2024-07-25
Altitude Changes the UV Equation
Most people think about UV protection as a summer beach concern. But UV intensity increases roughly 10 to 12 percent for every 1,000 metres of elevation gain. At the trailhead of a Rockies hike near Canmore or Kananaskis, you might be starting at 1,400 metres. Reach a summit at 2,800 metres and you are experiencing roughly 15 percent more UV than at the parking lot. Add snow or exposed rock that reflects UV back up at you, and the effective dose is even higher.
This is not trivial. Mountaineers who spend extended time above the treeline without proper eye protection develop photokeratitis, snow blindness, at rates that would surprise most casual hikers. It does not require a glacier. A long day above treeline on exposed rock under clear Alberta skies delivers a significant UV load, especially in summer when the sun angle is high and the day is long.
Lens Categories: Understanding the Numbers
Sunglasses are rated by lens category from 0 to 4 based on how much visible light they transmit. This is different from UV protection, which should be UV400 (blocking all UV up to 400 nanometres) regardless of category. The category tells you how dark the lens is, not how much UV it blocks.
| Category | VLT | Best For |
|---|---|---|
| 0 | 80-100% | Fashion, indoor, overcast (barely tinted) |
| 1 | 43-80% | Low sun, partly cloudy |
| 2 | 18-43% | Medium sun, general outdoor use |
| 3 | 8-18% | Bright sun, most outdoor sports |
| 4 | 3-8% | Extreme brightness: glaciers, high altitude snow |
Most everyday sunglasses are Category 2 or 3. For hiking below the treeline on a normal sunny day, Category 3 is appropriate. For hiking above the treeline, on glaciers, or on snow-covered terrain at elevation, Category 4 provides the protection your eyes need.
Important: Category 4 lenses are too dark for driving. They reduce visibility so much that they are illegal for road use in many jurisdictions. Do not wear your glacier glasses on the drive home. Carry a Category 2 or 3 pair for the car.
Side Shields: Not Just for Mountaineers
At altitude, UV and bright light come from every direction. It reflects off snow, rock faces, and even the atmosphere itself. Standard sunglasses leave gaps at the temples where this peripheral light floods in, bypassing your lenses entirely. On a bright day above treeline, you can feel this light hitting the sides of your eyes even with dark lenses on.
Side shields, those leather or fabric panels that attach to the temples and block light from the sides, are the traditional mountaineering solution. They work extremely well. They turn your sunglasses from a forward-facing screen into a complete enclosure that blocks light from every angle.
For casual day hikes below treeline, side shields are overkill. For anything above treeline, on exposed ridges, or near snow, they make a meaningful difference in comfort and protection. Many alpine-style sunglasses have removable side shields so you can add them when you need them and take them off for lower-elevation approaches.
Photochromic: The Hiker's Ideal Lens
Hiking involves more dramatic light transitions than almost any other outdoor activity. You start in a parking lot at dawn, climb through dense forest where the canopy blocks most direct sun, emerge above the treeline into full alpine exposure, and may cross snow or scramble on bright exposed rock. Then you do it all in reverse on the way down, often finishing in the dim light of late afternoon forest.
Photochromic lenses that adjust to these changing conditions are exceptionally well suited to hiking. In the forest, they lighten to let you see roots, rocks, and the trail clearly. Above treeline, they darken to handle the intense brightness and UV. The transitions happen naturally as you move through different environments, and you never have to stop to swap lenses.
Modern photochromic lenses in quality frames react quickly enough for hiking transitions. The move from forest to treeline is gradual enough that the lenses keep up easily. The concern about photochromic lenses being sluggish is mostly outdated, at least for current premium versions. Budget photochromic lenses can still be slow, so quality matters here.
A photochromic lens with a contrast-enhancing base tint, like a light amber or rose that darkens to a deep brown, gives you the best of both worlds. In the forest, the amber base enhances trail contrast. Above treeline, the darkened brown manages the brightness while still providing better terrain definition than plain grey.
Frame Durability and Weight
Hiking puts frames through mechanical stress that everyday use does not. You throw your pack down with the glasses on top. You stuff them into a lid pocket. Branches snag them. You drop them on rock. Frames need to survive all of this without breaking or losing their shape.
Nylon and grilamid (a type of nylon) frames are the standard for hiking eyewear. They are lightweight, flexible, impact-resistant, and hold their shape well across temperature extremes. Metal frames are heavier, can bend permanently, and conduct cold against your skin in winter conditions. Acetate (the material most fashion glasses use) is too brittle for the bumps and drops of trail life.
Weight Matters Over Long Days
On a day hike, the weight of your sunglasses is barely noticeable. On a multi-day backpacking trip where every gram counts, lighter is better. More importantly, heavier frames create more pressure on the nose and ears over long wearing hours. A 25-gram frame feels dramatically different from a 40-gram frame after eight hours on the trail. Lightweight frames with rubber nose pads that distribute pressure evenly across the bridge of your nose will feel best on long days.
Lens Material: Glass vs. Polycarbonate on the Trail
Glass lenses provide superior optics and scratch resistance. Polycarbonate lenses are lighter and virtually shatter-proof. For hiking, the choice depends on what kind of hiking you do.
For well-maintained trails, either works fine. For scrambling, bushwhacking, or anything involving technical terrain where you might fall or take a branch to the face, polycarbonate is the safer choice. It absorbs impact without shattering. Glass does not. A shattered glass lens near your eye in the backcountry, far from medical help, is a serious emergency.
For most hikers, polycarbonate is the practical choice. The optical clarity difference is minor, and the safety and weight benefits are meaningful.
Prescription Hiking Sunglasses
Clear vision on the trail is a safety issue. You need to see where you place your feet, read trail markers, spot wildlife (or wildlife hazards), and navigate terrain accurately. If you wear glasses, having your prescription in your hiking sunglasses eliminates the need to carry two pairs or constantly swap between clear glasses and clip-on shades.
A prescription photochromic lens in a lightweight nylon frame with removable side shields covers the full range of Alberta hiking conditions: forested valley trails, exposed ridgeline scrambles, and everything in between. It is one pair that handles it all.
For strong prescriptions, the lens thickness and weight increase, which is where high-index materials (1.60 or 1.67) help. They keep the lenses thinner and lighter, which matters for comfort on long days and for keeping the total weight reasonable in a wrap-style frame.
The Hiking Sunglasses Checklist
- UV400 protection is non-negotiable at any elevation
- Category 3 for below-treeline hiking in normal sun
- Category 4 or photochromic for above-treeline, snow, or glaciers
- Side shields (removable) for exposed alpine terrain
- Nylon or grilamid frame for durability and light weight
- Polycarbonate lenses for impact safety in the backcountry
- Rubber nose pads for comfort and grip during sweaty climbs
- Retainer strap for scrambles and exposed ridges
If you hike in the Canadian Rockies and want sunglasses that handle everything from the Grassi Lakes trail to a Ha Ling summit scramble, come in and we will match you with the right combination of lens, frame, and protection level for how you use the mountains.