The depth of the Antarctic ozone hole has been steadily increasing, but it is unlikely to become much deeper than in 1993. During October of that year most of the ozone between 12 and 20 km altitude disappeared. The ozone destruction process has so far been confined to this height range, probably because the air above and below is too warm for stratospheric clouds to form. The area affected by ozone depletion has also increased since the beginning of the effect in the 1970s, but is limited by the strong circumpolar circulation and cannot normally extend beyond 55° south. The 1994 hole was the largest yet observed, covering over 23 million km2. The shape of the hole is continually changing; sometimes it is circular and at other times very elongated. The duration of the hole has increased as well, because with less ozone present there is less warming of the stratosphere and the circumpolar circulation is more stable and lasts longer. Major volcanic eruptions, such as Mt Pinatubo in the Philippines which erupted in 1991, may put material which contributes to ozone depletion into the stratosphere.