A scale analysis, i.e., an estimation of typical values of important atmospheric and ionospheric parameters and time scales is performed for the meteorological type of winter anomaly at medium latitudes (40¿N). An interpretative model of the winter anomaly is developed by a correlation analysis on the basis of data obtained from the Western European Winter Anomaly Campaign 1975/76 at El Arenosillo, Spain, and a simultaneous winter anomaly campaign at Wallops Island, U.S.A. large number of rocked experiments performed in six previous winters in Spain are also analyzed and found to support the model. The model explains the buildup and decay of the winter anomaly by ombined action of temperature, turbulent downward transport of nitric oxide, and horizontal transport of nitric oxide by winds. The horizontal wind direction is not important for the picture. Mechanisms involved are fast enough to explain the rapid variations of the winter anomaly. Large-scale winter anomaly structure is linked to planetary wave number 2. The model picture is found to be compatible with quite a number of experimentally known features of the winter anomaly. There are hints to a more general validity of the model.