Air Check Values and Stacking, from the FAQ (Comment / Question)
Q. What is the Air Check value of clear (countryside, desert and winter) terrain? A. Zero.
Q. Do Air Check values stemming from underlying terrain accumulate with obstacles? A. No. Only the greater of these Air Check value applies. So a sandbag position over clear countryside hex will have an Air Check value of 1 (for the Sandbag), while a sandbag position over a forest hex will have an Air Check value of 2 (due to the Forest). The Air Check value may still be increased by the presence of adjacent enemy units, however.