Air Bubble Formation in Icicles

December 20, 2016

Icicles 3 (1)

December 2016 Viewer's Choice

Photographer: Glenn McCreery
Summary Author: Glenn McCreery

Icicles form and grow when cold water flows downward from an object, such as a roof or tree branch, in sub-freezing air temperatures. Air will dissolve in water whenever water and air are in contact. Although the solubility of air in water increases as water temperature is lowered, when water freezes the solubility of air suddenly becomes very low and previously dissolved air comes out of solution, forming bubbles. If the freezing is sufficiently rapid, with the water freezing radially from the outside walls toward the axis, air bubbles will be trapped in the interior. This usually results in a column of air bubbles oriented along the axis of the icicle, as shown in the left photograph, taken on January 27, 2016.

However, if the ice doesn't grow too quickly, the bubbles will be pushed forward in front of the ice surface, and so the ice will be clear and without bubbles. The right photograph, taken on February 12, 2016, shows icicles that are mostly bubble free, although in the third icicle from the left two bubbles are shown trapped just below the locally bulging surface. This indicates that the bubbles didn't quite have time to make their escape. 

Photo Details: Left - Sony NEX 6 camera; 16-50 mm; f/3.5-5.6 lens; at 24 mm; ISO 200, 1/80 sec. exposure; f/7.1. Right - Canon 5D II camera; 24-105 mm; f/4.0 L lens; at 97mm; ISO 400; 1/160 sec. exposure; f/16.