1800-2000 KHz (1.8-2.0 MHz ) or the 160 m band

The top band, as it is sometimes caled, suffers from extreme daytime D-layer absorption. Even at high radiation angles, virtually no signal can pass through the F layer, so daytime communication is limited to ground-wave coverage. At night, the D layer quickly disappears and worldwide 160 m communication becomes possible via F2-layer skip. Atmospheric and man-made noise limit propagation. Tropical and midlatitude thunderstorms cause high levels of static in summer, making winter evenings the best time to work DX at 1.8 MHz. A proper choice of receiving antenna can often significantly reduce the amount of received noise while enhancing desired signals.

Source: ARRL Handbook, 1993