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Radiosonde just unpacked

This is the radiosonde as it is unpacked from the shipping crate and put into position for calibration. Click on the picture for a labeled closeup of the radiosonde which is being readied for the ascent through the atmosphere. The temperature and humidity sensors, also called thermistors, come in sealed packages. The temperature sensor changes electrical resistance with changes in the temperature and the humidity sensor changes electrical resistance with the humidity. The wrappers are seen lying on the table in the foreground.

This is an old style radiosonde. Some of the newer ones are much smaller requiring smaller balloons and parachutes. The new systems also do all of the data processing by computer. I've chosen to use the old system because the steps involved are visible allowing you to see the intermediate data collection steps during the radiosonde ascent. These are not visible with the modern systems as all the data processing is done by a computer.

When the radiosonde transmitter is hooked up to the battery, it is placed in the calibration chamber. This image shows the radiosonde in the chamber. The batteries are on the outside to the left. The small fan is used to decrease the pressure in the chamber.
Radiosonde turned on

The radiosonde and the radio receiver are turned on, the chamber closed and the fan turned on for a simulated run. The temperature and humidity are read from the wet and dry bulb thermometers. There is also a simple resistor which is unaffected by changes in pressure, temperature or humidity in the radiosonde which is turned on occasionally. This resistor is used to compensate for any drift in the measurements. At this time, the position of the radiosonde is also read and the distance and position measurement electronics are set to the position of the chamber. The next image shows the chart recorder reading the temperature and humidity during the calibration.

As before, click on the image to see a larger view with labels.

Once calibrated the battery is packed in the radiosonde. The radiosonde is taken out to the shed where it is tied to a parachute. The parachute is in turn tied to the balloon. Both are bio-degradable.

The balloon is inflated with helium by putting it on a weight scales much like the one used in medicine. The only difference is that the balloon pulls up. When the balloon pulls up with just the right upward force, the scale turns the valve. Each radiosonde must pull upward with the same force so the speed of ascent through the air will be the same. This is important because the wind speed and direction are measured by knowing the exact positions of the radiosonde its trip upward through the atmosphere.

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Office of Climate, Water, and Weather Services
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Silver Spring, MD 20910
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Last Updated: March 26, 2004