Analyses are presented of experimental field strength values on two long mid-latitude circuits, Fort Collins to Hiraiso and Hawaii to Hiraiso on a frequency of 15 MHz, for the different seasons of the 1975–1986 solar cycle and for different ratios of the wave frequency to the MUF. It is shown that on average the field strength during the night and twilight increases with solar activity and the electron concentration and also with MUF increase or wave frequency decrease. Comparison of experimental field strength values with values calculated by the adiabatic invariant method for equinoctial conditions shows the existence of extra energy losses on the circuits. The losses arise when f < MUF during night and twilight. They are not related to radio wave absorption in the ionosphere. Their average value is 20 to 30 dB, and the values decrease with solar activity and electron concentration increase or with radio wave frequency decrease. Strong leakage of the radio waves into the skip zone for f > MUF is also observed during the night, sunrise, and sunset. The observed phenomena are explained in terms of multiple radio wave scattering by irregularities in the ionosphere. A similar, but smaller, effect also occurs during the day.