Friday, February 01, 2013

Fern frost

click photo to enlarge
Those of us who grew up in 1950s and 1960s Britain will remember frost patterns on the windows of the houses in which we lived. Sometimes they were on the outside but often we'd wake up in winter to find them on the inside. Double glazing and central heating have banished frosted windows for most people, but the absence of those two conveniences will result in the phenomenon even today.

They occur inside when there is air below freezing point outside and moist air inside that condenses on the cold glass as ice crystals. What has always fascinated me - and everyone I've spoken to about this kind of frost - is the form of the patterns that are produced. The most common type seem to be a shape similar to leaves or fern fronds. Quite how and why this shape results is a mystery to me, and a quick search doesn't produce a clear or detailed answer. It seems that the composition of the surface of the glass, particularly its imperfections, are contributory factors, but a fuller explanation is not easily found.

Today's photograph was taken on a recent early morning after a cold night. The icy patterns, commonly called fern frost, were on the outside of the windscreen of a parked car. The low sun that was obscured by trees was strong enough to produce a directional light that emphasised the details of the delicate "leaves".

photograph and text © Tony Boughen

Camera: Canon
Mode: Aperture Priority
Focal Length: 100mm
F No: f8
Shutter Speed: 1/100
ISO: 500
Exposure Compensation:  -0.33 EV
Image Stabilisation: On