Summertime opens up the high country to day hikers, backpackers, rock climbers, mountaineers, wildflower junkies, birdwatchers, stargazers, pilgrims, truth-seekers, and all manner of other visitors and refugees from the lowlands. These mountain ramblers aren’t just treated to brilliant sunshine and verdant alpine landscapes: They also find themselves up in the heights during prime thunderstorm season.
Mountains in summer are essentially storm nurseries. Head above timberline unprepared, or not sufficiently humble, and you can find yourself in perilous straits on any given afternoon.
Why are mountains such reliable stages for summer t-storms? Well, lots of reasons, but the simplest is that these high prongs and tables of rock heat up in the strong summer sun and warm the air above them to a temperature greater than the surrounding atmosphere. This creates the atmospheric instability to allow that mountain-heated air to rise (convection), and that rising air provides one of two key ingredients for storm formation.
The other is enough moisture to allow the creation of a cloud deep enough to make a thunderhead: in other words, deep enough to form ice crystals in its crown and to have enough precipitation swirling around in updrafts and downdrafts to electrify the whole affair and spark lightning.
Mountains as Storm Nurseries
A weather front may drive thunderstorms into your high-elevation playground, but here we’re mostly going to be focused on more local, “single-cell” disturbances: genuine mountain thunderstorms spawned in (and by) the high country.
Mountains generate such storms through a variety of processes. Most basic, again, is simple convection: Ground heated by the sun warms the air above it and causes it to ascend, potentially to the level where its water vapor condenses into clouds.
Here it’s worth sparing a few sentences to the concept of albedo (not libido, mind you, though it’s easy enough to make the Freudian slip). Albedo refers to an object’s relative ability to reflect sunlight: the higher the albedo, the more reflective the surface. Lighter-colored materials boast greater albedo, and therefore tend to be cooler than dark-colored ones. In the mountains, this means outcrops of darker rock (say, basalt) or subalpine conifer woods will heat up faster and more intensely than, for instance, pale granite or snowfields. These low-albedo zones may “seed” the local atmosphere with the instability for a storm.
Besides simple heating of elevated land from solar insolation, mountain terrain can brew up storms. An air mass shunted up a mountainside can (through what meteorologists call orographic lifting) form clouds and precipitation. Thermals rising up the shoulders of a mountain peak may converge at the summit and force up an air parcel to condensation level. A valley or gorge can channel airflow upslope to the same effect. Currents converging in the lee of an obstructing peak may pile into a thunderhead, too.
Fieldmarks of a Mountain Thunderstorm
Let’s say you’re headed into the high Wind Rivers or Uintas or Sangre de Cristos (one heck of a thunderstorm factory) for the day, or the week. How should you be scrutinizing that brilliant high-elevation sky in order to stay in the good graces of the mountains and one step ahead of any atmospheric turmoil?
Well, first let’s emphasize what many longtime hikers and climbers already well know: As a general rule, do your above-timberline carousing during the first half of the day and get back down into the timber by afternoon. Storms are more likely to arise in the latter half of the day once things have heated up sufficiently to provoke vigorous convection.
Key Into the Puffballs
By midday or early afternoon on a typical day in the mountains, you’re liable to see those familiar puffball cumulus clouds sailing along against the dazzling blue. When they’ve formed this late in the day, cheerily light-colored and broader than they are tall, these will often be nothing more than friendly “fair weather” clouds.
But maybe those puffballs pop up earlier, in mid- or late morning—first over peaks and ridges, then more widespread. Take note, innocuous as these first cumuli look: Their appearance in the sky so soon in the day suggests the atmosphere’s unstable enough to spawn a t-storm later on.
Such clouds are liable to form, grow, and dissipate over and over, with later versions gaining in depth. The lower in the sky they appear (and the earlier), the moister and thus more storm-ripe the conditions. Flat-topped cumuli suggest a stable layer capping further convective growth, at least for the time being—as does, more generally, a cumulus that’s wider than it is tall.
Clues to a tenser atmospheric arena? More columnar cumuli, taller than broad, with knobby-looking roofs. These are swelling cumulus (cumulus congestus), the forerunner to the cumulonimbus, sending up billowing towers called turrets. Turrets, which kind of look like fists punching straight up out of the cloudhead, reveal a hearty and growing cumulus that’s gunning to morph into a cumulonimbus.
Birth of a Thunderhead
As the swelling, turreted crown looms ever higher, it surpasses the freezing level and accumulates supercooled droplets and ice crystals. The shift to an icy dome manifests as a blurring to the cumulus profile, which in earlier stages (and in lower turrets) has a clean, sharp border. A cumulus tall enough to generate ice particles is said to be glaciating, and the visual hallmark is that spreading fuzziness to the cloud outline.
Glaciation marks a critical turning point in the t-storm life cycle: essentially the birth of a genuine cumulonimbus. (If you’ve ever had front-row seats to a swelling cumulus transforming into a cumulonimbus, boiling over with surging turrets and the whole mass in motion, the cloud can seem impossibly, overwhelmingly huge.)
A glaciating cloud often hastens its upward growth, towering to 30,000 or 40,000-plus feet. As the top hits the tropopause—the frontier between the lowest layer of the atmosphere, the troposphere, and the next higher realm, the stratosphere—it flattens out. This creates the striking anvil cloud characteristic of a full-grown cumulonimbus.
Showers & Lightning
A glaciated cloud also starts unleashing precipitation, as ice crystals grow big and heavy enough to tumble downward, glomming on water droplets as they do. If the air through which the precipitation falls remains cool enough, the ice may reach the ground in pellet form, aka graupel. Or it may melt and land as rain.
In an energetic thunderhead, strong updrafts may snatch plunging graupel and toss it back up into the cloud; pellets may fall and rise over and over, cloaking themselves in more droplets to form hailstones. You can often I.D. a hailstorm at a distance by the white hailshaft undergirding the cumulonimbus: Hailstones are more reflective than raindrops, hence the brighter look to their chute.
Hail’s a definite hazard, of course, but the leading threat during a mountain t-storm is lightning. That should be your prime cause for alarm upon observing a glaciating cumulus: As soon as precipitation is falling, you’ve got the potential for lightning.
Basically, the tumble and crash of precipitation stratifies electrical charge in the thunderhead, its icy heights positive and the heavy graupel creating a negative charge lower down. The negatively charged cloud base repels the typical negative charge of the Earth’s surface to create a localized positive charge below the storm, priming the whole area for some high-voltage crackling.
Most lightning spits within the thunderhead itself (as cloud-to-cloud/intracloud lightning), but a smaller share makes the freaky leap to terra firma. You see a brief but brilliant vein of electricity bridging sky and earth, but actually cloud-to-ground lightning involves a bidirectional sequence: a leader first lays out a road for electrons from cloud base to ground, then the flash comes as current surges along the reverse path as a return stroke—maybe a series of return strokes, interspersed with landward “recharges” called dart leaders.
Impressive as the complex mechanisms of a lightning strike are to dissect, you’re not going to be concerned about parsing them out in the field. Instead you ought to be seeking safe refuge: lower country, generally, and away from metal, soaked climbing ropes, and isolated trees or groves.
Clues to a Mountain Thunderstorm’s Approach
If you’ve got a sightline on a flashing, bellowing thunderhead a few drainages over, here are a few ways to gauge whether you’re in the risky path. A t-storm’s typically driven by midlevel winds, but the melee of clouds and air currents that often accompanies a storm can make recognizing the steering airflow challenging. But if you can see the cumulonimbus anvil, it typically points in the direction the tempest is traveling.
You can also key into lighting and thunder to plot the t-storm’s course. The boom of a thunderbolt covers a mile in five seconds. If the interval between lightning streak and peal is getting shorter—well, you’d better grease up those hiking boots and make yourself scarce.
In the convoluted topography of a mountainscape, though, you won’t always have a visual on an oncoming cumulonimbus. Say you’re in the lee of a peak or ridgecrest, perhaps hearing some distant thunder and wondering where you are in relation to the storm. If you see shreds of cirrus cruising high overhead, be wary: These could be wisps blown off the anvil cloud. It’s also possible you might see lower-level cloud rags wafting above: scuds, discrete “understory” clouds condensed in the updraft or downdraft of the thunderstorm and outpacing it by a bit.
Speaking of downdrafts, that diving rush of air from of a cumulonimbus (generated by its showers) can fuel a strong outflow that precedes the storm. If suddenly the winds whip up—you may scent ozone in the gusts—you may well have a thunderstorm upon you in minutes.
Sometimes you’ll see the overcast turn into a bizarre lid of pouch-like rumples: mammatus clouds. These warrant real concern for the mountain traveler, as they usually compose the belly of a thunderhead’s anvil. The fierce heart of the storm might be imminent.
Special thanks to Dr. C. David Whiteman, Research Professor of Atmospheric Sciences at the University of Utah and author of Mountain Meteorology, for his insight into high-country storms. Also exceedingly helpful as a reference for this article is Jim Bishop’s “Mountain Thunderstorms: Their Formation & Some Field-Forecasting Guidelines.”