Small itchy bumps on the arm in hot weather may be caused by cholinergic urticaria, polymorphic solar rash, eczema and other diseases. 1. Cholinergic urticaria: cholinergic urticaria is mainly associated with exercise, alcohol, spicy food, etc. The hot weather is also a trigger, patients will appear small but extremely itchy or even stinging wind, mainly involving the trunk and limbs. 2. polymorphic solar rash: polymorphic solar rash is currently considered to be a variety of causes of delayed photomorphic reaction, often in the sun 2 hours ~ 5 days after the local skin burning sensation or itching, damage to the erythema, pimples, nodules, etc., occurring in the exposed parts of the body, such as arms and other places. 3. Eczema: Eczema symptoms manifested as erythema, edema, papules, accompanied by obvious itching. Etiology and genetic, immune abnormalities, hormone level changes, etc., too hot or dry environment will also aggravate eczema will induce eczema, so the day heat arm pimples are very itchy, consider eczema caused. Itchy bumps on the arm in the heat of the day has more causes, it is recommended to consult a doctor for examination and a clear diagnosis.