While in the SEKI area, potentially spectacular views are often obscured and signs at some viewpoints explain the cause. This article cites air pollution data pointing to SEKI having the most polluted air of any national park in the U.S.: 
Several mysteries are explained in one statement:
Southerly breezes from the San Francisco Bay collect pollutants and push them to the valley's southern rim, where they bounce back north. Between Fresno and Visalia -- just below the park -- the warm air hits the cooler air pushing south and gets trapped in a swirling vortex called the Fresno Eddy...
According to some sources, Fresno is not the only vile vortex to be wary of: