Read a Mountain Sky Six Hours Ahead

Mountain forecasts are generated for a point at a given elevation and they are frequently wrong two thousand feet higher. Learning four cloud signals gives you a personal forecast that updates continuously, and in the mountains the cost of being caught above treeline in a thunderstorm is much higher than the cost of turning around early.

How to do it

1. Watch the direction of change, not the cloud

   A single cloud type means little. Cirrus thickening to cirrostratus over several hours means a warm front is coming, usually rain within twelve to twenty four hours. That same cirrus thinning means nothing is happening.

2. Take fair weather cumulus seriously after ten in the morning

   Small puffy clouds are normal. The question is vertical growth. If they are visibly taller each time you look, and the bases are darkening, you have convection building and a thunderstorm by early afternoon.

3. Treat a lenticular as a wind warning

   A smooth lens shaped cloud parked over a summit means very strong laminar wind aloft. Ridge travel will be brutal and gusts can knock you down.

4. Use pressure as the tiebreaker

   Falling steadily over three hours means deteriorating conditions, and a fast fall means fast deterioration. Many watches and phones show this. It is more reliable than any single visual signal.

5. Set a turnaround time and obey it

   In summer mountain ranges, be off exposed ridges by early afternoon. Pick the time before you start and treat it as fixed, because the decision made at the top with a summit in reach is never the good one.

6. Know what to do if it catches you

   Get off ridges and summits, away from lone trees, out of gullies that channel water, and spread the group out. Drop metal poles a short distance away. Get low and small on your pack.

Where people go wrong

• Trusting one forecast checked at the trailhead. It was made before you left and it was made for the valley.
• Judging thunderstorm risk by how it looks over your head. Storms build behind ridges you cannot see and arrive very quickly.
• Pushing a turnaround time because the summit is close. This is the most common contributing factor in mountain incidents.
• Sheltering under the only tree in a meadow. That is the strike target.