Why Kol-Tor is that colour

Glaciers do not melt tidily. As they move they grind the bedrock beneath them into a powder so fine that the particles are measured in microns — geologists call it rock flour or glacial silt. Meltwater carries it downhill into the lake, where it stays suspended rather than sinking.

Particles that small scatter short-wavelength light — the blues and greens — while longer wavelengths pass through and are absorbed by the water. What comes back to your eye is that particular opaque turquoise, quite unlike the transparent blue of a deep clear lake.

This is the part most visitors do not expect. The colour depends on how much silt is in the water, and that depends on how fast the glacier above is melting.

In late June and July, at peak melt, the lake is at its brightest and milkiest — almost a swimming-pool turquoise. By September, with less meltwater coming in and the silt slowly settling, the colour deepens into blue-green and the water looks clearer.

Neither is better. July photographs are more startling; September ones are more beautiful. If you want the postcard, come in July. If you want the light and the quiet, come in September.

Scattered light needs light to scatter. Under heavy cloud the turquoise flattens to grey-green and the lake looks like any other. It is worth being at the water in the middle of the day rather than at either end of it, which is why our Kol-Tor tour leaves Bishkek at half past six in the morning.

Not because it is dangerous, but because glacial silt is exactly as pleasant to drink as it sounds. Your guide carries water. The lake is for looking at.