We present an extension of the Garman-Klass volatility estimator that also takes into consideration overnight jumps. Garman-Klass-Yang-Zhang (GKYZ) volatility estimator consists of using the returns of open, high, low, and closing prices in its calculation. It also uses the previous day’s closing price.
http://tech.harbourfronts.com/trading/garman-klass-yang-zhang-historical-volatility-calculation-volatility-analysis-python/