What is Depth Hoar?

What is Depth Hoar? | Avalanche Safety 101

Welcome to our Avalanche Safety 101 Guide. In this article, we focus on a crucial aspect often overlooked by many outdoor enthusiasts: Depth Hoar. Understanding what Depth Hoar is, its formation, and its impact on avalanche safety is essential for anyone venturing into snowy mountain terrains. 

As we unravel the mysteries of the snowpack, you’ll gain valuable insights into why depth hoar is a key factor in assessing avalanche risks. Let’s start by answering the question: What is depth hoar?

Table of Contents

What is Depth Hoar? A Definition:

According to the American Avalanche Association:

“Depth hoar is a large, striated persistent weak layer that forms at the base of the snowpack.”

Specifically, depth hoar is made up of large, sugary snow crystals that deform within the snowpack, not on top of it (like surface hoar). 

Depth hoar is distinguished by its cup-shaped crystals, which result from a complex process within the snowpack called sublimation.

How Does Depth Hoar Form?

Depth hoar forms when there is a significant temperature gradient in the snow. This causes a vapor gradient as warm air moves up from the insulated ground up to the cold surface of the snow. Some moisture turns from a vapor into ice when it comes into contact with the colder snow layer, creating depth hoar.

This process occurs most often in cold, clear, and calm weather conditions when the snow surface is coldest, especially when the snowpack is still shallow early in the season.

What is Depth Hoar?

Depth Hoar Forms in Shallow Snowpacks

As snow falls and the snowpack gets thicker, the temperature gradient within it becomes less dramatic, and it becomes more difficult for depth hoar to form.

Over time, even depth hoar will settle, stabilize, and form bonds with the snow. However, this occurs very slowly, and depth hoard can continue to be a problem in the snow layer for several weeks before it finally fades away. 

Until this time, depth hoar will remain buried as a persistent weak layer that can cause dramatic, deadly avalanches without warning.

The Role of Depth Hoar in Avalanche Formation

Depth hoar crystals are large and angular and don’t bond together well in the snowpack. They act as a weak layer in the snowpack, making it prone to giving way under stress. Specifically, the facet snow structure allows it to collapse suddenly, which can trigger an avalanche.

When describing the threat of depth hoar, Author and avalanche expert, Bruce Tremper uses the metaphor of a layer of champagne glasses holding up a slab of glass. Because the layer is not shear resistant, as you load more snow on top of the glasses, they eventually snap at the stem, and the entire season’s snowpack comes crashing down.

Where is Depth Hoar Common?

Depth hoar is not a big issue in coastal ranges where snowfall is heavy and the snowpack gets thick quickly. This prevents significant temperature gradients in the snow that are required for depth hoar formation.

In the continental climate of Wyoming and Colorado, with thin snowpacks and lots of cold, clear nights, depth hoard can be a significant concern throughout most of the early and mid-winter. This is one reason why depth hoar is responsible for more deaths than any other type of avalanche weak layer in Colorado.

Identifying Depth Hoar in the Field

Depth hoar is very localized, often around rock outcrops and other areas where the snowpack is thinner. This is problematic, as these spots are often thought of as anchors and safe points – but when the problem is depth hoar, you get a chance, please y become a trigger.

The only way to identify depth hoar is to dig a test pit on the same slope and aspect you plan to hike, ski, or climb. Even this is not foolproof, as depth hoar varies considerably – even a few meters can make a big difference. When you are able, stop to dig multiple pits in a variety of spots to be certain the risk is low.

When looking at the snow, look for a layer of sugary, loose snow near the bottom of the snowpack. Depth hoar varies significantly depending on the conditions when it formed, but is usually visible with some practice and experience. If you spot depth hoar, seek a safer slope and give this one a wide berth.

Depth Hoar: Now You Know

Depth hoar is one of the most dangerous weak layers because it forms within the snowpack invisible to the naked eye when conditions are good, and persists for many weeks or months. When forecasts warn of depth hoar, take note of problem areas and give them a wide berth. Avoiding avalanche terrain is the only sure-proof way to stay safe when the problem is depth hoar. Learn more with our FAQs below; Safe travels on the trails!

FAQs

If we have not addressed your question below, leave a comment and we will get you an answer as soon as possible.

Q: What is depth hoar?

A: Depth hoar refers to a type of snow crystal found within the snowpack. It is characterized by large, cup-shaped, sugary crystals that form under specific weather conditions. These crystals are weak and can contribute to the instability of the snowpack.

A: Depth hoar is caused by a process called sublimation, where water vapor transitions directly from a gaseous to a solid state. This process occurs in cold, dry conditions, typically near the ground where the snowpack is in contact with the colder earth, leading to the formation of these unique crystals.

A: Surface hoar forms on the snow’s surface, resembling frost or feathery crystals, and occurs under clear and calm conditions. Depth hoar, on the other hand, develops within the snowpack, often at the ground level, and has larger, weaker crystals. Both can contribute to avalanche risk but in different ways.

A: Depth hoar facets refer to the individual crystals within a depth hoar layer. These facets are angular, large, and poorly bonded, creating a weak layer in the snowpack. They are a key component in understanding snowpack stability.

A: ‘Hoar’ in snow refers to frost-like ice crystals that can form either on the snow surface (surface hoar) or within the snowpack (depth hoar). It’s a general term for these types of crystals that are known for their weak structural properties.

A: Persistent weak layers are layers within the snowpack that remain unstable over a long period. Depth hoar often constitutes a persistent weak layer because its large, angular crystals don’t bond well with the layers above or below. This can lead to long-term instability in the snowpack.

A: Depth hoar causes avalanches by acting as a weak layer in the snowpack. When a stronger layer of snow forms on top of a depth hoar layer, the weight can cause the weak layer to collapse, triggering an avalanche. This is especially true if the snowpack is disturbed by external forces, such as skiers, snowboarders, or natural events.

Additional Resources

Alex Derr, Founder of The Next Summit

Alex Derr is an Eagle Scout, climber, and environmental policy expert located in Denver, Colorado. He created The Next Summit to help others stay safe exploring the mountains and advocate to preserve the peaks for the future. Follow him on Linkedin or Twitter or click here to contact him.

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Hi, I'm Alex!

In 2018, I watched in disbelief as dozens of people hiked into a storm on Longs Peak, unaware of the extreme danger. Soon after, I started The Next Summit to educate and empower the public to safely and responsibly explore America's mountains.

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