Smoke from forest fires can prevent clouds from forming raindrops. Soot particles in the smoke cause many small water droplets to condense, preventing the buildup of large droplets. Small droplets remain suspended in the atmosphere instead of falling as rain.

The image above, taken over the Indonesian island of Borneo, illustrates this phenomenon from two instruments aboard the Tropical Rainfall Measuring Mission (TRMM). The Visible and Infrared Sensor (VIRS) detects fires and shows the extent of clouds and smoke, while the Precipitation Radar (PR) detects cloud water droplet size and rainfall.

Smoke from the fires (red dots) in the upper right combined with clouds in the center of the image. TRMM's precipitation radar showed the small size (darker teal) of the water droplets suspended in these clouds compared to the larger (brighter teal) droplets in the clean clouds to the left. Only the clean clouds produced significant rainfall (blue).

Learn more about the effects of particles on clouds in Every Cloud had a Filthy Lining.
And the Tropical Rainfall Measuring Mission in the TRMM Fact Sheet

Image by Greg Shirah, NASA Goddard Space Flight Center Scientific Visualization Studio