A successful weather forecast requires: 1) a thorough diagnosis of the initial state of the atmosphere; 2) an accurate prognosis of the forces, energy, and motions of the atmosphere; 3) a coherent translation of that prognosis into sensible weather elements at or near the surface of the Earth where people live and work; and 4) effective communication of subsequent weather impacts to those people. Mr. Bob Ryan (a now retired broadcast meteorologist) published a 1982 essay in the Bulletin of the American Meteorological Society, stated elegantly the fundamental challenge of weather forecasting: “Imagine a rotating sphere that is 12,800 kilometers (8000 miles) in diameter, has a bumpy surface, is surrounded by a 40-kilometer-deep mixture of different gases whose concentrations vary both spatially and over time, and is heated, along with its surrounding gases, by a nuclear reactor 150 million kilometers (93 million miles) away. Imagine also that this sphere is revolving around the nuclear reactor and that some locations are heated more during one part of the revolution and other locations are heated during another part of the revolution. And imagine that this mixture of gases continually receives inputs from the surface below, generally calmly, but sometimes through violent and highly localized injections. Then, imagine that after watching the gaseous mixture, you are expected to predict its state at one location on the sphere one, two, or more days into the future. This is essentially the task encountered day by day by a weather forecaster.” Overcoming this formidable challenge starts first with a thorough diagnosis of the atmosphere, including analysis of a variety of quantitative and qualitative observations and cues. Observations of weather conditions at both the surface of the Earth and throughout the troposphere (the portion of the atmosphere where most weather occurs; within 6–12 miles of the Earth’s surface) are needed for accurate weather predictions. This is because the atmosphere essentially behaves as a complex and dynamic fluid. Weather balloons attached to lightweight payloads, containing weather instruments and communications equipment, help meteorologists diagnose the initial state of the troposphere and also provide valuable data for numerical weather prediction models.
The Weather Bureau (forerunner to the National Weather Service) began launching weather balloons routinely in the late 1930s. Today, over 90 stations are part of the upper-air network in the United States, with over 800 stations worldwide. Your NOAA/Florida Keys National Weather Service office in Key West is one of these stations! Our meteorologists launch weather balloons twice daily, at approximately 6:00 a.m. and 6:00 p.m. Eastern Standard Time (EST), and 7:00 a.m. and 7:00 p.m. Eastern Daylight Time (EDT). Occasionally, balloons will be released four times daily, usually when severe weather threatens (e.g., a hurricane). The payload tethered to the balloon is called a “radiosonde”, a small expendable instrument package containing sensors that measure pressure, temperature, humidity, and GPS position. These sensors are linked to a battery powered, 300 milliwatt or less radio transmitter that sends the sensor measurements to a sensitive ground tracking antenna (the ground tracking antenna is located inside the “white ball” behind the National Weather Service facility on White Street in Key West). Wind velocity data are obtained by tracking the radiosonde position in flight. The radio signals received by the tracking antenna are converted automatically to meteorological values, from which coded data are assembled for multiple elevations, and immediately transmitted to data users. All data are archived and available at the NOAA/National Centers for Environmental Information.
A typical National Weather Service weather balloon ascent will last about two hours. During that time, the radiosonde usually reaches an altitude of around 100,000 feet above ground. Horizontal distance traveled from the release point may vary from a mile or less to nearly 200 miles, depending on the winds aloft. During a typical flight over the Florida Keys, the radiosonde will reach subfreezing temperatures within 15 to 20 minutes after the release time! When released, the balloon is about five feet in diameter. It gradually will expand as it rises due to decreasing atmospheric pressure with increasing height. When the giant balloon (up to 25 feet in diameter) bursts in the bitter cold, rarefied air above the troposphere, about 99% of the mass of the atmosphere lies below. At this point, a small, orange-colored, biodegradable parachute will slow the descent of the radiosonde. Presently, data is not collected while the radiosonde descends. Eventually, the radiosonde will return to Earth. If you happen to find one, it is safe to handle. Please find the postage-paid mailbag (as noted in the instructions printed on the instrument), and send it back to the National Weather Service where it may be refurbished and flown again, lowering the cost of the weather balloon program.
If you catch sight of a rising balloon during the early morning or evening hours around Mid Town Key West, know that this old-fashioned, yet very effective data gathering operation remains essential for providing you with accurate marine weather warnings, advisories, and forecasts throughout the year. Indeed, these balloon observations will help you become marine weather-ready and stay safe!
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