The wavelengths of our radars are long compared to the diameter of the scatteres. We assume, scattering can be described by the Rayleigh approximation, where the backscattered energy is proportional to the sixth power of the diameter. However, in order to deduce the precipitation intensity from the measured radar reflectivity, it is necessary to have an estimation of the droplet size distribution (so-called droplet spectrum). Such droplet spectra can be determined with distrometers.


A classic distrometer was developed by Jürg Joss and Albert Waldvogel. The instrument works similar to a microphone: a small (and therefore slow) droplet transmits only a small momentum to the measuring surface. The larger (and therefore faster) the impinging drop is, the stronger the transmitted momentum. Each individual pulse is detected, so that the number density of the drops per volume can be determined on the basis of the known relation between sedimentation velocity and drop size (v(D)).

To minimize the dead time after a drop, the measured (analogue) signal is instantaneously used to bring the membrane back into its initial position by a second coil. Data are polled once a minute from the Joss-Waldvogel distrometer. Drop sizes are encoded by integer numbers between 1 and 255, which are then evaluated in 20 drop size classes.

While the Joss-Waldvogel distrometer requires a fixed relation between sedimentation velocity and drop diameter (which is only fulfilled by rain), Parsivel (Particle Size and Velocity) is capable of measuring drop size and velocity separately.

For this purpose, it transmits homogeneous laser beam of 28 mm width and 1 mm height from the transmitter head to the receiver head. If a Hydrometeor (generic term for particles of water in the atmosphere: raindrops, hail, graupel, snowflake, cloud ice, clouds drops) crosses the laser beam, it scatters a part of its energy: The received intensity is attenuated. For hydrometeors smaller than the beam height (1 mm), the reduction is proportional to their cross-section. For large particles, it is proportional to the horizontal dimension of the particle.

From the duration of the attenuation the fall rate can be derived, assuming the shape of the particles is well known. It is taken into account, that raindrops larger than 1 mm become more and more oblate. For particles of unknown form, the determined velocity is only a rough estimate. However, it turns out to be possible to detect different types of precipitation (rain, snow, sleet, hail, drizzle).


Fig.1: The diagram shows the drop spectrum of derived value Z and R for one day . Basis are minute value respectively.


Specifications of RD-69 disdrometer
Droplet diameter: 0.3 mm to 5 mm
Measurement surface: 50 cm²
Relationship between droplet diameter D and amplitude of the output signal:
Ucompr.: Ucompr. = 0.94 D1.47
  (Ucompr. in Volts, D in millimeters)
accuracy: +/- 5% of measured drop diameter
Operating voltage: 115/230 V AC, 50-400 Hz, 5 VA
Operating temperature: 0 °C to 40 °C
-transducer: 10 cm x 10 cm x 17 cm
-processor: 10 cm x 23 cm x 27 cm
-transducer: 2.4 kg
-processor: 1.8 kg
Length of cable between transducer and processor: 10 m
For Manufacturer: Fa. DISTROMET LTD, Basel

Fig. 2: The Joss-Waldvogel disdrometer

specifications of PARSIVEL-Distrometer
Wavelength of the laser diode: 780 nm
Power of the laser diode: 3 mW
Dimensions of the skylight (W x H x D): 160 mm x 1 mm x 30 mm
Measurement surface: 48 cm²
Diameter range of hydrometeors*: 0.2 - 25 mm
Speed ​​range of hydrometeors*: 0.1 - 20 m/s
weight: 15 kg
Power supply: 12 V, 600 mA
interface: RS 232
For Manufacturer: Ott Messtechnik, Kempten


Fig. 3: The PARSIVEL-disdrometer

* Hydrometeors = droplets and / or ice particles