you are most likely getting urticaria (hives). hives can be caused by the temperature change that you mentioned. you can try taking a long-acting antihistamine before these warm season exposures (loritidine zyrtec allegra) to see if it helps. urticaria caused by temperature changes may not be prevented by antihistamines however but it is worth a try. keeping your skin surface cooler would also seem to be a reasonable preventative; wearing clothing that "breathes".
during work out or a hot a summer day temporary red vesicles appears on my skin and they itches until disappears. why? . basically whenever my body temperature goes up i get several red vesicles/rash like thing on my arm back neck and some other part of my body. doesn't have to be exposed to sun it just happens. how can i prevent it?