Fog Monitor (FM-120)


A popular all-weather optical spectrometer that continuously measures particles in the 2 - 50 µm range.
Optional feature: Particle-by-particle data




  • Weather research
  • Studes of fog formation and dissipation
  • Visibility and light-extinction studies
  • Evaluation of the super-cooled droplets that lead to highway and power-line icing

The instrument’s anti-icing heaters and all-weather enclosure make it suitable for tower mounting. 

Photo at near right: the FM-120 at a research station in Jungfraujoch, Switzerland. Photo courtesy of Emanuel Hammer.

Photo at far right: the FM-120 in St. John's, in Northeastern Canada. Photo courtesy of Curtis Paxton.




The FM-120 uses state-of-the-art technology to size particles under a wide range of conditions. Its design offers the following state-of-the-art features:

  • Nickel-coated optical flow block to prevent static build-up
  • More robust electronics, with digitized signals from the photodetectors 
  • Universal voltage input for analyzer head 
  • Particle-by-particle data (optional feature)
  • Dynamic thresholding feature to account for small drifts in baseline signal
  • Raw data available for one particle in each sampling instance, information that serves as an important instrument diagnostic


Particle-by-particle (PBP) Data (Optional Feature)

The PBP option provides high-resolution information on particle light-scattering intensity. This in turn allows much more precise and detailed size distributions, as well as minimization of the well-known Mie scattering ambiguities.

The measurement of particle interarrival times provides both an independent measure of particle concentration and the capability of evaluating the fine-scale structure of fog. Evaluation of the interarrival times allows the study of inhomogeneities in the fog structure and analysis of entrainment and mixing. These important processes control the evolution of fog properties and affect visibility and regional climate change.

The PBP particle times are accurate to within a microsecond, an improvement by a factor of a million over the standard FM-120 data.



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

PADS allows the user to control the FM-120 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 FM-120
  • Monitor laser health and temperatures at various instrument locations
  • Play back data for post-flight viewing
  • Reprocess data with new parameters for additional analysis


Online FM-120 Software Manual



How It Works

The FM-120 is a forward-scattering optical spectrometer. For accurate sizing, the FM-120 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 FM-120 collects forward-scattered photons within an annular cone that is approximately 3.5° 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). 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 digital pulses. If the qualifier pulse is greater than half the sizer signal, the particle is deemed within the depth of field. The particle is then sized based on the amplitude of the sizer pulse. 




Parameter Specification
Measured Parameters Single-particle forward light scattering
Auxiliary Parameters Temperature
Derived Parameters Particle diameter
Particle number concentration
Liquid water content (LWC)
Effective diameter (ED)
Median volume diameter (MVD)
Particle Size Range Droplets with 2-50 µm diameter 
Number of Size Bins 30
Typical Sample Area 0.24 mm² 
Sample Flow Volume 1 m3/min
Sampling Frequency Standard Data Rate: Selectable, 0.04 sec to 20 sec
PBP Data Rate: No limit until particles are coincident
Refractive Index Non-absorbing, 1.33 (the industry standard for water)
Light Collection Angles ~3.5º - 12º
Data System Interface RS-232 or RS-422 serial interface
Up to 460 kB 
Calibration Verification Precision glass beads
Software Particle Analysis and Display System (PADS) software, included
Environmental Operating Conditions Temperature: 0 to 40 °C
Relative Humidity: 0 - 100%, non-condensing
Altitude: 0 - 4,000 meters
Weight 10.0 kg (instrument), 9.5 kg (pump)
Probe Dimensions 23 cm H x 28 cm W x 37 cm L
Power Requirements Universal input for analyzer head, 50-60 Hz
115-120V or 230V for pump (set at factory, but user-configurable)
200W (instrument), 400W (pump)

Specifications are subject to change without notice.



Included Items

  • Instrument
  • Laptop Computer
  • PADS Software
  • Sample Pump
  • Test Cables
  • Calibration Fixture
  • 30 micron glass beads
  • Custom Shipping Case
  • Operator Manual
  • One Day of Training at DMT Facility
  • One-year Warranty
  • Email and Phone Technical Support

Accessories (Purchased separately)

The FM-120 can be purchased with a swivel-head mount, shown at right. The swivel head ensures the instrument inlet is always facing into the wind.


How to Order

Contact DMT for pricing or more information.


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

Selected Bibliography

J. K. Spiegel, P. Zieger, N. Bukowiecki, E. Hammer, E. Weingartner, and W. Eugster, “Evaluating the capabilities and uncertainties of droplet measurements for the fog droplet spectrometer (FM-100).” Atmos. Meas. and Tech. Discuss., 5, 3333–3393, 2012. doi:10.5194/amtd-5-3333-2012

W. Eugster, R. Burkard, F. Holwerda, F. N. Scatena, and L.A.(Sampurno) Bruijnzeel, “Characteristics of fog and fogwater fluxes in a Puerto Rican elfin cloud forest.” Agricultural and Forest Meteorology 139 (2006), 288-306. doi:10.1016/j.agrformet.2006.07.008

Reto Burkard, Patrick Bützberger, and Werner Eugster, “Vertical fogwater flux measurements above an elevated forest canopy at the Lägeren research site, Switzerland.” Atmospheric Environment 37 (2003), 2979-2990. doi:10.1016/S1352-2310(03)00254-1.

Otto Klemm, Thomas Wrzesinsky, and Clemens Scheer, “Fog water fluxat a canopy top: Direct measurement versus one-dimensional model.” Atmospheric Environment 39 (2005), 5375 - 5386. doi:10.1016/j.atmosenv.2005.05.041.