It's really difficult to decide when you should and shouldn't work your horse. Many people believe you shouldn't ride if it's "way too hot" or "way too cold." I had a vet criticize me once for working my horse when it was around 10 degrees outside saying it can be damaging to their airways. After thinking about her statement, I had to agree with her. When we exercise outside in very cold temperatures, we have the ability to cover our mouths with a scarf which assists in warming and humidifying the cold, dry air we are breathing in. With that knowledge, I consider my horses' respiratory rates when I'm working during frigid winter temperatures. Obviously, their upper airways are much larger than ours, so there is more opportunity for the breathed in air to warm and humidify before entering the lower airwars. There is less chance of bronchospasm than for us, but the danger is still present. When I work them, I give very frequent breaks and do lots of walk work. I used to think I needed to keep them working to stay warm, but I've changed that, and instead, use blankets and keep them bundled while I ride (for me, too).
But what about when it's REALLY hot, like the last couple days? Unfortunately, we may have to compete in this weather. What is the best plan for working when the heat index is 90+ degrees and there are heat advisories? If I was at a show, I likely wouldn't scratch, so I figure I'd better know how my horses will handle the temperatures and how I will fair under the same conditions.
What's my strategy for preparing to ride when it's really hot? First, make sure the horse is comfortable. I find they are generally itchy this time of year. I groom really well and make sure all the itchy parts are addressed. I use tons and tons of fly spray. I have my preferences, but everyone likes different things, so use what you like. I don't sponge them off beforehand. I leave them cool and dry to start the work. If they are already sweaty from being outside, I may sponge their chest and neck, but I don't wet the back. I don't wrap the legs, preferring they stay cooler. If there is a real need for wraps, I choose leather, fleece lined boots over polos. I've found polos hold the sweat and shift too much for my comfort level when it's really hot outside. I also put sunscreen on white noses.
I cover myself in sunscreen. I make sure I am well hydrated by drinking lots of gatorade and water. I wear light cotton t-shirts rather than fancy Joules riding shirts and vests, opting for comfort over fashion (well, I always do that). I fill my boots with baby powder so they're a little easier to take off and stay fresher. For those who get "SwAss," I can't recommend "Anti Monkey Butt" enough. If you're riding in leather seated breeches, I'd just apply it rather than waiting for the chaffing to start.
What I can't tell my students enough is that when it's really hot, you need to be very efficient with your training. I keep my warm-up very short and to the point, using it to test reactions and fine tune aids. I have a plan for what we will work on for the day. Get right to the training piece and work to achieve exact reactions immediately. I use lines that cut the arena short if I'm working on lateral movements so I can repeat movements quickly rather than going from one side to the other and wasting valuable energy. I get as much done as I possibly can in 20 minutes, no longer. That way, when I go to a show and it's really hot, I know I can get the job done and not waste my horse in the heat.
After working, untack very quickly and get them in the shower. I hose for almost as long as I ride, making sure to really chill the legs. Scrape water off as you hose to keep the warming water away from the body. Whether I turn out or leave in the stall depends on the time of day, location of turnout and the horse's temperament.
Be sure to give electrolytes. Fran gives Boomer a bucket of Gatorade after each workout and he sucks it right down. It's also a nice treat! Offer free choice salt and added electrolytes in a meal.
Hope this helps!