What atmospheric condition is commonly associated with improved VHF propagation?

The atmospheric condition commonly associated with improved VHF propagation is temperature inversions. Temperature inversions occur when a layer of warmer air traps cooler air near the ground. This phenomenon can create a stable atmosphere that reduces turbulence and allows VHF signals to travel further than they normally would. Under these conditions, radio waves can reflect off the warmer layer of air and effectively "skip" over longer distances, leading to enhanced communication range.

Other conditions listed in the options, such as high humidity or high-pressure systems, do not specifically enhance VHF propagation in the same way. High humidity may affect the propagation characteristics of certain frequencies, but it is not a primary driver of improved VHF conditions. Stormy weather, on the other hand, often leads to increased turbulence and atmospheric disturbances, which are typically detrimental to radio wave propagation.