As I sit here watching my cat scratch, I can confidently say, your dog is probably allergic to what everybody else is, pollen of various sorts.
With cats, when they are allergic to airborne things and foods, they itch. I get her a steroid shot about 4 times a year and in between I had a compound pharmacy do up some transdermal benadryl or chlor-trimaton to keep the scatching to a minimum. There's nothing that can be done if the animal goes outside.
Check with a vet, I had to have my cat tested to see what she was allergic too, it turns out she's allergic to many foods that she had been eating and if it's green and outside, she's allergic to it. I joke that she's the only cat in the world who's allergic to herself.