Once again, mobile is the thing to optimise for, especially as it represents such a large part of the market.
Ad viewability is defined by the Media Rating Council and IAB standards and is counted when a minimum of 50% of the ad falls within the viewable space on a user's screen, and for one second or more.
There's a nice interactive viewability page to demonstrate more effectively of what's meant.
Here are four tips on ad viewability from Google Replace 320x50 ad units with 320x100 as revenue per thousand impressions (RPM) tends to rise when using large mobile banner ads, with the best practice putting the ad just above the fold.
Use 300x250 ads as they were devised to fit most mobile screens, and it's an advertiser preferred size, increasing the fill rate. Placed just below the fold could generate an approximate 50% viewability rate.
Reduce accidental clicks by moving ads at least 150 pixels away from content to improve user experience and decrease spam. 150 pixels is a starting point, so you should test it.
Use anchor ads and vignettes page-level ads that are designed for mobile devices. Anchor ads stay on the bottom of the page ass a user scrolls.
Also, there was an earlier post on viewability with an infographic Google AdSense: Four Ways to Improve Ad Layout Viewability [webmasterworld.com]