Monsoons - a change in wind bringing a change in weather

Over the past week persistent monsoon rain has caused severe flooding across many parts of Pakistan.  Some areas, such as the Swat Valley in the north-west, have been virtually cut off with aid-agencies having to rely on donkeys to transport their supplies.  On Friday, Nadim Ahmed of the Pakistan National Disaster Management Authority suggested 12 million people had been affected by the floods with 650, 000 houses destroyed over 132, 000 square kilometres.  Since then further rain has begun to overwhelm the Sukkur Barrage in Sindh province and rescuers are continuing to evacuate as many people as possible.

So what is a monsoon?  It is a wind that changes direction seasonally and often results in a ‘dry’ and 'wet’ season.  The wind itself can be thought of as similar to a larger scale version of the sea breeze – I hope you were all paying attention to Met Chat 6 weeks ago!  High pressure is established over one area; low pressure over another and, as the laws of physics like balance, a flow is established to try and equalise these pressures.  In a sea breeze these initial highs and lows are created by the difference in temperature between the land and the sea; in the South Asian Monsoon, these establishing factors are a little more complicated.  Often the wind blowing from one direction will bring more moist air to the region, increasing the chances of rain and resulting in a ‘wet’ season.  When the wind reverses direction drier air will reduce the chance of rain – the ‘dry’ season.  In some parts of the world the monsoon is fairly reliable, but in other parts of the world its onset and cessation can be less reliable.  In contrast to this year, areas of Pakistan suffered droughts in 2009 when the monsoon rains failed to arrive.