Google just rolled out an update to its Above Fold Page Layout algorithm (or "Top Heavy" update) that was launched in January this year, and is designed to reward sites with content above fold rather than ads.
Cutts tweeted "weather report" calling it "minor," and said it affects less than 0.7 percent of English language queries are affected --"Minor weather report: Update of goo.gl/OpIDL launching today. ~0.7% of English queries noticeably affected," Cutts tweeted.
Cutts' also included a link to the Google's launch post about the Page Layout algorithm, which explains:
"We've heard complaints from users that if they click on a result and it's difficult to find the actual content, they aren't happy with the experience. Rather than scrolling down the page past a slew of ads, users want to see content right away. So sites that don't have much content "above-the-fold" can be affected by this change.
If you click on a website and the part of the website you see first either doesn't have a lot of visible content above-the-fold or dedicates a large fraction of the site's initial screen real estate to ads, that's not a very good user experience. Such sites may not rank as highly going forward."
If you believe your site has been affected, Google confirm that webmasters can recover from this update by having their site modified and recrawled.
In the Jan post, Google advised, "If you decide to update your page layout, the page layout algorithm will automatically reflect the changes as we re-crawl and process enough pages from your site to assess the changes. How long that takes will depend on several factors, including the number of pages on your site and how efficiently Googlebot can crawl the content. On a typical website, it can take several weeks for Googlebot to crawl and process enough pages to reflect layout changes on the site."
Google also stated back in January that webmasters can use their Browser Size tool (now replaced with Browser-Size Analysis tool in Google Analytics) to see how much content vs. ads shows up above the fold.