The Indian Summer Monsoon and Mount Everest "Weather Window"
Compiled by May 9, 2004

Monsoon Definition:
A monsoon is a seasonal shift in the prevailing wind direction, that usually brings with it a different kind of weather. The persistent wind flow is driven by a warm air mass with low pressure at the surface that forms over the continent as it is warmed by the sun. Air from the relatively higher pressure air mass over the ocean flows toward the low pressure over land. Probably the most famous monsoon is the Indian summer monsoon, as it affects such a large portion of Asia. In May and June of each year, the dry northerly wind flow over India changes direction, and warm humid air from the Indian Ocean flows from the south, gradually overspreading the Indian subcontinent. Widespread torrential rains, and even severe thunderstorms, accompany the "onset" of the monsoon. The Indian Ocean version of the hurricane, called a "cyclone", can also occur and move ashore in association with the onset of the monsoon. These cyclones have at times killed thousands of people who live in the low-lying areas along the eastern coast of India and Bangladesh. A weaker version of this monsoon occurs over the normally dry southwestern United States in the later summer when more humid air, accompanied by thunderstorms, invades the region.

The Monsoon blows from South-West, but moves mainly in a North-Westerly direction.

Monsoon Influence:
The monsoon strongly affect Indian financial growth. 2002 was the driest season in a decade. 2003 was the best in 5 years.

The onset of the monsoon off the coast of Sri Lanka mid-May, often accompanied by Cyclones high up in the Bay of Bengal, will signal the coming of the Everest famous Weather Window.

The Weather Window is a period with no Jetwind, low general winds and little precipitation perfect for climbing above 8000 meter. The majority of all successful Everest summit occur during the Weather Window that occur between app. May 15 and Jun 8, when the Monsoon hits the Himalayas (Jun 6 for the Kanchenjunga area). The climbing season as defined by government climbing permits ends June 1.

The length of the Weather Window is app. 2 weeks. The Weather Window might be disrupted by incoming weather systems.

Successful Everest spring season summits 5 year:
1998       May 18               May 27              May 20      25 climbers
1999       May 5                 May 29              May 26      23 climbers
2000       May 15               May 28              May 16      23 climbers
2001       May 19               May 26              May 23      88 climbers
2002       May 16               May 30              May 16      77 climbers

Wind prediction charts:
Season                            Start date low wind period
2001 Spring Season        May 19
2002 Spring Season        May 23

Everest Autumn Weather Window:
By comparing charts AdventureWeather identified an autumn weather window. Check story,AnautumnEverestweatherwindowSep172003.shtml

Monsoon Weather forecasting:
Special Weather Report May 13 2003
- AdventureWeather

IMD - Indian Meteorological Department
- Long Range Monsoon forecast presented yearly mid-April
- Daily and Weekly Weather reports

NCMRWF - National Centre for medium Range Weather Forecasting
- Monsoon View Charts

Onset and withdrawal of Monsoon