Most long-term weather forecasts cover no more than seven to ten days; the further ahead into the future they look, the less accurate they are. But thanks to increasingly powerful supercomputers (forecasts are based on computer modelling), meteorologists are developing “subseasonal” weather forecasts that look as far as four weeks into the future. They rely on different factors than normal forecasts—ocean temperature and currents, soil conditions, global climate phenomena—and they do have some limitations: so much processing power is required that these forecasts are much less granular.