The spatial extent and duration of the winter anomaly, i.e., anomalous and large enhancement in ionospheric absorption of HF and MF waves occurring on groups of winter days at middle latitudes, are examined on the basis of fmin data at 18 stations in USSR, Japan, and North America during the periods November--February in 1971--1972 and 1972--1973, considering that fmin enhancement represents an increased ionospheric absorption of HF and MF waves. The relationship between the phenomena of winter anomaly and geomagnetic storm is also examined. It is found that the winter anomaly occurs simultaneously in an extensive daytime area at middle latitudes, at least in some scattered region over the area, whose longitudinal extent reaches at most ~100 ¿. The southern boundary of the anomaly seems to be controlled geomagnetically and located below 30 ¿ in geomagnetic latitude. Also it is shown that the winter anomaly seems to be associated with geomagnetic storms, mainly sc (sudden commencement) storms, because outstanding enhancements in wave absorption begin usually 1--3 days after the beginning of an individual sc storm and last for more than a few days with decreasing magnitude. It is suggested that the winter anomaly is ascribed to ionization enhancements in the lower D region due to precipitating electrons from the outer atmosphere, mainly from the slot region, associated with geomagnetic storms.