A volcanic winter is a reduction in global temperatures caused by volcanic ash and droplets of sulfuric acid obscuring the Sun and raising Earth's albedo (increasing the reflection of solar radiation) after a large particularly explosive volcanic eruption. Long-term cooling effects are primarily dependent upon injection of sulfide compounds in aerosol form into the upper atmosphere—the stratosphere—the highest, least active levels of the lower atmosphere where little precipitation occurs, thus requiring a long time to wash the aerosols out of the region. Stratospheric aerosols cool the surface and troposphere by reflecting solar radiation, warm the stratosphere by absorbing terrestrial radiation, and when combined with anthropogenic chlorine in the stratosphere, destroy ozone which moderates the effect of lower stratospheric warming. The variations in atmospheric warming and cooling results in changes in tropospheric and stratospheric circulation.