The hole is bigger than the area of North America

The "hole" is a severe depletion of the ozone gas layer in the upper atmosphere that protects life on Earth by blocking ultraviolet rays from the sun.

From 21-30 September, its average area was 27.4 million sq km (10.6 million sq miles) - bigger than North America.

It is caused mainly by human-produced compounds that release chlorine and bromine gases into the stratosphere.

The Montreal Treaty was designed to protect the ozone layer by phasing out the production of chemicals, such as chlorofluorocarbons (CFCs) in spray-can propellants, responsible for ozone depletion.

As a result of the protocol and its amendments, concentrations of ozone-depleting substances in the lower atmosphere (troposphere) peaked around 1995 and are decreasing in the troposphere and stratosphere.

If the stratospheric weather conditions had been normal, the ozone hole would be expected to reach a size of about 8.9 to 9.3 million square miles.

Unusual extent

The Ozone Monitoring Instrument on Nasa's Aura satellite measures the total amount of ozone from the ground to the upper atmosphere over the entire Antarctic continent.

In addition, scientists from US National Oceanic and Atmospheric Administration's (Noaa) Earth System Research Laboratory in Boulder, Colorado, used balloon-borne instruments to measure ozone directly over the South Pole.

Nasa launched its Aura satellite in July 2004

By 9 October, the total column ozone had plunged to 93 Dobson Units (DU) from approximately 300 DU in mid-July. Dobson Units are a measure of ozone amounts above a fixed point in the atmosphere.

More importantly, nearly all of the ozone in the layer between eight and 13 miles above the Earth's surface had been destroyed. In this critical layer, the instrument measured a record low of only 1.2 DU, having rapidly plunged from an average non-hole reading of 125 DU in July and August.

"These numbers mean the ozone is virtually gone in this layer of the atmosphere," said David Hofmann, from the Noaa's Earth System Research Laboratory.

"The depleted layer has an unusual vertical extent this year, so it appears that the 2006 ozone hole will go down as a record-setter."

Annual variation

Observations by Aura's Microwave Limb Sounder show extremely high levels of ozone-destroying chlorine chemicals in the lower stratosphere - about 20km (12.4 miles) high.

These high chlorine values covered the entire Antarctic region in mid to late September. The high chlorine levels were accompanied by extremely low values of ozone.

The size and thickness of the ozone hole varies from year to year, becoming larger when temperatures are lower.

Because of the international agreements banning ozone-depleting substances, researchers have calculated that these chemicals peaked in Antarctica in 2001 and have been declining.

However, many of them have extremely long lifetimes once released into the air. While there are year-to-year variations, scientists expect a slow recovery of the ozone layer by the year 2065.