Question

Why front portion of a wind vane has a smaller surface area than its rear portion

Answer 1

So that it can show deflection even with a little wind because when the surface area is reduced than the impact force . here by the wind increases and hence lets us know the reading with precision.

Hope u got that

Answer 2

Because by making the front portion smaller the rear portion has motors which convert air into electricity. Therefore the air get passes through front portion . Therefore the front portion are made smaller.