Walton has devised his own criteria for named heatwaves in the US, based on duration and extremity, on a one to five scale similar to hurricanes. Heatwave Chevron is classed as a four and is “historic”, Walton said. The meteorologist said he has a list of 20 oil and gas companies – including Exxon and Shell – for upcoming heatwaves and will turn to coal companies if he runs out of names.