Scotch mist. When the air is full of droplets so small and fine that they don't fall at all, just hang there. So it doesn't appear to be raining, but you will be soaked to the skin in ten minutes. An umbrella can't help you, nothing is falling. Only full top and bottom rain gear will keep you dry. But your face will still gather the water you walk through, blinding you, and then it runs down your neck. At camp, the tarp is just as effective as the umbrella, i.e. not at all, no matter how low it's pitched. Because the air is full of water, and where there's air, there's wet. You can see your breath at 20 Celsius.
This is why I own nothing made of down.