Which of these conditions could enhance radio wave travel through the atmosphere?

High sunspot activity can enhance radio wave travel through the atmosphere primarily because it increases the levels of ionization in the ionosphere. The ionosphere is a layer of the Earth's atmosphere that is crucial for radio wave propagation, particularly for high-frequency (HF) signals. During periods of high sunspot activity, solar radiation and solar flares cause an increase in ionization, which can lead to better reflection of radio waves back to Earth. This phenomenon can improve communication over long distances, as radio waves can bounce off the ionosphere and travel beyond the horizon.

In contrast, dense cloud cover, heavy rainfall, and low temperatures generally have a negative impact on radio wave propagation. Dense cloud cover can absorb or scatter radio waves, leading to weaker signal strength. Heavy rainfall can similarly attenuate radio signals, as water droplets interfere with the transmission. Low temperatures do not directly enhance radio propagation, and depending on the conditions, they can also lead to decreased ionization levels, negatively impacting HF communication. Therefore, high sunspot activity stands out as a condition that contributes positively to radio wave travel.