Volcanic ash clouds contain fragmented rock particles and volcanic glass (0.001–2 mm) that can reach the stratosphere and be carried thousands of kilometres by the jet stream. They are invisible to conventional weather radar.
They are extremely dangerous for aviation: ash melts at jet engine temperatures (~1,400 °C), solidifies in turbines, and can cause total engine failure. The Eyjafjallajökull eruption (2010) closed European airspace for 6 days. VAAC (Volcanic Ash Advisory Centre) units monitor and predict ash dispersion.