usefulness of shade cloth at different altitudes

I'm in a suburb of Denver, at a bit over 5000 feet of elevation, with a dry, sunny climate. The sunlight and UV radiation are intense. So is the radiant cooling at night.

I was wondering about building a shade structure over my whole back yard with 20% shade cloth. Would this benefit everything? Or would some plants like tomatoes not like it? In other words, would a decrease in UV damage compensate for 20% less light?

How much light can plants actually use before they are oversaturated anyway? I seem to recall hearing that even at sea level at lower latitudes there is a superabundance of light, thus the success of multi-story plantings there compared with failure of similar densities at high latitudes.

Also, is there any self venting fabric that can both keep a garden somewhat shaded while increasing temperatures at night?

A 20-33% shade cloth would adequately screen your backyard from satellites and prevent people from using Google Maps to see what you have and what you are doing.  If your local economy tanks, that would make your backyard food supply less of a target for theft as well.  Even if you don't actually grow anything under it, a shade screen is still a very good idea.

On the vegetative front, remember that dry winds are a dehydrating and desiccating force.  Having vertical 50-83% shade cloth on the side(s) of your property that get blasted by hot summer winds is just as useful as having 20-33% all across the top.  (Just be careful not to create any artificial wind tunnels.)

Note that photosynthesis always runs at the maximum possible speed.  Plants don't really exercise any form of restraint.  Ultimately one resource becomes the limiting factor.  Usually it's root access to water.  Shifting the bottleneck to sunlight would decrease the drain rate for moisture and allow greater ion diffusion of nutrients prior to uptake.  The plants would grow less but they would be more nutrient rich.  You'll notice this most with your leafy greens.  So lower quantity, but higher quality.  You get to decide which is more important to you and your family.

The higher the % your shade cloth is, the more it blocks out sunlight during the day and the more it blocks radiative cooling at night.  It will be warmer under a 66% shade cloth at night than it will be under a 33% shade cloth.  Both will be warmer than no cloth at all.  Line-of-sight to a clear, cloudless sky is a good recipe for frost.

Finally, long-term economics don't favour 'hi-tech' materials.  Avoid the trap of over-thinking this.  Go to a nearby nursery, ask them what shade cloth they use, and buy the same brand.  They've already worked out what the best product is to use — no point you wasting $$$ and years working it out for yourself.

I lived on the Front Range for quite a while at higher altitude. Denver gets hail. Also I know the frying of the UV. (if you are close to sea level and use SPF suntan lotion, add two to four to the number you are using to make up for the UV). Weather during summer was usually bake-your-brains by mid afternoon. And yes, using a pot holder on your steering wheel.

30% shadecloth will be your best friend and will give you about the best turnaround on protection and useful shade. Get woven not knitted, knitted will unravel everywhere. Any color will do. Woven will deal with hail a little better too. Windbreak to your prevailing wind will do a lot for the dry climate there as well. I know there are water issues that are getting worse, so preventing evaporation from wind will help in growing anything. At that altitude 'wind scald' is a definite issue, and can take out a tomato plant in an afternoon.

Rather than use fairly high percentage shadecloth I would use fencing and set additional grommets and fit tarping to the fence tightly enough not to flap. Flapping or loose tarp will wear out in no time. Properly fitted tarp would last me at least three years. I would get the huge tarp grommets (with setter) at Harbor Freight Tools, a couple of packages of the largest ones will do a pretty good length of fence tarp. I would get a foresty green with tan other side and put the green side to the neighbor. That way they had something civil to look at. Wire with galvanized tying wire or aluminum electric fence wire (TSC supply has a 17 gauge 100% non coated that's fairly cheap for a quarter mile spool). Just remember to be a little generous with the wire length and curl the ends back so nothing sharp to bite you later. And double loop through the grommet. My go-to windbreak fence materials are steel posts and calf panels (50" x 16' got at a farm supply place). Lay out the panel, put the posts in to match (usually one at each end and one in the middle) then wire the panel to the posts with either the posts on the lee side of the prevailing wind or on your side of view if you have neighbors. Put the tarp on the outside on the non post side. Cut tarp and fold over a few inches, measure, set grommets, add a few where they put some on the non cut edges, then attach.

I am now at a little lower altitude elsewhere and I'm still using 30% shadecloth. From about 5000 to about 7500 feet, that seems to be a useful % weave and is readily available. I bought some mesh tarps at Harbor Freight on sale, that are 30% and have had those installed on my screen house for the last three years and they have held up pretty well. The other piece for the high tunnel was bought online, cheap for huge, unhemmed. I am still hemming it with an Awl-for-All and the black tarpy seal strips off the top of cat food bags, saved a real fortune. Adding a double coin of tarp material (Purina Complete Cat Chow comes in tarp material bags) where I want to put a grommet, gives strength to the set.

My tomatoes and peppers outside struggle some even with afternoon tree shade. In the screen house they are lush and vigorous. The Harbor Freight tarps have taken heavy quarter sized hail and a few up to golfball and survived. (I am a volunteer storm spotter and the local NOAA gave me a 'hailometer' ruler with holes to measure the hail fall and use the correct terminology when calling one in).

Thanks Deb and Tim!

That's good to know.