Friday, September 3, 2010 @5:52 PM

Just for JJ, Sian Dan, Wan Sin, Pepsi and Egg.

Passage A for Summary

In this day of satellites and radar, sending up a balloon may sound like a crude way to forecast the weather—akin to licking one's finger or throwing some grass into the air to test the wind.


In fact, weather balloons play a crucial role in gathering information for today's forecasts, probing regions of the atmosphere that newer technologies can't reach. Balloons track temperature, pressure, relative humidity, and wind speed and direction (via GPS tracking) in the upper atmosphere—conditions that have drastic effects on the weather we experience at sea level.


Across the United States, National Weather Service (NWS) releases balloons twice a day, at 7 a.m. and 7 p.m. eastern standard time. In total the NWS launches about 200 balloons a day. Other countries send up their own balloons, for a total of about 2,000 flights a day worldwide. Weather agencies regularly share the information that those flights gather.


Soon after the first piloted balloons flew, in France in the late 1700s, pilots began to bring thermometers and other instruments aloft with them. In the 1800s some pilots even went to the extreme of observing weather from a balloon six miles (9.6 kilometers) high, suffering frostbite and the lack of oxygen in the process.


It wasn't until the 20th century that automatic weather measuring instruments were developed and sent up in unmanned balloons. But because balloons flew so high and encountered winds of up to 200 miles an hour (320 kilometers an hour), many of the readings were lost when the instruments crashed to the ground far from launch sites. The development and expansion of radio in the 1930s, however, allowed the instrument packages to transmit weather data back to Earth as it was collected. These packages became known as radiosondes, and gave rise to the global upper atmosphere observation networks still in use today.


The typical weather balloon is about six feet in diameter (two meters) and is filled with either helium or hydrogen. It carries a one-pound (half a kilogram) package of weather instruments and transmitters. Flights may last over two hours, reaching altitudes of 22 miles (35 kilometers), where the temperature drops to minus 130 degrees F (minus 90 degrees C). At that height the balloon, having swollen to about 20 feet (six meters) in diameter due to the low air pressure, tends to pop, cutting short the flight. The one-pound radiosondes do not hurtle to the ground, however—little parachutes bring them back down safely, complete with mailing instructions. Returned instrument packages can be retooled and reused. But even now, with GPS tracking, the NWS recovers only about 20 percent of the radiosondes.


During flight each radiosonde sends a constant stream of data to NWS—from 1,000 to 1,500 readings per balloon. All told, the NWS receives 200,000 to 300,000 readings from balloons daily—crucial for understanding the current state of the atmosphere. It's only by getting a complete picture of what the weather is doing now that forecasters can hope to say what it will do next.