There are several different atmospheric causes, of imperfections in the images seen in a telescope. The most serious are due to thermal convection from the ground, or the stirring of a stable layer at the ground by wind : choice of site and observatory design can reduce these effects. In the free air well away from the ground some of the largest inhomogeneities may be due essentially to large gradients of humidity, and certain cloud types indicate the mechanisms by which such inhomogeneities are established. Temperature fluctuations caused by pressure fluctuations in eddies close to the telescope are probably not serious, and can be avoided by observatory design.