My peppers were hit by a couple of light frosty mornings back in Nov but came thru ok. A more recent Dec dip down to 19F did them in. My squash plants never live to see cold weather.

I think squash plant tissues are much more tender than peppers - higher water content?

I don't think water content is an issue. Highly frost tolerant veggies don't have lots less water in their foliage than frost sensitive foliage does. I think the issue is what's in the water. It may be just sugars, but may also be certain proteins that render them frost tolerant. Also, some plants can simply withstand crystallization of internal liquids. Raw collard greens are 90% water, and they are HIGHLY frost tolerant.

Squash stems are usually hollow - I don't think that helps either...

I have seen exactly the same thing. Peppers last longer in my garden then anything else that is not supposed to survive a frost. I figure they a "desert" plant, it gets cold at night in the desert! I do not know why but I have noticed it every year - it always beats a dozen different things. It beats: tomatoes, potatoes, cucs, melons, beans, beets, all squash (winter and summer), corn, okra and that is off the top of my head. I built a cold frame I cover in winter and run a small heating pad and a couple light bulbs. Really cold nights I add blankets on top. I typically do not harvest my peppers until December every year this way.

I am reading in extension literature (see, for example, https://www.ontario.ca/page/effect-extreme-temperatures-tomato-and-pepper-crop) that peppers are actually LESS frost tolerant than tomatoes. That is, tomatoes are more likely to survive a light freeze than peppers. Not sure I believe that.

It occurs to me that moisture on the leaves could have an effect. To the extent that the surface of squash leaves are moister than peppers leaves, wind would chill them more readily. One might expect that leaves with a moister surface are also more likely to suffer from heat stress. Squash leaves certainly wilt in heat more than peppers do, presumably because water is evaporating from them more rapidly. It's evaporation that does cooling, so leaves that don't retain water as well will get colder in a windy freeze.

" That is, tomatoes are more likely to survive a light freeze than peppers. Not sure I believe that. "

Me neither. It's possible that it might be true for early spring transplants but for late mature plants in autumn the peppers seem to have the upper hand over tomatoes or almost any other summer crop.

There are micro climates. Being close to a heat source, like a cement wall might help. In a raised bed, the walls of the bed might protect the peppers from wind. Lots of factors might be at play. But the other thing is that different cultivars have different characteristics. If your peppers survived, but the squash did not, it might say more about the variety of each than a general rule about the plants.

Peppers are known to drop fruits or blossoms when the weather gets too cold, so, certainly, in the fall, repeated low temps might make it impossible for your pepper to ripen fruit, long before the plant itself succumbs.

Didn't peppers originate in the mountains of Central and S. America...?

And I do agree that a factor could be the moisture content/tenderness of leaves/stems. My pepper plants - when I used to be able to grow them - would develop very tough, woody stems. I had more than a few that lasted a couple of years, since we only very rarely experience freezes here.

Well, these peppers and these squash plants were far from any walls. The conditions were identical for both. But certainly if you have some large thermal mass next to a plant, like a brick or concrete wall, the temperature won't drop as fast.

With regard to leaf moistness, I'm betting that if you set aside a squash leaf and a pepper leaf to dry, the squash leaf would dry out first. Why? Because the surfaces are slightly more moist. Squash leaves are covered with these urticating hollow bristles that may conduct moisture. Pepper leaves are smooth and waxy. The leaves that are moister are ones that will be cooler.

It is well understood that plants can get frost nipped if the temperature is above freezing but if there is wind. That's because moisture on the plant evaporating cools it below the air temperature. If you stand out in the wind when you're all wet, you'll know it.

For sure my pepper survive light frosts when tomatoes die. This has happened several years.

Would be interesting to understand, among frost-intolerant plants, which are the most frost intolerant. That is, for a predicted mild freeze, who needs protection the most? Would be interesting to hear stories about THIS FROZE BUT THAT DIDN'T among plants considered frost-intolerant.

I know it was asked a couple weeks ago. But no, peppers didn't originate in the mountains. They originated in the Amazon and tropical basin lands of Brazil and Bolivia. Domistication spread from there. Potatoes are the mountain nightshades. Though domesticated in the Andes, most of the modern genetics grown around the world are derived from lowland domestics.