Last year 20 million people were affected by the worst flooding seen in Pakistan for 80 years [Daniel Berehulak/GETTY]

Last year's flooding in Pakistan is fresh in the memories of the local people and there are concerns that it could happen again. After all, no one predicted that the rains would be so heavy last year, so what's to stop the same thing happening this year?

Last year was an exceptional year. Floods submerged almost one fifth of the entire country and 20 million people were affected. The aid that came in arrived too slowly, and was hampered by the damage the flooding had caused to the roads and other infrastructure .

Meteorologically speaking, last year wasn't a normal year in Pakistan. The severe rains were actually triggered by the same thing that caused the severe drought in parts of Russia: a shift in the jet stream.

A jet stream is a thin ribbon of fast-flowing air, which rushes along approximately 10 kilometres above the earth. Most of us can usually ignore it, but every now and then it lets us know it's there; for example the jet stream over the Atlantic is the reason that flying from the US to the UK is so much quicker than the flight going the other way.

There are usually two branches of the jet stream over the northern hemisphere, one which billows over the Mediterranean and Central Asia, and the other which is slightly nearer the Pole. They usually meander north and south, without causing any problems.

One of the most important things that jet streams do is to dictate the storm tracks. Effectively they guide the rain systems, so if they ever get stuck in one place, one region of the globe will certainly know about it. This is what happened in the UK of 2007, when it never seemed to stop raining!

Last year, the polar jet stream increased in strength and deviated from its usual path. It shot up to northern Russia and looped back southwards towards Pakistan. This strange placement of the jet remained anchored in position for long enough to cause huge problems.

The storms, which would normally provide Russia's breadbasket with irrigation, were diverted north by the jet stream. As a result, Russia suffered its hottest summer on record, and was ravaged by wildfires.

The jet stream, still acting high up in the atmosphere, then guided cool air down towards Pakistan. The reason for the unusual position of the jet stream isn't exactly clear, but its timing was unfortunate, because at the same time, the temperatures recorded in Pakistan were above average.

Cold air sitting above top of warm, moist air favours the development of powerful thunderstorms, generating huge amounts of rain.

Could it happen again this year?

Due to the unusual nature of the meteorological conditions, flooding on the same scale is unlikely to happen again this year. However, some flooding is almost certain in some parts of the country. In fact flash-flood warnings have already been issued for some mountainous parts of the Punjab Province.

Pakistan is a region of the globe that often sees flooding during the monsoon, but adding to their problems is the changing landscape of the country. Recent deforestation has ensured that hillsides are less able to absorb water and are more susceptible to landslides.

Also, like many countries, more and more houses are being built on floodplains, which means that if a river manages to burst its banks, whole communities can be devastated.

Flooding on the scale of that seen in 2010 may be unlikely, but people are right to be on their guard.

Source: Al Jazeera