A question that comes up every summer is “How do horses deal with heat?” Most (European) riders are anxious, and I used to be one of them. But after spending time in Australia, I’m a lot more relaxed.
As a general rule, horses deal with heat better than people do. Dry heat is no problem at all, if anything, humidity is. In a European context, weather should not be an issue for healthy horses.
Of course there are things you can do when it gets hot. Either make sure there is shade on the pasture to give your horses the option to seek shelter from the sun, or don’t leave them out too long. If you have the chance, turn them out over night. Make sure they have access to water at all times. Don’t put boots on. It gets crazy hot underneath and the risk of damage to the tissues is bigger than a potential injury.
We had a very hot day earlier this week and it was actually hotter in the stable than it was outside, so I turned my horses out all afternoon even though there was no shade. At least outside there was some wind, while the stable was stuffy and the air was horrible. The fact that the horses that can access a paddock from their stables chose to stand outside in the sun (Snoopy was one of them), instead of staying inside, was a good indicator regarding what to do.
You can still work your horse when it’s hot, you simply adjust to the conditions. Work them early or late when it’s cool. Or don’t work them as long as you usually do, maybe half an hour instead of a full hour. Include lots of rest phases where you work them in walk. Another option is to simply do something less intense, like go for a slow hack or lunge them. Give them a good shower afterward until they have cooled down.
Overall, don’t obsess too much. As I’ve said earlier, horses deal with extreme weather conditions a lot better than humans do.