Know the signs of heat stress, heat exhaustion and heat stroke. It can kill. Our activities are for enhancing our health and fitness, not impairing them or worse. Some macho types think that more suffering in the sun, heat and humidity, even withholding from themselves fluids, is going to toughen them to the conditions. That's why short sprint events in hot weather are much less risky than endurance events.
Environmental temps affect the temperature gradient.
See more on humidity below. Our clothes must not hinder the heat-loss mechanism of radiation. We don't want to stop heat from being transferred from our skin surface to the environment. The unfit, and some special populations like children, will not be ready or able to effect the cooling mechanism of adequate perspiration. The obese also will also be less able to enjoy heat-loss, as they wear more naturally insulating adipose tissue like excess clothing.