Surface Hoar in the Italian Alps
March 25, 2014
Photographer: Elena Gissi
Summary Author: Elena Gissi
During a trek through the Italian Alps earlier this winter, I encountered a large area of snow cover coated with a layer of sharp crystals, mostly shaped like wedges or feathers. Although my first thought was to classify this phenomenon as a strange type of snow, it’s actually one of the forms of hoar frost -- more precisely surface hoar.
Surface hoar can form on a snowpack within a few hours overnight, under specific conditions:
- A clear sky at night, allowing the snow to effectively radiate heat to space and thus become very cold
- Sufficient water vapor to condense on top of the snow surface
- Very weak sunlight or none at all during the day, to prevent melting
- Very weak wind or none at all, to avoid breaking the crystals
- An open space not protected by trees, to maximize nighttime cooling
Despite the beauty of these frost crystals and the pleasant sensation of touching the carpet of ice points with the bare hand, this phenomenon is extremely dangerous when formed on steep slope. Surface hoar, in fact, builds a thin, fragile and disrupting layer between the snow mass underneath and any snow that falls upon it, leading to conditions that promote avalanches.
The hoar crystals that I sampled varied considerably in size -- some were a 1/2 in (1 cm) or more in diameter (bottom photo). The crystals pictured here were taken in two different occasions, but both in the area around the village of Pragelato, in the North-Western Italian Alps.
Photo details: Top - Camera Model: Canon DIGITAL IXUS 870 IS; Focal Length: 5mm; Aperture: f/5.6; Exposure Time: 0.0006 s (1/1600); ISO equiv: 80. Photo taken on November 17,2012. Bottom: Camera Model: Canon DIGITAL IXUS 870 IS; Focal Length: 5mm; Aperture: f/2.8; Exposure Time: 0.0031 s (1/320); ISO equiv: 80; Software: Microsoft Photo Gallery 16.4.3508.205. Photo taken on December 15, 2013.