The Nephelometer is a Weather Instrument used to measure the suspended particles in liquid or gas colloid. This is done through the light beam and the light detector, which is set on one side of the source beam on the Nephelometer. The light beam and the light detector are fixed at an angle of 90° with each other. The particle density becomes visible under the light reflected from these particles into the detector. The word Nephelometer has been derived from a Greek word nephos, which again came from nepheloid layer meaning cloud.

Nephelometer comes from the Greek word for cloud, nephos, cf. "nepheloid layer".

The amount of light reflected largely depends upon the physical and other properties of the particles like the color, shape, and reflectivity. The meteorologists work upon establishing a relation between the suspended solids and the turbidity.

Turbidimeter is those Nephelometers which are used for testing the quality of the water. It keeps a check on the light reflected off the particles and does not monitor the attenuation due to cloudiness. Nephelometric Turbidity Units or NTU are the units of turbidity in US.

The Gas-phase nephelometers are common among the meteorologists for the study of atmosphere. This device provides data on the visibility and atmospheric albedo. The instrument can also measure the smoke & other particles of combustion in the air. This form of the instrument is known as an aspirated smoke detector. The Gas-phase nephelometers can detect the extremely low particle concentrations and are thus used to protect highly sensitive and valuable electronic goods such as telephone switches and mainframe computers.