Extreme Temperature Changes
You may experience hives in response to external stimuli, such as a change in temperature. [9] If the temperature of your surroundings becomes too hot or too cold, your body may release the chemicals that cause this rash into your bloodstream. Sometimes this temperature change need not be external. High body temperatures also trigger hives in certain people. [10]
Sunlight Exposure
It is not very common, but some people develop urticaria on exposure to sunlight. Known as solar urticaria or sun allergy, this rare condition causes hives to appear on the skin within minutes of exposure to sunlight.