Mode conversion effects were observed on a long-distance VLF propagation path at mid-latitudes during two winters; 1974/75 and 1975/76. The occurrence of amplitude minima as a result of interference between modal components was found to be delayed in the sunrise transition during the D region winter anomaly. At the same time the depth of amplitude fading and the magnitude of phase step decreased and, sometimes, became indiscernibly small. These anomalous effects of mode conversion are interpreted as being due to the lowering of the nighttime VLF reflection height in excellent agreement with the high absorption of HF radio waves in the daytime. The implication is that the electron density in the D region is enhanced day and night during winter anomalous days. Dynamical processes in the middle atmosphere are likely to play an important role in the day-long enhancement.