Turbulence is a bad help for reaching distant mountains in a picture, and happens as different layers of air masses move along the atmosphere.
Under calm conditions, like when an strong anticyclone is affecting the area, air masses tend to be calm and lay without mixing, then allowing light to travel more efficiently.
This effect also causes fog in low levels of the atmosphere, a helpful situation as humidity and pollution will be kept there, extreme-cleaning air above.
An example of fog created under high pressure systems: Air above is extremely clean
As stated in previous chapters, the best conditions we have ever experienced have been under very high pressure conditions.
Good conditions can be achieved as well before a warm front, when hot air starts to get in in atmosphere’s higher levels, maintaining air masses calmed below and decreasing turbulence.
When air masses mix due to difference in temperature and density it creates upstreams that will blurry our images.
This effect can be due to:
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A low pressure system affecting the area
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Surface heat creating convection (specially under mid day)
In the image below, taken just before a cold front arrived with great amount of turbulence, we can see how the mountains are still visible, but not as clear as they would be if the atmosphere had been calmed.
Aneto (3.404 m. / 205 km.) as seen from Monte Caro (1.441 m.) under not so good conditions
Another good example of how turbulence can mess up a picture is this image, when the edges of the building are not straight but blurry.
Madrid (670 m. / 45 km.) as seen from Bola del Mundo (2.265 m.)
For more information:
Beyond Horizons 
