LIDAR sends pulses of coherent light up into the sky and measures scattered and reflected light from any particles or debris floating in the air. From timing the 'echoes' it can determine not only the presence of material but also measure its height and thickness.
Last week Adam Povey, of Oxford University's Department of Physics, was scrambled to STFC's Chilbolton Observatory in Hampshire to use a system, jointly operated by both Oxford University and Hovemere Ltd, to see if any sign could be detected of the ash cloud passing over southern England.
By last Friday his measurements could detect a thin layer of material at around 3km altitude (c10,000 feet up), part of which slowly descended over the following couple of hours before merging with echoes from other debris at the top of a convection layer (the Planetary Boundary Layer) around 3,000 feet up. Below that height, the air is full of all sorts of other debris, including human-generated pollution, so is difficult to untangle from the volcanic ash.
Since then Adam, part of Don Grainger's group at Atmospheric, Oceanic and Planetary Physics, and the team have continued to monitor the cloud and are publishing updates of the latest information and images online.
Andy Sayer, another member of the Oxford team, told me: 'From the satellites we're getting the 'big picture' of what's happened over Europe over the course of the past week or so, although because of the way the satellites sample the revisit time for any one location can be a couple of days.'
'From this we can estimate the light extinguished by the ash, and learn about the size of the particles, as well as measuring the amount of sulphur dioxide released during the eruption and where that's going.
'The lidar, on the other hand, is giving us a continuous profile of any ash at one site (Chilbolton) and we can see how distinct any layers are and how they're mixing with the rest of the atmosphere.'