Observing nights would be quiet sessions of laboratory measurement if the atmosphere did not severely limit both the quality and quantity of the performance to be expected from a night. The perturbations created by the atmosphere are called seeing. This is the principal obstacle to observation, but it is enough. Seeing…acts at two levels: at high altitudes as a result of strong winds in the upper levels of the atmosphere, and near the ground as a result of convection currents caused by the exchange of warm and cold air. The astronomer cannot do anything to diminish to high-altitude seeing, but he can act at ground level, particularly by creating air currents in the dome.

It is possible to know in advance, during the day, by observing natural phenomena, whether or not the night will be good. Total absence of wind is a good sign. If the leaves stay still at the ends of branches, even eucalyptus leaves that flutter in the slightest breeze, one can hope for good images. At Nice, the sea provides an excellent criterion: It should show large flat surfaces resembling those of oil, elongated like rivers; no white breakers on the shore; and a barely visible horizon. The sky ought not to be too blue at the zenith. If the smell of vegetation stays near the ground, and the smoke from house chimneys rises only slightly, or even falls back again toward the ground, the chances are that the images will be good. Of course, one can look at a star through the telescope in full daylight to test the quality of the sky, although daytime seeing differs from that at night. It is known that, in principle, the planets do not twinkle; but if the seeing is bad, Venus, Jupiter, Mars, and Saturn twinkle very slightly. Their twinkling is particularly noticeable to a short-sighted person who takes off his glasses. If he sees no twinkling when he has done that, that is a certain sign of good seeing.

The slit of the dome should be opened an hour before observing begins, and the instrument put in a nearly vertical position with the valve open. Long refractors are like balloons: They trap warm air, which rises and stays behind the objective. The air is cooled again by the tube and the objective, and forms slow eddies. This effect is particularly marked in the 18-m-long great refractor at Nice. It is one of the reasons why the tubes of refractors should be kept horizontal during the day. For the same reason, one should avoid letting the slits and sides of domes receive heat from the setting sun. It is good to ventilate the room before observing, in order to expel all the warm air. Avoid having too many people under the dome before observing; their body heat will not create a favorable environment for good seeing.

Danjon [French astronomer 1890-1967] advised sprinkling domes in order to cool them; this is not always possible. Plenty of grass kept well watered around a dome, and copses of shrubs, moderate the heat exchange. There should be no parking lots near the dome opening, as cars are sources of heat. In general, the natural vegetation of the site should be protected. Deforestation and too many buildings favor the creating of ascending currents of hot air at the beginning of the night. On mountain summits and high plateaus the image quality is better than at the sides. Double-star observation requires a still atmosphere; a light haze is not a nuisance. Strong winds, which clear the air and give a deep blue sky, are sources of bad seeing.