Weather Knowledge · 8 min
Clouds, lightning and mountain weather
Why this matters
Clouds are the forecast you can see from the trail, and lightning above treeline is the fastest-moving lethal hazard in New York guiding. Both are field skills, not classroom ones.
By the end of this lesson you will be able to
- Identify the main cloud types and what each predicts
- Apply the lightning safety rules, including the flash-to-bang count
- Explain lapse rate, wind chill and why summits are a different climate
Clouds worth knowing by name
| Cloud | Look | Means |
|---|---|---|
| Cirrus | High, thin, wispy "mares' tails" | Often the first sign of a warm front, 12 to 24 hours out |
| Cirrostratus | High milky veil, halo round sun or moon | Front approaching. The halo is a classic |
| Altostratus | Grey sheet, sun a watery disc | Rain within hours |
| Stratus | Low uniform grey, fog on the hill | Drizzle, poor visibility, hypothermia weather |
| Nimbostratus | Dark, thick, featureless | Steady rain or snow, already happening |
| Cumulus | Detached cotton, flat base | Fair, if they stay small |
| Towering cumulus | Vertical growth, cauliflower tops | Get off the ridge. This is a thunderstorm building |
| Cumulonimbus | Anvil top, dark base, rain shaft | Thunderstorm now. Lightning, downdraft, hail |
| Lenticular | Smooth lens over a summit | Strong winds aloft |
The one to act on early is towering cumulus. Once it has an anvil you are late.
Lightning
New York's summer pattern is afternoon convection: clear morning, cumulus building by late morning, storms between roughly 14:00 and 19:00. That pattern is why summit trips start early and why "we will be down by two" is a safety statement, not a preference.
Flash to bang. Count the seconds between the flash and the thunder and divide by five for the distance in miles. Five seconds is one mile.
30/30. If the count is 30 seconds or less, the storm is within about six miles and you should already be in a safer place. Wait 30 minutes after the last thunder before moving back into exposure.
- Get off summits, ridges, open ledges and exposed water immediately. Descend, and keep descending
- Get out of and away from water. Paddlers get off the lake and away from the shoreline
- Avoid the tallest trees and lone trees. Uniform forest below the ridge is comparatively good
- Avoid shallow caves and overhangs, where ground current jumps the gap
- Spread the party out, roughly 50 feet apart, so one strike cannot take everyone
- The lightning position: crouch on the balls of the feet, feet together, hands off the ground, on an insulating pad if you have one. It is a last resort, not a plan
- Put metal-framed packs and trekking poles down a short distance away
A person struck by lightning carries no charge. Start CPR immediately; cardiac arrest from lightning has an unusually good outcome when CPR starts fast.
Mountain weather
- Lapse rate. Air cools with height, roughly 3.5°F per 1,000 feet in moist conditions and up to 5.5°F in dry. A 4,000-foot summit is commonly 15 to 20°F colder than the trailhead before wind
- Wind. Summits and cols accelerate wind. Wind chill on Marcy in November can be lethal on a day that is merely unpleasant in Keene Valley
- Orographic lift. Air forced up a mountainside cools and condenses, so summits make their own cloud and their own rain. Valleys can be sunny while the ridge is in cloud all day
- Diurnal wind. Valley breezes flow uphill by day and cold air drains downhill at night, which is why the valley bottom is the frost pocket
Practical upshot: forecast the summit, not the trailhead, and carry the layers for the summit even when the parking lot is warm.
From the field
11:40 on an exposed ridge, cumulus that were flat at nine now have hard cauliflower tops. Flash-to-bang on the first strike is 25 seconds, five miles. You turn the party immediately, descend into the timber, spread out, and sit it out. Thirty minutes after the last thunder you move. You do not summit, and nobody argues, because you told them at the trailhead this could happen.
This lesson is part of the NYS Licensed Guide Exam course. Open it in the course to answer the retrieval checks that go with it, track what you have read, and quiz the whole unit. Lessons, quizzes and the glossary are free.