MotownWebGuy's post in Popups was marked as the answer
They are called Vignette ads. Its a new ad format that is becoming more common on websites.
Sorry, but they help pay for the expense of running this website.
From Google Adsense:
Vignette ads are full-screen ads that appear between page loads.
Vignettes are displayed when the user leaves a page, rather than when they arrive on one, so the user doesn’t have to wait for them to load.
We (Google) limit the number of vignettes per user to maintain a good user experience.
MotownWebGuy's post in Popups was marked as the answer