You can't hide ads; it's in the AdSense terms and conditions under "5. Prohibited Uses":
You shall not, and shall not authorize or encourage any third party
to: ... (ii) edit, modify, filter, truncate or change the order of the
information contained in any Ad, Link, Ad Unit, Search Result, or
Referral Button, or remove, obscure or minimize any Ad, Link, Ad Unit,
Search Result, or Referral Button in any way without authorization
Moving the ad to the footer area for narrow screens using media queries might be safer.