Although many pokemon are said to control the weather, most of them, such as Groudon or Kyogre, belong as much to myth as fact. It is not uncommon for a pokemon to create weather conditions through techniques like Rain Dance or Sunny Day, but these conditions are typically short-lived and highly localized. And then there is Dragonair.
For all the Weather Institute's research into machines and Castform, they could not exceed the weather control machine which existed naturally in the form of a pokemon. The crystals of Dragonair give it a remarkable control of the weather not just in its immediate vicinity, but stretching out for over a mile. Fiercely temperamental, enormously shy and vain, in the wild they brought drought to crops or answered prayers for rain, perhaps giving an origin to many religious customs. They have at times been used for warfare in recent years, for a Dragonair battalion can dampen any rocks or flames, or simply flood away the enemy's food supply. Had they been tamed in an earlier, more warlike age, one fears to think of the devastation they could have wrought.
Today, peace has come to Kanto and Johto, and Dragonair are used to keep the weather pleasant. If it rains too much they bring sunlight, if there's drought they bring rain, and they have largely ended the days of blazing summers, endless storms, and freezing winters. It is a rare weather pattern which a group of Dragonair can not vanquish, and these severe conditions are said to come from the gods themselves; perhaps they truly do, for no other explanation is forthcoming.
If one wishes to understand what life was like before Dragonair, they need only venture to Hoenn, where these pokemon are seldom found and the roads filled with endless rain or sand.