The thing about weather apps is that the more often you use it, the more it costs the developer to supply data to the app. And it costs even more for an app like Weather Up that puts the data-intensive maps at the forefront (there’s a reason lots of weather apps hide the maps).
Ads are seen every time the app is opened, so as long as the ads pay a bit more than the data costs, we can make a little money. We are a business after all. If we offer a one-time purchase to disable ads (and enable all the great PRO features), data costs would slowly eat away at the profit every time the app is opened. We’re in this for the long haul and think 50¢ a month (or $5/year) is a reasonable price for users and profitable enough for us to not only maintain the app, but keep improving it.