Fog Monitor – DMT (FM-120)

The FM-120 Fog Monitor, developed by Droplet Measurement Technologies (DMT), is a highly sensitive optical instrument designed to measure the microphysical properties of fog. Widely used in fog research and weather modification programs, the FM-120 provides detailed observations of fog droplet size distribution and liquid water content (LWC).

Key Features:

• Measurement Principle: Forward-scattering spectrometry using laser-based detection
• Laser wavelength: 658 nm
• Droplet Sizing Range: 2 – 50 µm
• Number of Size bins: 30

• Size Resolution:1 µm bin width for droplets ≤ 14 µm, 2 µm bin width for droplets > 14 µm and ≤ 50 µm
  • Sampling Frequency: 1 Hz (data later averaged to 1-minute intervals)
  • LWC Calculation: Based on Mie scattering theory, assuming spherical water droplets with known refractive index

The FM-120 determines droplet size by analyzing the angular distribution of laser light scattered by individual droplets. Using Mie theory, the instrument assumes the particles are spherical and composed of liquid water. Liquid Water Content (LWC) is derived by integrating over the measured size spectrum.

Its high temporal resolution and fine droplet classification make the FM-120 ideal for studying fog microphysics, evaluating visibility, and supporting model validation and forecasting tools.

   Applications:

• Fog microphysical research
• Weather modification and cloud seeding support
• Validation of satellite-based fog detection
• Low-visibility hazard assessments

For more information, refer to:

• Our published study: Atmospheric Research (2024)
• Manufacturer details: DMT Fog Monitor FM-120

Figure 3. Fog Monitor – DMT (FM-120).