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Buried under the snow of following years, the coarse-grained hoar frost compresses into lighter layers than the winter snow.

As a result, alternating bands of lighter and darker ice can be seen in an ice core.

The proportions of different oxygen and hydrogen isotopes provide information about ancient temperatures, and the air trapped in tiny bubbles can be analysed to determine the level of atmospheric gases such as carbon dioxide.

Cores are drilled with hand augers (for shallow holes) or powered drills; they can reach depths of over two miles (3.2 km), and contain ice up to 800,000 years old.

The physical properties of the ice and of material trapped in it can be used to reconstruct the climate over the age range of the core.

It can make some snow sublimate, leaving the top inch or so less dense.

When the sun approaches its lowest point in the sky, the temperature drops and hoar frost forms on the top layer.

Ice cores have been studied since the early 20th century, and several cores were drilled as a result of the International Geophysical Year (1957–1958).

Depths of over 400 m were reached, a record which was extended in the 1960s to 2164 m at Byrd Station in Antarctica.The bubbles disappear and the ice becomes more transparent.The weight above makes deeper layers of ice thin and flow outwards.These include soot, ash, and other types of particle from forest fires and volcanoes; isotopes such as beryllium-10 created by cosmic rays; micrometeorites; and pollen.The lowest layer of a glacier, called basal ice, is frequently formed of subglacial meltwater that has refrozen.The cuttings (chips of ice cut away by the drill) must be drawn up the hole and disposed of or they will reduce the cutting efficiency of the drill.

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