Cloud Droplet Probe (CDP-2)


A compact, lightweight cloud particle spectrometer that measures particles in the 2 to 50 µm range.
Special Features: Particle-by-particle data (optional)




  • Atmospheric and cloud research
  • Weather modification
  • Aircraft icing studies and certification
  • Hurricane and storm research
  • Agricultural and industrial spray characterization

Photo at near right: the CDP-2 mounted on a tower at a wind farm in Canada. The probe is being used to test icing conditions that could pose a danger to the wind turbines.

Photo at far right: the CDP-2 mounted on the Cloud Lab, a blimp designed to survey atmospheric conditions at low altitudes over a prolonged period of time.




The CDP uses state-of-the-art technology to size particles under a wide range of conditions. The latest CDP design offers improved housekeeping processing and several other new features:

Recent studies have shown that particle fragmentation can lead to overestimating particles in certain size ranges by a factor of up to 1000. While the original CDP already improved dramatically upon older instruments like the FSSP in this regard, the new anti-shatter tips and wetless windows reduce particle artifacts further.
The instrument automatically adjusts both sizer and qualifier signals to adjust for temperature drifts and ensure accurate particle sizing.
This change protects the instrument from power and ground fluctuations.


Particle-by-Particle Feature (PBP)

PBP is an optional feature that provides precise information on particle scattering intensity and inter-arrival times. The standard CDP only lists particle times with one-second resolution. PBP provides particle times that are accurate to within a microsecond—an improvement by a factor of a million.

PBP data are useful when investigating small-scale cloud structure to identify mixing and entrainment, drop breakup and coalescence, and micro-scale turbulence.



The Particle Analysis and Display System (PADS, right) is optional software that provides a user-friendly virtual instrument panel.

PADS allows the user to control the CDP and display real-time data and logs. For instance, the program enables the user to do the following tasks: 

  • Sample and record data
  • View particle volume and number concentrations, as well as Median Volume Diameter (MVD) and Effective Diameter (ED)
  • View LWC as measured by the CDP
  • Monitor instrument parameters like CDP laser current and various electronics voltages
  • Play back data for post-flight viewing
  • Reprocess data with new parameters for additional analysis

Online CDP Software Manual


How It Works

The CDP is a forward-scattering optical spectrometer. For accurate sizing, the CDP accepts and sizes only particles that pass through a region of the laser beam with uniform power. This region of the laser is called the depth of field. 

As particles pass through the laser beam, light scatters in all directions. The CDP collects forward-scattered photons within an annular cone that is 4° to 12° from the laser beam. The collected light is then directed onto a 50/50 optical beam splitter and finally to a pair of photodetectors, referred to as the sizer and the qualifier. (See figure below) There is a mask in front of the qualifier detector to define the depth of field. The edge of the depth of field is defined by the points where half of the light scattered from a particle is blocked by the mask.

The photodetectors then convert the photon pulses into electrical pulses. The pulse from the qualifier is multiplied by two, and if the resulting signal exceeds the pulse from the sizer, the particle is deemed within the depth of field. The particle is then sized based on the amplitude of the sizer pulse. 

See the CDP-2 Operator Manual for more information.


Parameter Specification
Measured Parameters Single-particle light scattering
Derived Parameters Particle diameter

Particle number concentration

Liquid water content (LWC)

Effective diameter (ED)

Median volume diameter (MVD)
Particle Size Range 2 – 50 µm
Number Conc. Range 0 - 2,000 particles/cm3
Typical Sample Area 0.24 mm2
Number of Size Bins 30
Air Speed Range 10 - 250 m/sec
Sampling Frequency Selectable, 0.04 to 20 seconds
Refractive Index Non-absorbing, 1.33 (the industry standard for water)
Light Collection Angles Optical design: 4° - 12°

Optical performance: 1.7° - 14°
Laser 658 nm, up to 50 mW
Data System Interface RS-232 or RS-422 serial interface
Calibration Precision glass beads
Routine Maintenance Window cleaning and glass bead calibration check
Recommended Service Annual cleaning and calibration at DMT service facility
Software Optional Particle Analysis and Display System (PADS) software
Power Requirements System Power: 28 VDC at 2A

Anti-ice Power: 28 VDC at 12A
Environmental Operating Conditions Temperature: -40 to 40 °C

Relative Humidity: 0 - 100%, non-condensing

Altitude: 0 - 50,000 feet (0 - 15,000 meters)
Weight Probe: 1.37 kg / 3.0 pounds (standard version); a lightweight UAV version is also available

Electronics box and header cable: 0.82 kg / 1.8 lb
Probe Dimensions 26.7 cm L x 17.5 cm W x 21.6 cm H (10.5” L x 6.9” W x 8.5” H)
Electronics Box Dimensions 17.8 cm L x 8.9 cm W x 5.1 cm H (7.0” L x 3.5” W x 2.0” H

Specifications are subject to change without notice.

Included Items

  • Instrument
  • Shipping Case
  • Operator Manual
  • One-year warranty
  • One day of training at DMT facility

Accessories (Purchased separately)

  • PADS software and laptop
  • Spinning pinhole for alignment and calibration check
  • Canister adapter—allows the CDP to be used with conventional cloud probe mounting canisters


How to Order

Contact DMT for pricing or more information.


Phone: +001 303 440 5576
Fax: +001 303 440 1965

Selected Bibliography

S. Lance, C. A. Brock, D. Rogers, and J. A. Gordon, “Water droplet calibration of the Cloud Droplet Probe (CDP) and in-flight performance in liquid, ice and mixed-phase clouds during ARCPAC,” Atmos. Meas. Tech., 3, 1683–1706, 2010. doi:10.5194/amt-3-1683-2010. Link

M. W. Gallagher, P. J. Connolly, A. Heymsfield et al. "Observations and modelling of microphysical variability, aggregation and sedimentation in tropical storm cirrus outflow regions." Atmos. Chem. Phys. Discuss.,11, 23761–23800, 2011. doi:10.5194/acpd-11-23761-2011.Link

J. Crosier, K.N. Bower, et al, “Observations of ice multiplication in a weakly convective cell embedded in supercooled mid-level stratus,” Atmos. Chem. Phys. Discuss., 10, 19381–19427, 2010. doi:10.5194/acpd-10-19381-2010. Link

D. Rosenfeld, W. L. Woodley, D. Axisa, E. Freud, J. G. Hudson, and A. Givati (2008), "Aircraft measurements of the impacts of pollution aerosols on clouds and precipitation over the Sierra Nevada," J. Geophys. Res. , 113, D15203, doi:10.1029/2007JD009544. Link