Moire Christmas
I was sitting in a meeting this afternoon and suddenly there was a bit of a swoosh and a pile of snow flew past the window. This is a common occurrence in Norway when the snow slips off the roofs. If you visit a city you will see signs warning that you are going to be hit on the head by snow falling off the roof. Its an interesting projectile problem when it happens, the first bit isn't going very fast so drops vertically, as the sliding snow picks up speed it falls further and further from the wall but not very far since the angle of the roof means that the horizontal component of velocity isn't that big. One of my students wrote an EE on it some years ago attempting to find a function for the distribution of snow.
I took this picture to illustrate the blog entry but when I made the image smaller an even more interesting phenomenon was revealed. Can you see the pattern on the wall in the middle of the photo? The wall actually is made of vertical planks. I think this is a sort of Moiré fringe due to the slight angle between the rows (or columns) of pixels in the camera and the planks. I will have to experiment with different camera angles to see if there is any change.