  Convection in a thin layer of liquid (gas) with temperature dependent
viscosity between poorly heat conducting boundaries is studied within framework
of the Proctor-Sivashinsky model. This model is examined in order to study both
the flow pattern formation and the second-order structural phase transitions as
between patterns with translational invariance as well as between structures
with broken translational invariance but keeping a long-range order. The
spatial spectrum of arising patterns and estimation of their visual
defectiveness are analyzed. The relation between the density of pattern defects
and spectral characteristics of the pattern is found. We also discuss the noise
effects on the formation of pattern defects. The influence of temperature
dependence of viscosity on the process of pattern formation and structure
transformations is also discussed. It is shown that the temperature dependence
of viscosity inhibits structural transition from regular rolls to square cells.
