[A] lot of people ask what it takes to make it rain in California, and the answer relies on a bewildering variety of inputs, some of them very complex and very far away. One of the most intriguing, and least understood, is a weather phenomenon called the Madden-Julian Oscillation, or MJO. The MJO is a large tropical system of clouds, rainfall and wind patterns—imagine a traveling storminess—that takes 30 to 60 days on average to work its way across the globe. The system typically begins forming over the equatorial Indian Ocean and travels eastward, flanked on both sides by dry regions.

Related articles: 

• Discover Magazine: Climate Change Is Intensifying the Global Water Cycle 
• Cornell Chronicle: ‘Earth is transitioning’: Models suggest more megadroughts