 Monsoons impact millions of people every year. Farmers rely on rain for their harvests, and lives are threatened by flooding or landslides. And yet, it remains very hard to predict monsoons early and accurately. European researchers have now devised a way to estimate the monsoon season in India. This innovative approach uses a rare isotope, Beryllium 7. The forecasts are not only more accurate than traditional methods, but also available earlier, which could give governments and residents more time to prepare. This unusual weather-tracking approach works because of how air circulates on earth. Each hemisphere features three large-scale patterns, or cells. The Hadley, Farrell, and Polar Cells, where two cells meet is a convergent zone. Monsoons, seasonal shifts in wind that trigger heavy rain, happen at the Intertropical Convergent Zone, or ITCZ. Monsoons are seasonal because the earth's tilt affects the ITCZ's location. In the summer, sunlight is strongest and warms the air more to the north. That coincides with India's rainy season. In the winter, the ITCZ and hence monsoons move south. Beryllium 7 comes into play because it can trace what's happening in the atmospheric cells. It's created in the upper atmosphere when cosmic rays bombard nitrogen and oxygen atoms. The isotope attaches to aerosols and is transported in atmospheric cells. Beryllium 7 can then be detected by ground monitoring stations and therefore help predict monsoons based on the movement of the zones. Beryllium concentrations are high, right under the Hadley Farrell Convergent Zone, where air moves downward but low in the tropical area under the ITCZ. It turns out that when these fluctuations are plotted from stations in Russia and Australia, the Beryllium 7 concentrations follow opposing trends. When the concentration at one location increases, it decreases at the other. The researchers found that they can predict the start of the monsoon more than seven weeks in advance. With an accuracy of three days, that's an improvement over current methods, which predict monsoon arrivals only two to three weeks ahead with an accuracy of plus or minus five days. Similarly, monsoon withdrawals become predictable with this method about six weeks in advance and accurate within plus or minus one week. The longer lead time could save lives and help India's economy. The researchers also hope to apply their novel technique to other monsoon regions and other seasonal weather phenomena influenced by the movement of the atmospheric cells.