FINCH monitors the atmospheric INP number concentration in situ. A sample flow of 2 liter/min of ambient air is introduced on top of a vertically oriented thermostatted flow tube of 4l volume. Here it mixes with a flow of warm, humidified air and a flow of cold air to produce a fog that travels down the flow tube towards the optical detection unit at the bottom. The cylinder wall and the mixing flows are temperature controlled. Temperatures and humidities are adjusted to produce the desired undercooling and supersaturation in the range of -10 to -40°C and 0-105% relative humidity, respectively. INP are activated and ice crystals grow during the approximate 10 seconds of transit to the optical particle counter at the bottom.