Abstract: An auxiliary air cooling unit cools and dehumidifies air entering an air-conditioned building. The auxiliary unit operates in response to increased temperature and/or moisture content of the outside air to prevent air of undesirably high moisture content from entering the building. As the auxiliary unit operates selectively during periods of increased cooling load, it avoids or reduces problems of over-cooling of the air in the building by the main air-conditioning unit and reduces the need for periods of inefficient reduced-power operation of the main unit.