These are two of Blue Book’s standard default explanations, (also referred to by Blue Book scientific consultant Dr. J. Allen Hynek, as a “poverty of hypotheses”). Firstly, if only one radar system paints a UFO, it is always possible that it was a result of a malfunction. The second requires some basic understanding of atmospheric physics. In the lower atmosphere, air (gas) temperature cools as it rises and the pressure decreases. This is referred to as an adiabatic lapse rate. Under certain conditions, a warmer (less dense) air mass moves over a cooler (denser) air mass, inverting the normal adiabatic lapse rate. In some instances, electromagnetic waves propagating from radar can be refracted off the temperature inversion boundary layer to the ground, (then back to the original source) resulting in anomalous radar returns. Thus, if radar paints a UFO, and an inversion was present at the time, it is always possible that it was a result of anomalous propagation.