Alpine Glow

Alpine glow, also known as Alpenglow, is a striking natural optical phenomenon observed in mountainous regions, where the peaks or upper slopes of mountains are illuminated with a reddish, pink, or golden hue, typically seen just after sunset or just before sunrise. It occurs when the Sun is below the horizon and its light is scattered and reflected in the atmosphere, casting a warm glow on high-altitude landscapes.

Meaning and Origin

The term “Alpenglow” originates from the German word Alpenglühen (Alpen – Alps, glühen – glow), referring to the brilliant reddish illumination often observed on the peaks of the European Alps. Although the phenomenon is most famously associated with the Alps, it can be seen in any mountainous region under suitable atmospheric conditions, including the Himalayas, the Rockies, the Andes, and other high ranges across the world.

Conditions Required for Alpine Glow

For Alpine glow to occur, several specific conditions must coincide:

1. Sun Below the Horizon:
   • The Sun must have set or not yet risen, positioned just below the horizon so that direct sunlight cannot reach the lower atmosphere or the observer’s position.
2. Clear Atmosphere:
   • The air must be clean and free of dust or heavy moisture to allow scattering of sunlight across the atmosphere.
3. High Elevation:
   • Mountains or elevated terrains are required to intercept and reflect the indirect, scattered sunlight that creates the effect.
4. Reflective or Snow-Covered Surface:
   • Snow, ice, or pale rock surfaces enhance the visibility of the glow by reflecting light more effectively.
5. Right Angle of Illumination:
   • The scattering of sunlight at a low angle produces the characteristic red or pink hue, as shorter blue wavelengths are scattered out and only longer red wavelengths remain visible.

Mechanism of Formation

Alpine glow results from indirect solar illumination rather than direct sunlight. Its appearance is caused by two main optical processes — Rayleigh scattering and reflection/refraction of light in the lower atmosphere.
1. Before Sunrise or After Sunset:

• When the Sun is below the horizon, its light travels through the atmosphere at a very low angle.
• Shorter wavelengths (blue and violet) scatter out, while longer wavelengths (red and orange) bend and reflect off atmospheric particles and clouds.
• These refracted rays illuminate the mountain peaks from below the horizon, creating the reddish or pinkish hue.

2. During Twilight:

• As the Sun moves further below the horizon, the effect gradually fades.
• The glow is strongest when the Sun is between 4° and 6° below the horizon, corresponding to the period known as civil twilight.

Types of Alpine Glow

Alpine glow can occur in two main forms, depending on whether it involves direct or indirect sunlight:
1. Direct Alpenglow:

• Happens when the Sun is still slightly above the horizon and directly illuminates the mountains.
• The light is warm and golden, transitioning to pink or red hues as the Sun sets.
• Commonly seen in late afternoon or early morning on clear days.

2. True (Indirect) Alpenglow:

• Occurs when the Sun is completely below the horizon.
• The glow is produced purely by scattered and reflected light, without any direct sunlight.
• Appears as a diffuse red or pink illumination on the peaks or clouds above them.

Colour Variation and Duration

The colour of Alpine glow changes with time and atmospheric composition:

• Early Stage: Golden yellow or orange hues dominate as sunlight grazes the atmosphere.
• Peak Intensity: Deep pink, rose, or crimson tones appear at twilight.
• Late Stage: Colours fade into violet or bluish shades as night sets in.

The phenomenon usually lasts only a few minutes, though under ideal conditions it can persist for up to 15–20 minutes.

Best Regions to Observe Alpine Glow

While the phenomenon can occur in any mountainous region, it is most frequently observed in areas with high, snow-covered peaks and clear atmospheric conditions:

• The Alps (Europe): Especially visible on peaks such as Mont Blanc and the Matterhorn.
• The Himalayas (Asia): Mount Everest, Kanchenjunga, and Nanda Devi often display striking Alpine glow at dawn and dusk.
• The Rocky Mountains (North America): Well-observed in Colorado and Alberta.
• The Andes (South America): Prominent on snow-capped Andean peaks during the dry season.
• New Zealand and the Southern Alps: Offer vivid examples during clear evenings in winter.

Scientific Explanation of Colour

The reddish tint of Alpine glow results from selective scattering of light in the atmosphere:

• When sunlight travels a longer path through the atmosphere at low angles, shorter wavelengths (blue, green) are scattered out by air molecules.
• The remaining red and orange wavelengths dominate the reflected light that reaches the mountain peaks.
• Snow and ice further amplify the effect by acting as reflective surfaces that diffuse the coloured light across the landscape.

Difference Between Alpine Glow and Sunset/Sunrise Glow

Significance and Appreciation

Alpine glow holds both scientific and aesthetic significance:

• In Meteorology: It provides clues about atmospheric clarity, particle concentration, and moisture levels.
• In Geography and Photography: It is a subject of fascination for photographers and travellers, symbolising the beauty and tranquillity of high mountain environments.
• In Culture: Poets and artists have long celebrated the Alpine glow as a symbol of purity, peace, and the majestic serenity of nature.