UK storms
Storms results from the circulation of air in the atmosphere which is similar to the working of a heat engine. Getty

Strong storms will become stronger while weak storms become weaker, and the cumulative result of all the storms will remain unchanged under global warming, says a study led by atmospheric physicists at the University of Toronto.

The team quantified the way in which increase in water vapour from global warming influences the strength of atmospheric air circulation.

The atmosphere it turns out will adapt to hotter, wetter climate.

"We know that with global warming we'll get more evaporation of the oceans," said Frederic Laliberte, a research associate at U of T's physics department and lead author of a study published in Science.

"But circulation in the atmosphere is like a heat engine that requires fuel to do work, just like any combustion engine or a convection engine."

The warmer the air mass, the more water it takes up from the surface. As it reaches the Equator, it begins to ascend through the atmosphere, cooling as it expands and sending heat out into space.

Since cool air can hold less moisture than warm air, condensation occurs, releasing heat. When enough heat is released, air begins to rise even further, pulling more air behind it producing a thunderstorm.

This is the way heat and moisture is redistributed between the Equator and the North and South Poles.

The scientists concluded that the increase in water vapour from global warming was making the process less efficient by evaporating water into air that is not already saturated with water vapour.

This inefficiency limited the strengthening of atmospheric circulation, though not in a uniform manner.

Air masses that are able to reach the top of the atmosphere are strengthened, while those that cannot are weakened.

How warm was this summer?
Heat waves in urban areas were most severe in the last four decades according to a study. Reuters

Urban heat on the rise

In a related study on global warming, an international team has found that urban areas have been experiencing significant increases in heat waves over the past 40 years.

The prolonged periods of extreme hot days have significantly increased in over 200 urban areas across the globe between 1973 and 2012, with the recent four years seeing the worst of heat waves, says the study said to be the first to focus on extreme weather on a global scale.

Lead author of the research, Professor Vimal Mishra, from the Indian Institute of Technology (IIT), Gandhinagar, said: "Our results show significant increases in heat waves and the number of hot days and warm nights, and at the same time declines in cold waves and extreme windy days in many urban areas over the last 40 years.

"We also find that the number of changes in precipitation extremes was modest, which is somewhat surprising as our previous work showed a predominance of increases in precipitation extremes in major US urban areas."

The study, undertaken by researchers at the IIT, Northeastern University, University of California, Los Angeles, and the University of Washington, shows that over the same period, more than half of the studied areas witnessed a significant increase in the number of individual extreme hot days, whilst almost two-thirds saw notable increases in the number of individual extreme hot nights.

The results appear in IOP Publishing's journal Environmental Research Letters.

Using data on rain, air temperature and wind speed from the National Climatic Data Center (NCDC), the study looked at areas with a population greater than 250,000 (around 650 areas) and then refined the list based on the area's proximity to a data station and the availability of complete weather records.

The results also showed a general decline in cold waves, and around 60% of urban areas experiencing a significant decline in extreme windy days.

The team is now examining the impact of climate and weather extremes in urban regions on critical lifeline infrastructure, as well as on urban and coastal ecosystems and marine life.

Heat waves in Australia, Japan, China, New Zealand and Korea during 2013 have been directly linked to climate change.