Fading out

Published: Thursday 30 September 2004

-- Short Wave & Medium Wave Transmission

Has it been difficult, of late, to tune in to radio stations such as Radio Ceylon and bbc? Has the crackle or signal fade-out, as you tune in to medium- and short-wave stations on your radio, increased? Have you been wondering why? Now, an international team of researchers comprising scientists from the Pune-based Indian Institute of Tropical Meteorology (iitm) have the answer. The culprit, they say, is changes wrought in the upper atmosphere due to global warming.

There exists, at an altitude ranging from 50-300 kilometres (km) in the upper atmosphere, a layer called the ionosphere. This layer is vital for radio and television transmission, particularly for the long-distance variety. Ideally, medium- and long-wave signals from radio and tv stations bounce off electrons in the ionosphere, and return to the Earth. Medium waves reflect from the d region (50-100 km); short waves from regions e and f (100-300 km). But because there has now occurred a decrease in the density of ionospheric electrons, the ability of short- and medium wave signals to reflect has been affected: the signals, consequently, have weakened.

But why this decrease in electron density? The team led by iitm scientist Gufran Beig undertook a five-year study that used global temperature records for the last three decades. The team found that the temperature of the ionosphere has decreased by 15C in this period. As one travels upwards in the atmosphere, the temperature gets cooler and the atmosphere shrinks: such a degree of cooling in the ionosphere as the study found has led to unnatural shrinkage, affecting, in turn, electron density.

The team has won the World Meteorological Organisation's prestigious Norbert Gerbier-mumm international award for 2004 for their study of the impact of global warming on the upper atmosphere.

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