Thanks for the info on the barometer; (I guess I've never carried one) -

But here's some info on recognizing dangerous clouds and storms:

Last August, coming from Mineral King into Big Arroyo, when I was coming over Black Rock Pass, I noticed clouds building up in a very visually striking pattern, as I ascended.

I grew up in the midwest (Illinois), so I am very familiar with how thunderheads LOOK. But I transplanted to California; and I actually don't often see thunderheads near where I live now. When there are storms, they usually roll in as a low-altitude marine-layer first.

. . . so as we came down into Little Five Lakes, this cloud-pattern began to pile up into a familiar "popcorn" stratocumulus shape, DARK underneath! ... and rolled over and covered the whole sky. Then we heard thunder. By then, we were into some trees and boulders, and we got rained and sleeted on.

The next day was clear - but as we were climbing up onto a 12,000' peak (Eagle Scout Peak) near Kahweah Gap, I started to see that pattern of clouds gathering, the same as I had seen about the same time-of-day as the day before (1pm or so). Just - above the ridgelines and peaks, hazy clouds were forming and disappering right before our eyes, on and off, for about an hour. Amazing and hypnotic to watch. (this was happening over OUR peak, as well as the Kahweahs, across the Arroyo valley).

I figured this was the pre-cursor to another afternoon stormfront, and I was right.

I thought I had more time - but I was wrong.

We dilly-dallied on the mountain, bouldering, for about another hour before the clouds came in solid. Above 11k', we were fogged. Had it been a storm, at that time, it could have gone badly for us. Luckily, the worst we got was some light sprinkles on the way down. Nothing indicated to us that we had any dangerous conditions imminent, so we didn't crouch down at any point.

Now: realize - even the wispy clouds forming rapidly in that pattern like that can generate lightning. It's the updrafts and downdrafts within the clouds at the 10,000 - 20,000 - 50,000' level that are theorized to generate lightning. Lightning can even come out of a clear-blue area of the sky (under storm-forming conditions; where there is moisture, updrafts, cool+warm air mixture, etc.) So, there was some risk in my decision to remain on the mountain; but getting down (instead of sheltering) had to be a priority, once we were fogged in.

Wind and rain and hail are scary, and they can be dangerous, of course. But lightning kills the most.

In general, (but not as an absolute) - summer storms tend to form in the afternoon, when the daytime heat has warmed the air, and caused it to rise, generating updrafts, mixing with cooler, drier air at higher altitudes, forcing the moisture to condense. This pattern can repeat day after day after day, if conditions are right.

Moreso in the mountains, because you have runoff water, and moist soils concentrated in the valleys, and that moisture evaporates in the heat, and the prevailing winds pushes it up and over the terrain to higher elevations (which was what likely caused the "dancing clouds" effect I was seeing, along the ridge at Blackrock Pass). That rain gets dumped, the next valley over.

This is why it is important, when you're planning a day-hike up to a peak, to get up to the summit as early as possible, and get off the peak generally by noon or 1pm.

Obviously, you can't predict weather like clockwork like that. But sometimes, summer weather can get into very predictable daily patterns.