I wrote about volatility clusters in earlier posts. Another phenomenon, which I observed in the last years is what I call "volatility pre clustering". A fancy term for a simple pattern: prior to intermediate term market tops, volatility started to rise for two or three weeks while prices turned sideways. Take a look at the following chart:
I measured short-term volatility using a 5 day average true range (ATR) of the daily moves. Note how ATR started to rise before major sell-offs in January and April 2010. It is almost like a car hitting a bumpy road before falling off the cliff.
This is probably not an exclusive structure, so declines can happen without pre clustering as well. However, its occurrence would send a strong warning sign. As you can see from the following chart, volatility hasn't risen so far, so no red flag from this pattern: