The market's so-called fear index is ticking up as we enter what's historically been the rockiest month of the year for stocks.

The CBOE Volatility Index, known as the VIX, spiked as much as 9.5 percent on Tuesday near a session high of 14.35. The index, which is used to measure expected volatility in the stock market over the next 30 days, was close to its highest level since August 16. A low reading means relative calm in the markets while a higher number shows more uncertainty.

September has been the worst performing month of the year for the Dow Jones Industrial Average and the S&P 500 since 1950, the worst for the Nasdaq since 1971, and the toughest for the Russell 1000 and Russell 2000 since 1979, according to data from "Stock Trader's Almanac."

In a note to clients this week, technical analyst Andrew Thrasher also pointed to the seasonal downturn, and said September "has historically seen more weakness in mid-term election years than non-mid-term years."