Forget satellites and balloons - this is how you tell the weather accurately!

It's a common enough reason why people like geocaching: "It takes me to interesting locations". Well I was only intending to nab Rickmansworth Weatherstone because it was in Hertfordshire, a county I don't think I've ever been to before, let alone cached in, and I read the cache description thinking it was one of those "here's an interesting nugget of history for you" types. If they're longer than two paragraphs, I don't tend to read them, to be honest, but this cache seemed to be a bit elusive. What was so special about this "weatherstone" that makes it very accurate? How does it work? The plaque will explain, the cache description promises. 

So I went to the location, and found the stone:

 

And how on earth does this work? What does the stone do? If it's so perfectly accurate at judging the weather, why does the Met Office not have one? Perhaps because it's less of a predictor and more of an observer:

Even so, I love it!