The Penna basin has a dry tropical monsoon climate. The upper basin of the Penna is semi-arid, with summer temperatures of 25-45 °C and winter temperatures of 18-35 °C. Most of the rainfall is provided by the moisture-bearing southwest monsoon, which provides rain to most of India between June and September. The Penna basin lies in the rain shadow of the high Western Ghats ranges, which prevents much moisture from reaching the region. Post-monsoon cyclonic storms in the coastal region produce additional rain during September and October. The winter northeast monsoon, which provides rain between December and March, provides little rain to the upper basin and slightly more to the lower basin. It varies greatly from year to year in south India, sometimes bringing powerful cyclonic storms with flooding and high winds. The mean annual rainfall is 550 mm/year in Anantapur, and 900 mm/year in Nellore. The Penna basin suffered from a prolonged drought in the 1990s, which caused much misery among the regions farmers and generated political demands to build an aqueduct to bring water from the Krishna River to Rayalaseema.