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The rain in the plains fall mainly on the weekends

Murphy's Law holds true for the weather, at least in White Plains in the state of New York: the rain does fall mainly on the weekends. Saturdays, it has been found, have been 22 per cent rainier than Mondays in recent years. Moreover, air pollution may be the reason.

Randall Cerveny and Robert Balling, of Arizona State University, reviewed weekly climate fluctuations recorded at Sable Island - just east of Nova Scotia and downwind of the US eastern seaboard - from July 1991 to January 1995. They found that concentrations of ozone and carbon monoxide in the air peaked late in the week. That cycle correlated strongly with the rainfall patterns observed upwind, in the vicinity of White Plains. The investigators concluded that the higher levels of aerosolized pollutants in the air may help to trigger rainfall.

The greenhouse effect: blame the trees?

Researchers working in Costa Rica have discovered disturbing evidence that rising temperatures have markedly slowed the growth of tropical trees over the past decade. The slowdown may explain calculations suggesting that tropical forests, which are usually thought to take up carbon dioxide, have actually added billions of tons of the greenhouse gas to the atmosphere each year during the 1990s. As such, they would be a net source of carbon dioxide comparable to the combustion of fossil fuels. The trend could worsen global warming: as the mercury rises, tropical forests may dump yet more carbon dioxide into the atmosphere, causing still more warming.

At the beginning of their study, Deborah Clark and David Clark of the University of Missouri and their colleagues found that the growth of trees in Costa Rica fluctuated considerably from year to year. These fluctuations corresponded strikingly with unaccountable rises and falls in carbon dioxide from an unknown source in the 1980s and 1990s. The apparent lesson is that the varying annual growth rate of tropical forest trees could account, in large part, for these carbon dioxide increases.

Trees ordinarily take in carbon dioxide and release oxygen during photosynthesis, the process that turns sunlight into sugars. However, trees, like animals, also release some carbon dioxide as a byproduct of respiration. When growing vigorously, trees take up more carbon dioxide than they produce. If the growth slows, the balance shifts.

The scientists also found that the rate of tree growth was strongly linked to average temperature, slowing down in warmer years. Higher night-time temperatures seemed to force the trees to respire more, thus promoting the release of carbon dioxide. Because warming does not increase photosynthesis, the imbalance in carbon dioxide grows.

The annual excess of carbon coming from the tropical forests has been estimated at more than 4 billion tons in some recent years. In contrast, the combustion of fossil fuels is responsible for an estimated 6.5 billion tons.

This really won't hurt a bit

Invisibly small needles might take the sting out of shots for patients who need frequent doses of some drugs.

Insulin and other protein-based drug molecules cannot be taken in the form of pills because the digestive system would destroy them. Yet because these drugs must be taken often, perhaps several times a day, traditional hypodermic injections are inconvenient and painful.

Engineers at the Georgia Institute of Technology have come up with a solution: microneedles. These tiny needles are made by the same microfabrication techniques developed for etching integrated circuits into microchips. They are far thinner than a human hair and leave prick marks only about one micron (about a half a millionth of an inch) in diameter. At that size, they are just large enough to allow drug molecules to flow through. Because the microneedles are extremely short, they penetrate only the outermost, nerve-free layer of skin - so they are painless.

Stop, you're killing me

It's a familiar argument: even if cigarette prices were hiked up, addicted smokers would continue to buy them. But in fact, a new study from the US shows that this may not always be the case. Researchers at the Centers for Disease Control and Prevention reviewed 14 years' worth of data from national health surveys and found that lower-income, minority and younger smokers were all particularly responsive to price increases. A 10 per cent price jump, they estimate, would inspire 25 per cent of Hispanic smokers, 10 per cent of African-American smokers and nearly 1 per cent of non-Hispanic white smokers to kick the habit.

In the audio spotlight

Researchers in both the US and Japan are fast developing systems that will project narrow beams of sound from audio speakers. These sound systems could revolutionise the way audio is used in public spaces. Public address systems would be able to target their messages to recipients standing in specific places without disturbing anyone else. Museums, for example, could play descriptions of exhibits that could be heard only by individuals standing in front of a display.

The long-standing difficulty in projecting sounds narrowly arises from basic physics. Usually, the angular size of a beam is determined in part by the wavelength of the sound. Sending out a tight beam of audible speech would normally require a loudspeaker the size of a small building.

Acoustical engineers can sidestep this limitation with parametric arrays, which were developed for underwater sonar systems. These arrays set up vibrations in water, letting the water itself generate low-frequency waves from high- frequency ones. In the early 1980s, Japanese scientists demonstrated that they could use this technique to direct audio in a tight beam, using a compact array of ultrasonic transducers.

Still, the approach languished until about two years ago, when engineering student Joseph Pompeii of the MIT Media Lab and Elwood Norris of the American Technology Corporation in San Diego began tinkering on the problem independently. Pompeii's parametric array can supposedly project a beam of sound only three degrees wide for almost 200m (660ft). Both systems have had some problems with distortion of sound, particularly at low frequencies. According to Norris, however, the latest versions are clear enough for playing music.

! All items are adapted from 'Scientific American' magazine. Visit Scientific American's website at Copyright 1998, Scientific American, Inc. All rights reserved.