Dear Editor: Forest fires in the West are burning with higher intensity than ever before due to rapid heating of the planet. Climate scientists have discovered that a big factor in this is the vaporpressure deficit (VPD). VPD is the difference between the actual amount of moisture in the air and the amount of moisture the air could hold if it were saturated. When the difference is large the air pulls more water out of trees and other plants, causing the forest, sage and grasslands to get tinder dry. Climate change increases VPD because warmer air holds more moisture.