In my experience (IME), once the nights stay above a certain temp (more important then day temps - IME), the pollen stops flowing. Then, the flowers fail to develop fruit until the temps drop, and again the pollen flows.

In my garden, we seem to skip spring...the days get hot, but the nights stay cool. I have fruit set until the nights stay warmer.

I have extended the season by planting early (with protection - wall o waters), planting deeply and mulching heavily. It seems to keep the ground cooler for a bit.

New crop varieties have been developed that are heat tolerant. These varieties perform better in the heat because they possess more heat shock proteins (hsp).

A few varieties such as:
Amelia
Atkinson
Cosmonaut Volkov
Georgia Steak
Marion
Ruby

Here is a link that might be useful: Best Juicy Tomatoes

Fourth of July Hybrid never stopped setting for me. We went to triple digit August 1 and pretty much stayed there for six weeks. It is still setting and my nights are now in the 30s.

It's all about the night temps.

Agree that once air temps, and perhaps even more so the humidity reach a certain point, it will make little difference what varieties you plant. The pollen will be tacky and non-viable. It is just the nature of tomatoes.

Daytime temps can be moderated somewhat by providing shade for the plants and the humidity in and around the plants can be modified to a small degree with heavy mulching and careful watering. But the benefits are small. And unless you grow in an environmentally controlled greenhouse you have no control over the night air temps.

The so-called heat-tolerant varieties may help to a small degree but even they are not immune to the effects of humidity and none are noted for their taste anyway.

Like austinnhanasmom said above, many gardeners have found they get the best results by planting much earlier than what was once considered "normal planting time" under cover/protection as needed. That way you get the benefits of lots of fruit set before the heavy heat moves in.

It works for me.

Dave

Here is a link that might be useful: Blossom drop FAQ

In my experience, beefsteaks are hopeless in extreme heat. Round tomatoes have been more productive for me, and cherries seem way less hampered by the heat.

Some of my better-producing, tasty round tomatoes here in hell are: Burgundy Traveler, Prue, and Thessaloniki. My best bet for beefsteak tomatoes from the sauna is Mule Team.

Here is a list of varieties that have had good fruit set in high temps from a grower in Alabama who has this problem every year. Ami

Stump of the World
Kosovo
Bill's Berkley Pink
Neves Azorean Red
Cabernet (hybrid)
JDs Special C Tex
Black Krim
Indian Stripe
Marianna's Peace

I've found that there is a certain time each morning when the nightly dew has dried and sun's heat is still not raging. Around here, it's usually around 10 AM. During heat waves, this is when I go thru the garden shaking blossoms. I can't be totally sure, but I think it helps. My theory is to catch the pollen just as it dries out and just before the temperature gets out of control.

The other benefit is that I get a good look at what's going on with each plant (critters, diseases, etc.) Now, I do realize that I can do this any time I want to because I'm fully retired.

For those that have heat problems frequesntly, I suggest you arrange for some kind of shade, as mentioned above.

Ted