'Some decades tend to be hotter'

Dr O P Singh, deputy director general of meteorology at Regional Meteorology Centre in Delhi, resists attributing the rising mercury this season simply to factors like urbanisation and greenhouse gas emissions.

He also dismisses the notion that the average Delhi temperature has risen over the past decades.

Excerpts from an interview with Avinash Singh Sudan:


Do you think over the years the average temperature has risen in the Delhi-National Capital

Region?

It will not be correct to say that the average temperature has risen over the past decades. The studies which say there is an increase in average temperature may be based on a specific decade where the temperature was on the higher side.

To study the variations like temperature or rainfall, two major techniques are used. First come the inter-annual variations that are observed from one year to another year.

Second, a change that is observed over a longer period and is of a permanent nature. In between the two, there are epochal variations that are measured over a period of 30-40 years.

In some epochs the temperature will be on the higher side and in other epochs it may be on the lower side. So after filtering out the inter-annual and epochal variations, whatever remains will indicate the future trend.

What are the causes of rise in temperature and the persistence of heat wave in the capital and adjoining areas since May 18, 2013?

The main factors that have contributed to the heat wave conditions are absence of western disturbance or thunderstorm activity, the advection of hot north-westerly winds from desert region.

Advection means the transfer of heat or matter by the flow of a fluid, especially horizontally in the atmosphere. And finally, the subsiding air due to the anti-cyclone in north-west India.
A right combination of these factors will lead to rise in temperature.

The temperature has reached the 45 degree mark in May
itself. Is there something to worry about?

It is nothing unusual as in 2010 Delhi witnessed a prolonged heat wave spell around the same time. Atmospheric circulations and atmospheric conditions are not the same every year.

There are large variations from one year to another year in meteorological parameters like temperature and rainfall. Moreover, the all-time record for the highest temperature in Delhi is of 47.2 degree Celsius, recorded way back in 1944.

At that time the urbanisation in Delhi was not to the extent as is seen nowadays, yet the heat wave condition was such that the recorded highest temperature became an all-time record. The current rise in temperature is no way near to that.

Even though the effect of urbanisation and emission of greenhouse gases has an impact on the meteorological variables like temperature, yet combination of atmospheric factors also plays a major role in the rise in temperature.

Is the rise in temperature in Delhi-NCR a result of global warming?

No, it is not correct to establish a one is to one correspondence between global warming and rise in temperature of a specific place like Delhi as there are a large number of factors that contribute towards an increase in temperature.

Global warming is a reality. It is an established fact that the average global temperature including average sea surface temperature has increased.

Greenhouse gases like carbon dioxide have increased in the atmosphere globally over the decades.

A study needs to be undertaken with a long time series data to establish the contribution of global warming on the temperature of Delhi-NCR. It will not be correct to establish the rise in temperature of Delhi-NCR on the basis of short range data set.

Does India Meteorological

Department provide advisories to government to help it tackle the rise in temperature?

IMD’s role is to provide weather forecast and meteorological data to different users. There are other agencies like disaster management authorities which make use of the weather forecast and other inputs in preparing guidelines to mitigate the effect of the heat wave.