Stealth And Detection In The Desert
In general, the maximum distance in desert terrain at which a creature can succeed at a Perception check to detect the presence of others is 6d6×20 feet; beyond this distance, elevation changes and heat distortion in warm deserts makes sight-based Perception checks impossible. The presence of dunes in sandy deserts limits spotting distance to 6d6×10 feet. The scarcity of undergrowth or other elements that offer concealment or cover makes using Stealth more difficult.
Source
Core Rulebook pg. 397
