The eXtra Large Aperture Scintillometer (X-LAS) is an instrument designed for measuring the path-averaged structure parameter of the refractive index of air over horizontal path lengths from 1 km to 8 km. Structure parameter measurements obtained with the X-LAS and standard meteorological observations (air temperature, wind speed and air pressure) can be used to derive the surface sensible heat flux.

The X-LAS optically measures intensity fluctuations (known as scintillations) using a transmitter and a 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 X-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 X-LAS can be related the sensible heat flux.

Compared to traditional point measurements the X-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 X-LAS has important applications in energy balance and also water balance studies, because the surface flux of sensible heat is linked to evaporation.