How a stagnant weather pattern gave rise to destructive fires in Russia and floods in Pakistan.
Normally the jet stream in the Northern Hemisphere carries weather fronts over Russia in four or five days. But late in the summer of 2010 conditions were anything but normal. A large-scale, stagnant region of high pressure developed and lingered over western Russia for about a month. The rare weather pattern—known to meteorologists as an Omega blocking high—split the jet stream in two, causing winds to flow around the high, an area of descending warm air, in a horseshoe-shaped pattern similar to that of the Greek letter Omega (Ω). The high blocked the normal progression of weather fronts and produced droughts and unusually warm temperatures that fueled a rash of fires near Moscow. As Russia burned, the same blocking pattern kept a low-pressure area over northern Pakistan. The cool, rising air of the low generated torrential rainfall and destructive flooding in northern Pakistan when it clashed with warmer air from the high. In the visualization below, look for the warm air from the persistent high-pressure zone over Russia (shown in yellow and red) and the cooler air from the low-pressure zone (shown in blue) just north of Pakistan.