Horses are built better for dealing with cold weather than with hot weather. In winter we know that if it isn't too cold for us, it isn't too cold for them.
But the same thing does not apply in hot weather.
Horses have far less skin surface area relative to their large body mass than humans do. Since internal heat is mostly dissipated through blood brought to the skin surface when superficial vessels dilate, and since horses have less skin surface per pound of body mass to be cooled, they are less efficient at cooling down, and can more easily suffer heat stress and heat stroke.
Horses also lose more salt through sweating than humans do, since our sweat is isotonic and theirs is hypertonic. The major salt lost in their sweat is sodium chloride, with significant potassium chloride loss also common, and those should be provided when horses are sweating heavily. Light salt for humans combining sodium chloride with potassium chloride can be provided in loose form along with keeping a white salt block (sodium chloride) available at all times next to the water trough. Or, one of the equine electrolyte supplements which also contain calcium and magnesium can be provided free choice. Always be sure there is plenty of fresh drinking water available at all times, especially if you are adding electrolytes in their feed
The drought and lower humidity benefits thermoregulation by allowing more rapid evaporation of sweat, which speeds removal of heat at the body surface. More heat is carried off with the sweat as it turns to vapor. When air is already saturated by humidity, sweat can't evaporate and is much less effective as a cooling mechanism. If you can provide fans to move the air, it will promote evaporation and cooling.
Superficial blood vessels dilate to bring more blood to the body surface for cooling by radiation, and cold hosing rapidly cools the blood being brought to the body surface, which cools the body core as that blood is recirculated.
If you use a sweat scraper to scrape the water away as it absorbs heat and then add more cold water repeatedly you can significantly speed the cooling process.
If you do ride on these hot days, you should measure rectal temperature each morning to establish his baseline, and then measure it after you ride or anytime you suspect he may be stressed by the heat. Rule of thumb dictates that if his rectal temperature rises to over 103 F, you should get him into the shade and begin cold hosing immediately and keep cooling his whole body until the temperature drops below 101 F.
Offer cool drinking water during rides and immediately after a workout. Thirst can shut down in a dehydrated or overheated horse within minutes, so if you wait too long, the horse may refuse to drink.
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Add........................ Allowing horses to drip dry significantly slows the cooling process as the water that has absorbed the heat is not rapidly evaporated away from the skin. The effect is similar to that of soaking in a tub of warm water..It is always best to scrape warm water away as you add more cold water, so if the horse is already overheated, the quickest way to remedy it includes scraping the accumulated warmed water away.