Pakistan got Russia’s rain in summer 2010,

In summer 2010, devastating floods in Pakistan had by August covered one-fifth of the country. Meanwhile, 2010 brought Russia’s hottest summer on record, along with Russian drought, crop failure and wildfires. Dr. Jeff Masters, director of meteorology for the website Weather Underground, explained to EarthSky that the 2010 floods in Pakistan and drought in Russia were related via fast-flowing currents of air – high in Earth’s atmosphere – known as the jet stream.

Jeff Masters: The drought in Russia was caused by a jet stream pattern that took the jet stream far to the north of Russia, and kept low pressure systems that usually go over the country from dropping their rain.

At the same time, part of the jet stream veered south, he said.

Jeff Masters: So this meant that since the jet stream was looping far to the north, in order to counterbalance that it had to loop someplace else far to the south. And that place … was over Pakistan.

The jet stream looped over Pakistan as the yearly monsoon rains were occurring. The monsoon consists of air currents rising over heated land, which lets moisture-laden air flow in from the oceans. Masters said it was hot in Pakistan this summer also. So the monsoon was unusually heavy.

Jeff Masters: When you have hot air like that, it tends to have more water vapor. So now we had an exceptionally strong flow of moist air off the oceans that had a much higher water content than usual. And that’s a recipe for heavy rainfall and heavy flooding.

EarthSky asked Masters what percentage of Pakistan flooding was due to the jet stream, in contrast to the extreme heat and unusually heavy monsoon.

Jeff Masters: A rough guess would be to say that three fourths of the problem was due to heavy monsoon, and maybe one quarter was due to the fact that we had this jet stream pattern in place.

Masters said another factor was at work in causing Pakistan’s floods in 2010. He said in recent years Pakistan’s deforestation and development around rivers have left fewer roots to take up and hold water, and more concrete to create storm water runoff.

Jeff Masters: Pakistan is prone to these kinds of floods and probably more so in recent years – because flooding is not just the result of heavy rainfall. It’s also the result of land use changes.

He said that as people build levees to control rivers, there are usually more homes and buildings built in the flood plain. That adds to the potential for floods to devastate entire communities.

September 6, 2010

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