51动漫

THE scourge of farmers around the globe, El Ni帽o can upset
astronomers鈥� observations as well. Its dramatic warming effect in the Pacific
may draw enough water into the atmosphere to dim views of the cosmos at infrared
wavelengths, even on the clearest nights.

Astronomer Jay Frogel of Ohio State University in Columbus studied data from
nearly 200 nights of observations at the Cerro Tololo Inter-American Observatory
in Chile, spanning 1978 to 1992. Each time, he monitored about a dozen
鈥渟tandard鈥� stars of known brightness. Changes in the apparent brightness of
these stars betrayed hourly and daily fluctuations in the clarity of Earth鈥檚
atmosphere.

Over longer timescales, several patterns emerged in the measurements for
near-infrared wavelengths. Most notably, the standard stars appeared slightly
dimmer during the southern hemisphere summer, from December to February. That
was no surprise鈥攚arm air holds more water vapour, which absorbs infrared
radiation.

But Frogel also found a link with episodes of El Ni帽o. Strong El
Ni帽os draped a moist veil above the observatory, dulling stars by as much
as 2 per cent compared with 鈥渘ormal鈥� years. The analysis will appear in
Publications of the Astronomical Society of the Pacific.

Frogel suspects that observers more distant from El Ni帽o鈥檚 warm water
pool would see a similar influence鈥攅specially now, during the century鈥檚
strongest El Ni帽o. 鈥淔or observational programmes that require an accuracy
of 1 per cent or better, this effect must be taken into account,鈥� he says.