This is a 90 hour time lapse <video> of the effects of the ionosphere's DLayer on medium wave (ex. AM radio) radio frequency propagation. The DLayer is a day light occurrence, the result of the sun's UV radiation knocking electrons off molecules' atoms in the atmosphere between 30 to 60 miles altitude. During this period HF(10 MHz and less) skywaves are completely absorbed. Once the the sun sets the electrons recombine with the molecule's atoms and the DLayer disappears. RF signals are then reflected off the much higher (around 170 miles altitude) FLayer, making long distance reception of AM radio stations not receivable via ground wave (line of sight) possible, often referred to as skipping. This can been seen in both the waterfall and spectrum as day turns to night and vice versa. Note the occasional appearance of RFI on the left side of the waterfall from a light dimmer in my house early in the morning and at various times during the evening. Also note the switch to high power of local AM stations during the day and switch back to low power at night. This is done to avoid drowning the RF waves at night as almost every radio station would reach almost every AM radio over large areas. The following snapshots show day time reception (first image) versus night time reception (second image.)
Active DLayer - Only Local AM Stations 
Inactive DLayer - Long Range Reception AM Stations