section_path: Climate/Precipitation
section_name: Precipitation
paragraph_text: Precipitation
  
  The Himalayan hydroclimate is crucial for South Asia, where annual summer monsoon floods impact millions.
  
  A critical influence on the Himalayan climate is the Southwest Monsoon. Variability in monsoon rainfall, influenced by local Hadley circulation and tropical sea surface temperatures, is the main factor behind wet and dry years. This is not so much the rain of the summer months as the wind that carries the rain. Different rates of heating and cooling between the Central Asian continent and the Indian Ocean create large differences in the atmospheric pressure prevailing above each. In the winter, a high-pressure system forms and remains suspended above Central Asia, forcing air to flow in the southerly direction over the Himalayas. But in Central Asia, as there is no substantial source for water to be diffused as vapour, the winter winds blowing across South Asia are dry. In the summer months, the Central Asian plateau heats up more than the ocean waters to its south. As a result, the air above it rises higher and higher, creating a thermal low. Off-shore high-pressure systems in the Indian Ocean push the moist summer air inland toward the low-pressure system. When the moist air meets mountains, it rises and upon subsequent cooling, its moisture condenses and is released as rain, typically heavy rain. The wet summer monsoon winds cause precipitation in India and all along the layered southern slopes of the Himalayas. This forced lifting of air is called the orographic effect.