The project was started (and still being partly coded by) one of the most successful Czech internet entrepreneurs, currently ~16th wealthiest Czech out there, his fortune estimated at ~$.7bn, so this venture seems to be self funded.

I can dig up an interview I read with him (in Czech), but I can't recall him talking about the financial side of things. He rarely talks to the press, so it's hard to come by this sort of information.

If you're on the webpage, under the top left corner hamburger menu the first item is "Donate". Not sure how they've done this year, their goal listed is $550k but for me it shows 0% which I find hard to believe in December. I blame my browser lockdowns.

They also have apps with in-app purchases to donate as well.

My preferred weather widget on Android (Meteogram Pro) links out to Windy if you tap the top center of the widget, it's very handy.

They crowdsourced something like 500k EUR this year too