A Technical Description of the Dropsonde
Abstract: A dropsonde is an expendable weather reconnaissance device that was created by the National Center for Atmospheric Research. It was designed to be dropped from an aircraft at a high altitude over open waters in order to measure storm conditions of a hurricane, or other tropical systems, as it falls to the surface of the ocean. Within its short but effective journey, the dropsonde transmits a vertical profile of the atmosphere’s temperature, barometric pressure, and wind data. This paper will describe the development and technological advancement of the dropsonde by various major weather service organizations. It will also describe the structure of the dropsonde and details about its functionality and usage. The dropsonde is an important device for the prediction of tropical weather activity and its remarkable efficacy during peak hurricane season will also be discussed in this paper.
Technical Description:
With global climate change increasing its detrimental effects each year, extreme weather events, hurricanes in particular, have become more prevalent than ever before. Meteorologists warn the public about hurricanes and their disastrous impact whenever the atmospheric winds shift them towards land. People are told how much damage to prepare for based on the hurricane’s wind speed, atmospheric pressure, sea surface temperatures, and the amount of rainfall that will arrive with its landfall. But how exactly are these quantitative measurements determined? After all, a hurricane develops in open water and there is no physical human interaction with it until it makes landfall- unless a “dropsonde” is used by hurricane hunters for successful forecasting.
A dropsonde is a weather device that is dropped out of an aircraft by hurricane hunters at a specific altitude above a hurricane and with the force of gravity, it drops down the eye of the hurricane and onto the surface level of the Earth. It has a satellite-based Global Positioning System (GPS) attached to it that collects data of the surrounding atmosphere during the device’s descent and using radio transmission, the data is then relayed back to the aircraft. The dropsonde is essential for data collection during field projects by hurricane hunters because it is otherwise extremely dangerous and difficult to fly an aircraft into the eye of a hurricane and collect quantitative data in regards to the conditions of the storm.
In the early 1970s, the National Council for Architectural Registration’s (NCAR) Earth Observing Laboratory (EOL) developed the first version of the dropsonde and used them in various experiments for tropical weather studies in the Atlantic Ocean. Through the 1980s and 1990s, the NCAR continued developing and improving the dropsonde by creating and dropping over 7,000 devices into areas of the ocean with tropical weather activity. In 1987, a digital version of the dropsonde replaced the original analog version and in 1993, a GPS was incorporated into the dropsonde for more advanced data collection. Hurricane research was scarce relative to what meteorologists know about them today but in 1982, the National Oceanic and Atmospheric Administration’s (NOAA) Aircraft Operations Center used the dropsonde for research which has led to the abundance of knowledge about hurricanes that is available now. In the same year, the dropsonde was adopted by hurricane hunters in the United States Air Force Air Weather Service for hurricane weather forecasting and it is still used by them for the same purpose today.
The anatomy of a dropsonde includes several components that contribute to its vertical descent through the atmosphere. In terms of dimensions, a standard NCAR GPS Dropsonde is designed in a cylindrical shape and is 40 centimeters long, has a diameter of 5.8 centimeters, and it weighs 350 grams. It is composed of various strong but light-weight metals including copper and steel in order to help it withstand the high wind speeds inside the hurricane. At the top of the device is a square-cone parachute that helps to increase stability and slows the free-fall with the guidance of air-resistance in order to record more accurate readings over a longer period of time. There are small air vents that are attached to the parachute so air can enter through it ten seconds after release from the aircraft. A shock cord, which reduces stress when the chute opens, connects the parachute to the shaft of the dropsonde, which contains all of the technological components that help collect and relay data. These include the GPS receiver, which collects data from GPS satellites to calculate wind speed and direction. The receiver is attached to a microprocessor that digitizes data from the three sensors at the bottom of the shaft that sense atmospheric pressure, humidity, and temperature at each location of the descent. The microprocessor also controls the radio transmitter which relays quantitative humidity, temperature, pressure, and GPS (wind) measurements to the aircraft every 0.5 seconds. A battery pack is also located at the bottom of the dropsonde, which provides power for up to one hour of flight. It is critical to load each dropsonde with a new battery prior to flight in order to ensure that it provides accurate data for the entire length of its descent.
During a typical research flight, about thirty dropsondes are released over the course of about 10 hours which means one dropsonde is released every 20 minutes. The more dropsondes that are released, the more accurate that the conclusion from the data will be because any critical patterns in the data will become more obvious. A dropsonde is usually released about 40,000 meters from the surface of the ocean and it ascends at roughly 28 meters per second. Once a dropsonde is released from an aircraft and it enters the eye of the hurricane, its parachute is released and its GPS receiver immediately detects the latitude and longitude points every 0.25 seconds in order to determine the wind speed and wind direction of the storm. Each time quantified data is obtained, it is labeled as a “data point.” Determining the wind speed of the hurricane allows meteorologists to categorize the severity of the storm and predict how much destruction it may cause. The wind direction of the storm allows predictions to be made about the direction that the storm is projected to follow and if it will make landfall. The sensors of the dropsonde are then activated to measure pressure, temperature, and humidity and with assistance from the parachute, a vertically-oriented profile of atmospheric thermodynamic data, which analyzes the relationship between temperature and atmospheric pressure, is determined. The microprocessor is then activated and it digitizes this data, which then allows the radio transmitter to relay the data back to the aircraft. The profile of atmospheric thermodynamic data allows meteorologists to determine how much rainfall that the hurricane will produce upon landfall. Lower atmospheric pressures and higher temperatures, which leads to higher humidity, causes more rainfall.
More than one thousand dropsondes are used to gather information on storms during each hurricane season and this comes at an economic cost. Dropsondes cost between $700 and $800 each and since they are used for open-ocean work, they simply sink after they reach the surface of the ocean which means they are only for one-time use. However, the NOAA feels that this cost is still absolutely necessary due to the dropsonde’s remarkable efficacy in gathering storm data. According to quantitative data analysis of the efficacy of a dropsonde, the device typically requires about 20 minutes to fall from the aircraft to the surface of the ocean. Temperature, pressure, and humidity are each data points that are transmitted every 0.5 second, or two times per second. There are 1200 seconds in the 20 minute fall and if 1200 is multiplied by 3 data points and then multiplied by 2 (2 times per second), the total is 7200 data points. Wind speed and wind direction data is transmitted 4 times per second so 2 data points multiplied by 4 and then multiplied by 1200 totals in 9600 data points. The sum of 7200 and 9600 data points is 16,800 data points per dropsonde. If this sum is multiplied by 30, since hurricane hunters typically use 30 dropsondes per research flight, then a total of 504,000 data points are obtained in each research flight. This is a monumental number that produces a high-resolution profile for researchers and can allow them to provide exceptionally accurate data analysis for the conditions of the hurricane that could not have otherwise been provided. Hurricanes also develop rapidly and the fact that over 500,000 data points can be obtained within only 10 hours also allows researchers to be time efficient with the release of a storm’s predicted impact upon landfall for the public. Since it travels downward vertically and the device is in direct contact with what it is measuring, the dropsonde determines precise readings of the atmosphere in a process called in-situ sensing. A commonly used in-situ sensing device is a thermometer. Collecting in-situ data, specifically vertical profile data, during severe weather is extremely difficult but the dropsonde eases the process tremendously.
Dropsondes are used within a global network of daily radiosonde launches through which countries share data with each other about atmospheric conditions within their land and continental waters. All of these launches, including dropsondes, use a system of global satellites to relay digitized data back to researchers. Satellites orbit around the Earth to provide location points and without them, the GPS of the dropsonde would not be able to determine longitude and latitude points of the hurricane and in turn, the projected pathway of the hurricane would not be determined. Although dropsondes are typically released from aircrafts controlled by pilots, NASA occasionally releases them from an unmanned aerial drone called a Global Hawk. These drones can fly in the air for over 18 hours and this allows for more data to be collected than typically collected during hurricane hunter flights.
The hurricane seasons over the past five years have been so hyperactive that more dropsondes have been released within this time period than in the entire decade before. The detrimental impacts of climate change are provoking tropical activity in ocean basins all over the world and hurricanes are progressively and rapidly intensifying which causes extensive damage and loss of life on land. As a result, dropsondes are more important for hurricane forecasting now than ever before as they can allow people to prepare for impact. Prior to the development of dropsondes, people could not prepare properly or evacuate in time when a destructive storm was approaching. With surface winds inside hurricanes exceeding 230 miles per hour and causing turbulence that can tear apart airplanes, there truly is no other instrument that is capable of such a difficult feat. The dropsonde is a perfect example of how technology can do the work that humans are physically incapable of doing while simultaneously saving the lives of humans with the information that they provide.
Works Cited
Chiles, James R. “Dropsondes Go Where Even Hurricane Hunters Fear To Fly.” Air Space Magazine, 21 September 2017. www.airspacemag.com/daily-planet/dropsondes-go-where-even-hurricane-hunters-fear-fly-180964982/. Accessed 17 September 2020.
Earth Discovering Laboratory. “What Is A Dropsonde?” National Center for Atmospheric Research, 2010. www.eol.ucar.edu/content/what-dropsonde. Accessed 17 September 2020.
Kinghorn, Jonathan. “What On Earth Is A Dropsonde?” Air Worldwide, 21 August 2015. www.air-worldwide.com/Blog/What-on-Earth-Is-a-Dropsonde-/. Accessed 17 September 2020.
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