Why did radio transmissions often work better at night than during the day?

Radio transmissions often perform better at night because of the behavior of the ionosphere, a region of the atmosphere that reflects radio waves back to Earth, allowing them to travel beyond the horizon.12

During the day, solar radiation causes the lower atmosphere to become ionized, creating the D layer of the ionosphere. This layer absorbs medium and lower-frequency radio waves, which limits transmission range.12

At night, the absence of sunlight causes the D layer to dissipate, reducing this absorption and allowing radio waves to reach higher, more reflective layers of the ionosphere—such as the E and F regions—more effectively. Guglielmo Marconi was the first to discover this phenomenon, observing that radio signals could be received over significantly greater distances at night than during the day.12