The Large Aperture Scintillometer (LAS) is an instrument designed for measuring the path-averaged structure parameter of the refractive index of air over horizontal path lengths from 250 m to 4.5 km. Structure parameter measurements obtained with the LAS and standard meteorological observations (air temperature, wind speed and air pressure) can be used to derive the surface sensible heat flux.

The LAS optically measures intensity fluctuations (known as scintillations) using a transmitter and receiver horizontally separated by several kilometers. The scintillations seen by the instrument can be expressed as the structure parameter of the refractive index of air.

The light source of the LAS transmitter operates at a near-infrared wavelength of 880 nm.

At this wavelength the observed scintillations are primarily caused by turbulent temperature fluctuations. Therefore the measurements obtained with the LAS can be related the sensible heat flux. Compared to traditional point measurements the LAS operates at spatial scales comparable to the grid box size of numerical models and pixels of satellite images used in meteorology, hydrology and water-management studies. The LAS has important applications in energy balance and also water balance studies, because the surface flux of sensible heat is linked to evaporation.