Why does glacier ice look blue?

Glacier ice looks blue because ice, like water, soaks up red light, and a glacier is so thick and so clear that the light has a very long way to travel before it escapes. By the time it comes back out, the red end of the spectrum has been absorbed and what reaches your eye is blue.

You can see the same thing in water. A glass of water looks colourless because a thin layer takes almost nothing from the light. In large quantities, though, water absorbs the other colours more efficiently than blue, so deep water looks blue. Ice is made of the same molecules, and the effect is the same.

So why is the ice in your freezer cloudy white, not blue? Ordinary ice is full of tiny air bubbles, and they bounce light around and turn it white. Deep in a glacier, the pressure of the snow above squeezes the bubbles out and makes the ice dense and clear. With the bubbles gone, light can pass deep into the ice, absorption adds up, and the blue appears.

People often say the blue is the same as the blue of the sky, but that is wrong. The sky is blue because of scattering. Glacier ice is blue because of absorption, the same reason a deep lake is blue.

The colour is fragile. Once blue ice is exposed to warmer air, cracks open in the surface and fill with air, and within hours the surface turns white. The deep blue returns only when the ice breaks or rolls over to show a fresh face. In January 2011, tourists at Tasman Glacier in New Zealand watched an iceberg roll over and reveal startling blue ice that had been kept away from air underwater for months.