Adsense on website sidebar question

Hi, I was wondering... If you have adsense ads on your website. say in the sidebar. Does it matter how high or low down the page you have the Ad?

Like, do you get revenue based on how long the ad is in the view-able area of the browser? Say you have the ad up high and the visitor immediately scrolls passed it. Is this any different then if the ad is right next to the content.

Im not sure if the question makes any sense. But im not talking about clicks. Im assuming the answer is... it doesnt matter where the ad is as long as it loads somewhere on the page.

or is there a timer that tracks and knows if the ad is in the visitors screen.

As far as I'm aware cpm advertisers pay every time their ad appears, regardless of time. So as long as the visitor allows time for the ad to be served, when scrolling, the advertiser is billed and so you get paid. Whether they view for a second or an hour.

Purely from an ads served perspective, there will be an advantage to having the ad at the top. This will mostly benefit crappy websites that the visitor doesn't stay on for very long. However: with regard to time spent on page and bounce rate I would think this would have a negative impact on SEO