The Republican period is shown as maroon, while the united Empire is purple. The Eastern and Western Roman Empires are represented as wholly distinct from 405 AD onwards, with the West as blue and the East as green. At the final phase there is no blue on the map at all.Via
Science in GIFs: A Mesocyclone Supercell at West Point Nebraska
Why is this not a tornado?
Technically, a “tornado” is a rotating vortex cloud that touches the ground. Needs a funnel first. We’re looking at a “mesocyclone” which is the rapidly rotating core of a supercell from which tornadoes can form. Now, a tornado could form from this storm at any time, so it would have a tornado warning attached to it. These kinds of rotation are what is seen on radar.
Science in GIFs: Refraction of light through a glass of water
There’s more than refraction to the Reverse arrow illusion, however.
Just because light bends when it travels through different materials, doesn’t explain why the arrow reverses itself. To explain this, you must think about the glass of water as if it is a magnifying glass. When light goes through a magnifying glass the light bends toward the center. Where the light all comes together is called the focal point, but beyond the focal point the image appears to reverse because the light rays that were bent pass each other and the light that was on the right side is now on the left and the left on the right, which makes the arrow appear to be reversed. Via