 meters with cold hands.
