Natural air conditioning, involving heating with solar energy and summer cooling to the night sky, is accomplished with ceiling ponds and movable insulation. Pertinent international aspects of climate, economies, mores and needs are related to industrialized and to developing countries. Tested throughout 15 months, in Phoenix, Arizona, when temperatures varied from below freezing to 115°F days, extensive data were obtained for very hot-dry and hot-humid climates. The Sky Therm system met these weather types with six means for modulating ambient conditions: solar heating, heat capacity, nocturnal radiation, water evaporation; radiation, evaporation and fan-coil operation; and evaporation, fan-coil and pond-blower use. Typical results are presented with a chart indicating that average daily room temperatures between 70° and 80°F can be maintained throughout a normal year, in Phoenix, without supplementary heating or cooling. During low dewpoint periods, water evaporation is not required until maximum temperatures exceed 100°; until 105° is exceeded, no electricity is needed. With dewpoints above 50° and dry-bulb maxima over 100°, both evaporation and electricity were used. In most developing countries nearer the equator, ample heating with horizontal pond collectors can be expected at normal altitudes. Acclimatization of people to somewhat higher summer temperatures may make the use of electricity and water evaporation unnecessary. The required plastics are being increasingly produced in industrializing countries, but cost may restrict use of the new method to buildings of higher priority than those to which the system would be applied in industrialized countries. Dual effects obtainable simultaneously with the same movable insulation include keeping one building or room warm and another one cold. The efficiency of a solar water-heater (or a solar still) can be improved by covering it at night with insulation moved from over a container in which water is cooled (or frozen) by nocturnal radiation; this water is maintained cold throughout the day when the insulation is positioned over it.