In Minnesota, standards call for a maximum Reid vapor pressure (RVP) of 9.0 in winter to aid cold starts, and a maximum RVP of 7.8 in summer to minimize potential vapor lock.

The vapor-pressure deficit was extremely high across the West, he said. That explains part of what’s been so dangerous about this month’s fires: They have grown explosively.

In Denver, the atmospheric pressure is only about 12 pounds per square inch, compared with 14.7 pounds per square inch in Los Angeles. With that much less pressure, you don’t need to apply as much ...