The chart below represents the difference between the high and low of each 30 minute period (9:30-10:00, 10:00-10:30, etc.) divided by the opening price of that time period. The average range of each 30 minute period over the entire year was 0.79% As you might expect, the final thirty minutes of trading was the most volatile with a range of 1.25%

The average daily range of the SPY in 2008 was 2.89%, well ahead of the 11 year average of 1.77%