Penitentes: where art thou?

 Hidden in amongst the ice penitentes above is PVL PhD student Giang Nguyen! Original image "Penitentes ice formations at the southern end of the Chajnantor plain in Chile in 2005." credit: ESO, https://www.eso.org/public/images/img_1824/

By Tue Giang Nguyen

A great deal of my research recently has been dealing with atmosphere-surface interactions. In conjunction with my survey for dunes on the Martian polar cap, I’ve also been looking for surface features called penitentes. For the uninitiated, penitentes are ice and snow blade structures common in tropical alpine regions such as the Andes and the Himalayas. Although Darwin thought that these ice blades were sculpted by the wind, later glaciological research proved otherwise.

Penitentes are formed by uneven heating and subsequently sublimation of an icy surface. Imagine a pair of mirrors held together forming a v-shape with their reflective surface pointing up. Now from above, if you shine a laser onto the v-shape mirrors, you will probably see that the light will bounce between the walls sending light towards the bottom of the v. This dynamic of light bouncing off the reflective side walls and concentrating towards the depression is how the ridges and troughs of penitentes take their shape. As the trough receives more heat and sublimate more water, it deepens while the side walls receive less heat and sublimate more slowly.

The formation of penitentes is dependent on incoming sunlight. Therefore the higher the sun, the easier it is for penitentes to grow. The literature indicates that penitentes can grow up to 55ºN normally; beyond this latitude their growth would require special overcast conditions and sloped terrains to help penitente formation. And although Canada is stereotyped as The Great White North, I don’t think I’ve actually ever seen penitentes.

Toronto is located 43º N which falls in the latitudinal range where penitentes can grow “normally”. However, the “normal” conditions for penitente growth involves constant sunlight, low humidity, temperature close to 0º C, and light winds. And though I wish for the weather to be like this for the upcoming days, winter is rarely ever so nice.

 Low humidity is almost a guarantee for most of the cold winter season. It’s the combination of sunny, warm temperature and light wind that is particularly rare for Toronto’s and, similarly, Montreal’s winter. Those living in Toronto and Montreal know how intensely cold and windy sunny winter days are. This kind of weather is often caused by a high-pressure system in the polar region migrating south-eastward bringing absolute dread to anyone who has to be outside. However uncommon the “ideal” conditions for penitente formation are, late fall and early spring might be the time that’s ripe for these ice structures.

I’ve read a report of penitentes forming near Boston back in 1939. From March 21-24, clear skies along with low humidity and near 0º C temperature after intense snowfall caused penitentes to form. And since Boston is relatively close to Toronto (720 km) from a global perspective, it’s entirely believable for Toronto to experience similar conditions. Maybe the reasons penitentes are so elusive, to me at least, has to do with human activities.

I think you’d agree that we’re more urbanized since 1939. Maybe an undisturbed field of snow is actually really hard to come by when you’re living in the biggest city of Canada. Maybe penitentes form all the time in the prairies whenever the chinook winds come barrelling in, and no one notices (though light winds do seem to be required for their formation). Or maybe people do notice but it’s just so uneventful that it’s not even worth talking about. People might even mistaken penitentes for sastrugi.

Sometimes when you can’t find something, you’ve got to make them yourself. With enough snow, you can grow your own penitentes in the backyard. Shining light onto fresh snow overnight and small jagged ice blades may appear the next morning. If it worked in Germany in the 1940s, it should work now. The things to pay attention to are the wind conditions and humidity. Wind can act to erode or ablate the snow/ice surface while humidity can make the air saturated which inhibit the sublimation of the troughs.

Ultimately, I don’t think I’ll get to see penitentes anytime soon. I will, however, occasionally look at a snow field and scan to see any grooves or waves starting to take shape. And you, dear reader, can now take part in hunting for these ice structures as well. And like the stars in the skies, you might be better off looking for penitentes out in the country. With the upcoming winter with its timely abundance of snow, keep an eye for penitentes – I know I’ll be doing so.