Where are you located? Both HD and Lowe's here sell shade cloth by the yard. Even Walmart carries several things in the fabric department that will work. And you can order it online from any of the greenhouse supply suppliers.

Or you can just use an old sheet, even a small tarp. The advantage to the shade cloth is it still lets air and water through while still creating a somewhat cooler air zone between the plants and the sun exposure.

Shade cloth comes in various %of shade, % of sun allowed. What % you need again depends on location. The higher the % of blocked sun the greater the temp reduction. Each 20% increase in shade equals approx. 5 degrees in temp reduction (depending on winds and humidity) but keep in mind you are also blocking the sunlight needed by the plants so you have to strike a balance.

Google Images pulls up lots of pics of how to rig if you Google 'garden shade structures'.

Dave

I am in Pennsylvania (5B) and have about one month, July or August, where air temp in the garden well exceeds 90 degrees for about 4 hours a day for maybe 4 or 5 days in a row. I would like to reduce the temperature around my tomatoes to below 85 degrees.

90F, for 4 hours is not that bad, IMO. I wish I had that kind of climate. So what you need just a small reduction of direct sun.

But remember, that when they say "high will be 90F" that is in the shade with adequate air flow. So by shading you cannot lower the air temperature but prevent if from getting higher than 90F.

Tell me again the relationship between temperature and humidity relative to tomatoes. I would guess the tomatoes do less transpiration if the humidity is higher. And thus can stand the higher temperature a little better?

Relative Humidity (not absolute humidity) relates to ability of a wet/moist body to loose moisture and cool off (wet bulb effect). When RH is high, it becomes harder for the water molecules to go from liquid state to vapor form, where it takes some heat away with it in the process. In humans that causes a discomfort in high temperature , because we cannot cool off. We sweat to get our cooling system going. But when RH gets high that system of cooling is partially disabled.

I don't know how this principle works in plants words. But I know that hot dry air/winds ( Like Santa Anna) can dehydrate plants ( by loosing too much moisture in a very low RH) .
Obviously, there is an optimum middle ground that plants can benefit most. Any extreme is not desirable.

Carolyn gave an explanation a few posts down - heat (over 95) and excessive humidity (over 80%) cause blossom drop and "clumpy" pollen.

I've also linked an article (with a more detailed explanation) from U of DE.

Here is a link that might be useful: U of DE Tomato pollination/weather article

Lets say relative humidity is normal and the wind is not blowing. The air temperature is 95 degrees and the sun is beating down on the tomato plant. There is adequate moisture in the soil. The soil temperature is say 75 degrees. Is transpiration cooling the plant or keeping the plant temperature down some?

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Plants can cope with that for a while fine. But may not set fruits. Then it would depend on how many hours that condition will continue. Normally 95F high lasts just for a few hours (that is the peak). But sun can shine on all day. supplying more heat beyond 95F of surrounding air. Plants have ability to cool off by transpiration as humans do. But we are looking for a favorable condition. Thus comes the shading, reducing hours of direct sun. Here is an argument "for" limited direct sun. Plants don't need all that ( like 10 ++ hours of direct sun) for photosynthesis. In cool climate it can be beneficial as a heat source but in hot climate it can be detrimental.