First, identify which months are the wettest and driest. By looking at the vertical axis, you can see that the bar with the highest amount of rainfall in inches will be the wettest. This bar will be blue. To locate the month with the least amount of rainfall, this will be indicated by a bar that falls under zero and will be the biggest red bar. Using this information, you can subtract the wettest month's value from the driest months value and obtain your result.