Dust To Rain

Red%20Spots%20Are%20Dust%20And%20Emissions.jpgOn my recent walk up the first ridge in the Margallas, I could not help but notice a blanket of smog, and dust suspended all over the capital. Ironically, Islamabad is considered to have some of the cleaner environments in the country. That hypothesis needs to be validated though. However, dust is an inherrent phenomenon in this region’s climate anatomy. Surprizingly this dust has other reasons apart from the recent increase in traffic to a greater degree. And interestingly that reason is the heavy monsoon rains this area receives. It’s more of the chicken and egg story. Are monsoons responsible for the dust, or the dust responsible for monsoons? Read on just in case you want to understand what really goes behind the scenes.

A monsoon is a seasonal shift in wind direction that alternately brings very wet and then very dry seasons to the sub-continent and much of Southeast Asia. Increased dust aerosols blowing in from western China, Afghanistan, Pakistan and the Middle East coupled with black carbon emissions from northern India accumulate in the pre-monsoon late spring in the atmosphere over the northern and southern slopes of the Tibetan Plateau. When the dust absorbs the sun’s radiation, it heats the surface air hovering above the mountainous slopes of the region. The heated air rises and draws warm, moist air in to northern India from the Indian Ocean, which helps create more rainfall. As the air warms and moves upward, new air is drawn in to take its place, which is also warmed – creating a process like a pump that pulls heated air upwards. The “heat pump” effect actually starts the wet monsoon season prematurely in northern India, leading to a longer rainy season.

Due to heavy rains, the pelting large water droplets, heavier dust particles settle lower on in the soil layer, leaving the relatively light weight particles at the top. When the monsoon is over, these particles are blown up into the atmosphere, and due to their lighter weight rise to new heights.

William Lau, research scientist at NASA’s Goddard Space Flight Center, Greenbelt, Md. and his team studied the aerosols using computer models. They found aerosols in the form of dust lofted from the desert surface and transported to the monsoon region can heat the air by absorbing the sun’s radiation, altering the Asian monsoon water cycle.

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