Final answer:
Unlike Jupiter and Saturn, Uranus has no measurable internal heat source, resulting in a much colder atmosphere. Uranus's stable and cool atmospheric conditions are due to its smaller internal heat contribution compared to the larger gas giants.
Step-by-step explanation:
Uranus has a much colder atmosphere than Jupiter/Saturn.
Uranus and Neptune are significantly less massive than Jupiter and Saturn and thus have much smaller atmospheres proportionate to their cores. Due to Uranus's lack of a substantial internal heat source, in contrast to Jupiter and Saturn which have major internal heat sources, Uranus's atmosphere ends up being much colder. These internal heat sources in Jupiter and Saturn contribute to warmer atmospheric temperatures. Voyager measurements unveiled that Uranus's atmospheric temperatures are only slightly warmer on the dark winter side compared to the hemisphere facing the Sun, which indicates the minimal impact of internal heat on Uranus's overall atmospheric temperature. This distinct lack of internal heating results in a more stable and cooler atmosphere. In essence, the stark temperature difference between Uranus and the larger gas giants stems from the former's inability to generate substantial internal heat.