Cloudbursts: India
This is a collection of articles archived for the excellence of their content. |
Contents |
In brief
[ From the archives of the Times of India]
‘Steep rise in cases of extreme monsoon rain’
What makes a cloud rain?
While the mechanism of cloud formation is well understood, what exactly causes a cloud to produce rain remains a meteorological mystery. Air temperature drops with altitude, hence warm air rising in the atmosphere cools and then condenses on to specks of dust or soot, tiny salt crystals and other microscopic particles floating in the air. These particles are called cloud condensation nuclei (CCNs). There are various meteorological models which use variables like wind speed, air humidity and so on, but the inner mechanics of clouds remain mysterious.
What is a cloudburst?
A cloudburst is an event in which heavy rainfall occurs over a localized area at a very fast rate. The area typically doesn’t exceed 20-30 sq/ km while the rainfall may reach the level of 100 mm per hour resulting in flashfloods as was witnessed in Uttarkashi and Ukhimath this year and in Leh in 2010. Typically, a cloudburst in India occurs during the monsoon season over the Himalayan region, northeastern states and the Western Ghats. It can also occur over the plains, but such occurrences are rare.
What makes the Western Himalayas more susceptible to cloudburst?
Cloudbursts become frequent during the monsoon season. It is believed that they occur because of rapid lifting of the monsoon clouds by the steep orography of the region. The clouds get vertically lifted and these convective clouds can extend up to the height of 15 km above the ground. This process is called the “cumulonimbus convection condition” which results in formation of towering vertical dense clouds. The lifting is usually dynamic and this causes thermodynamic instability resulting in a cloudburst. It is also believed that in the Himalayan region, the cloud which is being lifted rapidly also is accompanied by soil moistened by earlier precipitation. This soil perhaps acts as an additional source of moisture and might also have a role in the frequent cloudbursts in the region.
Have extreme rainfall incidents increased over the years?
A 2009 study carried out by R M Khaladkar, P N Mahajan and J R Kulkarni of the Indian Institute of Tropical Meteorology notes that there is an increase in extreme monsoon rain events in India over the past 50 years. The extreme point rainfall event (EPRE) for a particular weather station isdefined as the highest 24-hour rainfall reported in a particular month during the entire period of data availability. The study reports a steep increase in the highest 24-hour rainfall in recent years. Of the 165 stations covered under the study, many experienced an alarming rise in EPRE after the 1980s. In many cases, the intensity increased by 40% to 370%. The study points out that there might be certain uncertainties in the climate model, but these extreme events might have a link with global warming. The rise in surface temperatures might be causing more evaporation and might also be increasing the moisture holding capacity of the atmosphere, hence leading to extreme rains.
