A further potential disadvantage of FACE is its dependence on continuous air movement. During daylight hours the continual flux of solar radiation and resulting convective currents ensure that still periods are rare, except around dawn. However, at night, still conditions commonly occur. Some FACE systems mix pure CO2 into an airstream which is then pumped into the plots at the release points. This flow of CO2-enriched air moves air into the plot under still conditions. These systems can therefore enrich the atmosphere under still conditions. However, still conditions also result in a climatic inversion, where cold air forms at the surface overlain by warm air. Pumping air into the plot brings the warm air to the surface thus disrupting the inversion. Enrichment can be achieved under still conditions, but only by significantly altering the microclimate. The system described by does not predilute CO2 but releases pure CO2 at supersonic velocity through minute nozzles into the wind. The energy of these turbulent jets generates a predilution of the CO2 before the wind carries it back over the treatment plot. This system depends completely on some air movement and cannot operate under perfectly still conditions.