What is this "crystal ball" located at a meteorological station?
It's a Campbell-Stokes sunshine recorder, used to record the times at which the sun is shining. It acts as a lens, focusing the sunlight onto a piece of card. If the sun is shining, the focused beam will burn a hole through the card at that point.

Here's a closer view of a sunshine recorder in Wendelstein, Bavaria:

Source: Wikimedia Commons

And here's a used recording card with holes burned through it:

Source: Wikimedia Commons