Fog Monitor

This optical spectrometer continuously samples droplet-laden ambient air and produces size distributions in the range from 2 µm to 50 µm.


Rely on the only all-weather ground-based instrument capable of measuring particle size distributions (PSDs) in the 2 µm to 50 µm size range – critical for understanding the evolution of fog, and useful for calculating number concentration, liquid water content, and extinction/visibility.

Get high-resolution information on particle light-scattering intensity with the PBP optional feature. This allows much more precise and detailed size distributions, as well as minimization of the well-known Mie scattering ambiguities.

Access raw data for one particle in each sampling instance. Use this information as an important instrument diagnostic.

Depend on automatic correction for drifts in polarization signals through the dynamic thresholding feature.

Particle light-scattering

  • The FM-120 is a forward-scattering optical spectrometer that accepts and sizes only particles that pass through a region of the laser beam with uniform power.
  • Forward-scattered light is collected, directed through a 50/50 optical beam splitter, then measured by photodetectors to determine size and depth of field.
Measured Parameters Single-particle forward light scattering
Auxiliary Parameters Temperature Pressure
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-422 serial interface up to 460 kB
Environmental Operating Conditions Temperature: -40 to 40 °C Relative Humidity: 0 - 100%, non-condensing Altitude: 0 - 4,000 meters
Weight 18.5 kg (instrument), 13.0 kg (pump & hose)
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)

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

The Lifecycle Care Program ensures FM-120 integrity and performance vital to your research mission for 3 years or 3 calls (whichever comes first). When conducting complex and time intensive laboratory or field measurements it is critical to avoid down time, and ensure data collection accuracy. The program consists of 3 key deliverables for each year or call:

  • Instrument Logistics
  • Instrument Integrity
  • Instrument Performance

Lifecycle Care Program Includes:


  • Shipping to and from Droplet Measurement Technologies facility
  • Customs handling, including CARNET ATA if required


  • Incoming instrument evaluation and health check


  • Software and firmware validation
  • 100%-unit functionality test
  • Calibration data
  • Calibration report
  • Certificate of calibration

At times, our technician might discover an instrument issue, which might not be covered by the Lifecycle Care Program or by our product warranty. In such case we will reach out to you with a service & repair estimate to allow you to provide us with the next steps of action.