Several factors influence how much energy it takes to maintain normal body temperature. Air temperature, humidity, wind velocity, solar radiation, and precipitation are all factors that affect how much energy is expended to maintain normal temperature for animals that are exposed to these elements.
We know that horses use more calories to stay warm in the winter, as cold weather has been estimated to increase horses' digestible energy (DE) requirements about 2.5% for each degree Celsius below -10° (14° Fahrenheit) per the 2007 National Research Council's Nutrient Requirements of Horses. The “thermal neutral zone” for horses is estimated to be from about 5°C/40°F (lower critical temperature) to 25°C/77°F (upper critical temperature).
It takes horses about 21 days to adjust to a higher or lower ambient temperature, with most of the adjustment taking place in 10 to 14 days and more adjustment taking place over a longer time.
We do not have good horse data for the impact of conditions above the upper critical temperature. If we assume even a 0.5% increase in DE for each degree C above the upper critical temperature (a fairly conservative estimate based on known cold weather changes and other species information), and if the ambient temperature is 35°C/95°F, the horse would need 5% more DE at the higher temperature just to maintain body weight. And as most horses do not necessarily eat more at higher temperatures (some actually consume less food), the higher DE requirement for just maintenance might be expected to produce weight loss just from the extra energy required to keep cool.