1 Radiosondes 1.History of upper air measurements 2.Radiosondes (sensors, calibration, telemetry,multiplexing) 3.The Vaisala radiosondes 4.Special radiosondes.

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Radiosondes

1. History of upper air measurements

2. Radiosondes (sensors, calibration, telemetry,multiplexing)

3. The Vaisala radiosondes

4. Special radiosondes (Ozone,atmospheric electricity, radioactivity)

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EuropeanUpper Air stations

• Operational stations launch 4 times per day

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Requirements for upper air

measurements:(1) To make accurate measurements of important atmospheric parameters (usually temperature, pressure and humidity) above the surface

(2) To send this information back in as close to real-time as possible

[(1) and (2) usually achieved by making a profile measurement, with a balloon carried instrument, but aircraft data is also used.

(2) was once achieved using sondes which dropped something (the lizardsonde), or even exploded (the crackersonde) when a certain condition was fulfilled.]

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Kite-carried sensors

Handbook of Meteorological Instruments (Part 2: Instruments for upper air observations), HMSO, 1961

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Dines’ Kite Meteorograph

Handbook of Meteorological Instruments (Part 2: Instruments for upper air observations), HMSO, 1961

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Dines’ balloon meteorograph (1907-1939)

Aneroid capsule (pressure)

Silvered recording plate

Bimetallic strip (temperature)

Hair humidity element

Handbook of Meteorological Instruments (Part 2: Instruments for upper air observations), HMSO, 1961

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Radiosondes

Small and compact radio transmitters allow the data obtained by a sensors carried on a balloon to be transmitted back to a receiving station.

First successful radiosonde in the UK was the “Kew” Met Office sonde, in use from 1939.

Improvements followed, and the “Mark2” was used from 1945 up until the 1960s

Radiosondes require:

Sensors (with an electrical output)

Radiotelemetry (the data transfer system)

Batteries (which will work at low temperatures)

…balloons and parachutes…

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SensorsThe sensors must ultimately provide an electrical output, which can be turned into a frequency for the radio transmission. Mechanical sensors are coupled to transducers to achieve this.

An example is a pressure sensor. Small mechanical variations in an aneroid capsule are used to move an iron core within an electrical inductor. The inductance changes, which leads to a change in an audio frequency, transmitted directly over the radio link.

Other sensors used include

Temperature: bimetallic strips (mechanical), resistance wire (electrical)

Humidity: hair or gold beaters skin (mechanical), the “humicap” (electrical)

Pressure: aneroid (mechanical or electrical)

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Radio TelemetryA simple (carrier) radio wave requires a change (modulation) to be applied for information to be transmitted. This is usually either AM (amplitude modulation) or FM (frequency modulation)

AM

FM

“Information”

Transmitted signal

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MultiplexingIf more than one signal is required, and in a radiosonde, three different signals (humidity, temperature and pressure) are usually sent, the radio transmitter has to be switched between the three sensors in turn.

This is called multiplexing.

If the three signals are sufficiently different, or the order of switching is known, the individual signals can be recovered.

Multiplexing switch

Handbook of Meteorological Instruments (Part 2: Instruments for upper air observations), HMSO, 1961

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Mk2 MO radiosonde

Multiplexing switch driven by wind mill

sensors

Thermionic valves in radio transmitter

receiver

Handbook of Meteorological Instruments (Part 2: Instruments for upper air observations), HMSO, 1961

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Calibration•Radiosonde sensors have to be calibrated if they are to produce accurate measurements over a range of conditions.

•Calibration requires the sensor to be exposed to the full range of variation they will receive in service, but in a controlled environment.

•The results of a calibration are used to construct a response function, which is an equation used to link the values found by a sensor to the magnitude of the parameter it is sensing.

•The precise response function is unique to each sensor, and is used by the receiving computer to turn the data received into meaningful physical values.

•The response functions are typically polynomial functions, with many coefficients to cover the range of values required. These coefficients are supplied with each radiosonde.

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Calibration

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Mk3 MO radiosonde

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View of Mk3 sonde

Thermometer (resistance wire)

Polystyrene housing

rotary multiplexing switch

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Vaisala RS80 Radiosonde

(Vaisala)

Temperature sensor

Relative Humidity sensor (“humicap”)

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RS80 Specification

(Vaisala)

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“Windfinding”

If the location of a radiosonde is known, and recorded, its direction of motion can be determined from a set of the locations.

This allows the wind directions to be found, often referred to as “windfinding”. The profile of wind direction and strengths can therefore also be plotted.

The location of a radiosonde can be found by different methods:

•Tracking it with radar

•Using a Global Positioning System (GPS) receiver on the sonde to send back its location

•Using the LORAN positioning system on the sonde to send back its location

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GPS satellite system

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RS90 radiosonde

(Vaisala)

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RS90 specification

(Vaisala)

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Special radiosondesRadiosondes can carry a variety of sensors, either instead of, or in addition to, the standard meteorological sensors for temperature, pressure, and humidity.

Atmospheric properties which have been extensively with modified radiosondes include:

Ozone

Atmospheric electricity (the charges and electric fields within clouds and thunderstorms)

Radioactivity

Radiosondes for measuring the profile of ozone in the atmosphere are known as Ozonesondes.

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“Kew-Oxford” OzonesondeContains an ozone cell in which an electrolytic reaction occurs, using potassium iodide. When ozone is passed through iodine is formed, which causes a small current to flow.

From Brewer and Milford, Proc Roy Soc, 256 1960

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Ozone profiles

From Brewer and Milford, Proc Roy Soc, 256 1960

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Atmospheric electricity radiosondes

Electric field probe-

measures change in voltage with height, “Potential Gradient”

Haze layer

Venkiteshawaran S.P. Measurement of the electrical potential gradient and conductivity by radiosonde at Poona, India, pp89-100 In Smith L.G. (1958) Recent advances in atmospheric electricity, Pergamon Press

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Charge distribution in thunderclouds

Stolzenburg et al. (1998)

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In-cloud measurements

Harrison R.G. Rev Sci Inst 72, 6 pp2738-2741 (2001)

Balloon-carried disposable instruments have been designed at Reading to make in-cloud measuerements. These have:

•Detected charged particles emitted in aircraft exhausts

•Found thin and persistent, highly-charged layers

Sensor responds to changes in charge

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RS80 radioactivity sonde

(Vaisala)

Carries Geiger tubes, sensitive to beta and gamma radioactivity, as well as standard temperature, pressure and humidity