Counterflow Virtual Impactor

Designed by the University of Stockholm and the Met Office, the Counterflow Virtual Impactor is used to collect cloud droplets, removing them from the airflow and evaporating the water to leave a residual particle and water vapour. The water vapour is quantified and the the residual particles are quantified and sized.

The Counterflow Virtual Impactor (CVI) is a cloud sampling probe. CVI has two modes of operation, Counterflow mode (CVI, cloud mode) and Aerosol mode. Instruments are fitted to the rack itself, and the sample stream is also distributed to other racks on the aircraft.

CVI Mode

Samples cloud droplets above a certain size – The Cut Size, generally ~10μm (equivalent aerodynamic diameter)
Removes the droplets from the ambient (atmospheric) air
Evaporates the liquid water droplets to leave a residual cloud condensation nuclei and water vapour

Aerosol Mode

Acts as a normal inlet, taking in aerosol particles and ambient air, and distributing the sample to instruments:- Condensation Particle Counter (CPC), Passive Cavity Aerosol Spectrometer(PCASP), Lyman Alpha, water vapour detector.


Cloud particles from 0.1 µm upwards


CVI Inlet CVI Rack in cabin



Elisabeth Ostrom, MetOffice

Paul Barrett, MetOffice

Additional information