[Formendacil]

Perhaps this may be a tad too serious an answer for such a question, but I think we've seen the start of the correct answer in saying "it is Melkor's evil at work in the world".

To expand on that, if one reads the essays in HoME X Morgoth's Ring, I believe it is in the fourth section (the one that deals specifically with various Melkor Morgoth related topics), one will find the writings that gave the book its name.

Unfortunately, I don't have my copy at home with me, but it's back up at college, but I can recap:

Basically, in the same way that Sauron disseminated his power into the Ring, and used it to control things (specifically, the other Rings), Morgoth disseminated his power on a much broader scale: throughout all the physical matter of Arda. Tolkien adds that nothing (possibly excepting Aman) is free of the "taint" of Morgoth. Some parts of matter are more tainted than others; gold, for example, is a very strongly tainted element, hence why Sauron used it to make the Ring, as opposed to silver, but all matter has some Morgoth-element in it.

In the same way that the Sauron-element in the Ring gave it a "consciousness", so to speak, which we see manifested in the way it seeks to return to him, and betray its wearer, it is logical to assume that the Melkor-element in Arda is similarly still working towards his goal. And that goal, ultimately, we are also told in the same section of HoME X, is "the destruction and annihilation of Arda". Morgoth is unable to ever create or control all things, for he is not Eru, and as he becomes more evil, he becomes more blindly destructive.

In this light, I think it is very easy to see hurricanes, earthquakes, or the like as the manifestations of the Melkor-element in Arda blindly raging in destruction.

Sorry about no direct quotes, but my set of the HoME is in Edmonton, and I'm three hours away on a three week holiday at home.