Most people check the weather for a hike exactly once: the town forecast, the night before, for the temperature at the trailhead. Then they get 2,000 feet up, the wind picks up out of nowhere, the temperature drops fifteen degrees, and they’re suddenly having a genuinely different day than the one they planned for.
Temperature drops roughly 3.5°F for every 1,000 feet of elevation gain under normal conditions — a rule of thumb called the lapse rate. A mild 70°F trailhead can mean a 50°F summit on a moderate peak, and that’s before wind is factored in at all. Ridgelines and summits are also dramatically more exposed to wind than a sheltered valley trailhead, which means the actual experienced temperature (see: wind chill) can be another 10–15 degrees colder than the raw number suggests.
Elevation-adjusted forecast, not just the nearest town. If your app only gives you the valley number, do the lapse-rate math yourself, or look for a summit-specific forecast if the peak is popular enough to have one.
Wind speed at elevation, specifically. Trailhead wind and ridgeline wind are frequently unrelated numbers. A dead-calm parking lot can sit below a genuinely dangerous exposed ridge.
The hour-by-hour, not the daily summary. Mountain weather is famous for building through the day — clear morning, building clouds by early afternoon, and a real risk of afternoon thunderstorms in many ranges during summer. This is exactly why experienced hikers start early and aim to be off exposed terrain by early afternoon.
A daily high/low for the valley tells you almost nothing about what you’ll experience 3,000 feet up, four hours after you checked it.
If there’s any real chance of thunderstorms in the forecast, plan to be off any exposed ridge or summit before early afternoon. Mountain thunderstorms build with the day’s heating and are one of the genuinely dangerous, fast-developing hazards that a morning forecast can flag well in advance — but only if you actually check it before committing to a route.
Don’t check the weather for your hike — check the weather for your elevation, at the time you’ll actually be there. The trailhead forecast is a starting point, not the whole story, and the gap between the two is exactly where hikers get caught out.