full text of the classic FAA guide
Haze layers not visible from the ground are, at times, of concern at high altitude. These layers are really cirrus clouds with a very low density of ice crystals. Tops of these layers generally are very definite and are at the tropopause. High level haze occurs in stagnant air; it is rare in fresh outbreaks of cold polar air. Cirrus haze is common in Arctic winter. Sometimes ice crystals restrict visibility from the surface to the tropopause.
Visibility in the haze sometimes may be near zero, especially when one is facing the sun. To avoid the poor visibility, climb into the lower stratosphere or descend below the haze. This change may be several thousand feet.