Hiding desktop ads on mobile devices

I’ve been struggling with this for the better part of the week, and haven’t seemed to find a solution through the wordpress drums, so here it goes:

Simply put, I want my horizontal header ad near the top of my website (contentdope.com) to be hidden on mobile devices — it’s obviously to long to be seen properly on a phone. I’ve tried the hidden and ‘display:none’ codes, css and <div>, and I’m obviously too green to understand how to implement the code properly. Please help!