Finding an illegal, indoor growing operation in a big city can be like looking for a needle in a haystack. But Dutch police say there’s one tell-tale sign to look for during the winter months. If you see a house that doesn’t have as much snow as its roof as its neighbors, then it’s probably a marijuana grow op, they say. 

Those roofs tend to be snow-free because the high-powered lamps commonly used for cultivating cannabis indoors throw a lot of heat. So much so that they can literally melt the snow and ice on their rooftop. So authorities in regions where cannabis remains illegal often hunt for illegal grow ops by scanning the skyline for homes that stick out like a sore thumb.

And to illustrate how obvious those grow ops can be, they shared a pic of a home that was caught engaging in cannabis cultivation.