Weather Balls are one of the most central elements of Nami's Clima-Tact weapon in the One Piece series. These special balls are capable of producing several atmospheric effects, thereby allowing Nami to change atmospheric conditions around her in the circle of combat. The concept of Weather Balls was introduced along with the Clima-Tact during the Alabasta arc and represented a significant power-up in Nami's combat abilities.
Usopp's first Clima-Tact used three kinds of Weather Balls: Heat Balls, which were red, Cool Balls, which were blue, and Thunder Balls, which were yellow. Upon release, each ball produces a certain kind of atmospheric effect: the Heat Ball will create warm air, the Cool Ball will create cool air, and the Thunder Ball will produce static electricity. Mixing these balls in different ways brings Nami numerous different weather phenomena at her fingertips.
The mechanics of the Weather Balls are based on some kind of pseudoscientific explanation of meteorology in the One Piece world. After Haredas, Nami's mentor from Weatheria, it is known that weather balls could hold different kinds of weather with clouds in them. In this concept, one has the ability to create local weather effects; something which Nami uses as a weapon in her fighting style.
The results of the Weather Balls often turn out something visual to look at in the anime and manga. For instance, when Nami uses a Weather Ball in the anime and manga, it is normally depicted by a colored sphere coming out of her Clima-Tact, followed by a weather effect. For example, Thunderballs were often shown creating small bolts of light around the wand, like lightning or electrical discharges.
Besides the mentioned uses, Nami showed several techniques using weather balls. Using the Cloudy Tempo technique, she rolls together Cool Balls and Heat Balls to generate fog to blind herself, so that she can view clearly. The Thunderbolt Tempo technique involves rolling Thunder Balls to form strong attacks in lightning.
Nami's skill with the Weather Balls increases tremendously after the time skip. Her training on Weatheria has allowed her to make much more complex and powerful weather phenomena. This is reflected in all of her new techniques, showing an incredibly greater understanding of atmospheric science in the One Piece world.
The concept of Weather Balls does not apply only to Nami personally. On a grander scale, the scientists of Weatheria, across the One Piece world, have been successful in holding and manipulating weather in several forms. An example would be that they could integrate wind into ropes, much like how the Weather Balls work. This is one of those worldbuilding details that adds depth to the concept and provides an omen for future developments in weather-based technology from the series.