This is a picture of my son, who came home with itchy back and a raised rash, immediately I wrote his name (ALI) on his back above the urticarial rash with my finger tip (nail).
The picture was taken about 20 minutes after. It is Physical Urticaria due to heat because he is complaining of itchiness when exposed to hot weather where temperature reaches 43 degrees Celsius at night in Kuwait these days. He got used to the cold weather in Nebraska (USA) where he is studying and where temperature is around 20 degrees Celsius below zero most of the year (according to him).
Next day the rash disappeared completely (he took one tablet of antihistamine) and he has a clear back (following picture).