Welcome to Health Care Magic.
Based on the description given, you might b suffering from Solar Urticaria
It is a type of physical Urticaria occurs commonly due to exposure to UV rays and sometimes even visible sun light ( Photoallergens).
can be present in all kinds of weathers. It depends on the area where you reside.
If you are living in an area with less ozone layer covering or an area of high altitude, then the chances of UV radiation is high.
Apart from the environmental factors some people have skin sensitive to UV rays.
develops with in a matter of minutes and would disappear once you are indoor ( it might take 10 mins to few hrs). Rash is commonly seen on exposed parts that is why you are experiencing rashes on face and neck.
There are various modalities of treatments like antihistaminics, desensitisation, immuno suppression etc.,,
Please see a dermatologist
and take his advice on the type of treatment that suits you.
Wish you a healthy life