Question

Which one of the following spectral ranges is the most suitable for distinguishing snow from cloud?

• A. 1.55 μm - 1.75 μm
• B. 0.45 μm - 0.52 μm
• C. 0.52 μm - 0.60 μm
• D. 0.76 μm - 0.85 μm

Hint:

Snow and cloud look almost identical in the visible bands; look for the wavelength range where ice absorption makes snow reflectance drop sharply while cloud stays bright.

Answer 1

The Correct Option is A

Step 1: Recall why snow and cloud look similar in the visible range.
Both snow and cloud consist of countless small ice or water particles that scatter visible light very efficiently, so in the visible bands, roughly 0.45 μm to 0.60 μm, both appear bright white with very high and nearly equal reflectance. Any spectral range inside the visible window therefore cannot separate the two.
Step 2: Recall the ice absorption feature used to separate them.
Ice has a strong absorption feature in the Short-Wave Infrared (SWIR) region around 1.5 μm to 1.8 μm. Snow, being made of ice crystals, absorbs strongly in this range, so its reflectance drops sharply.
Step 3: Contrast this with cloud behaviour in the same range.
Clouds are mostly composed of liquid water droplets (or, for high cirrus, much smaller ice crystals with different optical path lengths) that remain highly reflective in this SWIR window because the absorption feature that suppresses snow reflectance is far weaker for typical cloud particle sizes and optical thickness. This creates a strong reflectance contrast between snow (dark) and cloud (bright) specifically in this band.
Step 4: Match this to the given options.
Option (A), 1.55 μm to 1.75 μm, falls exactly in this diagnostic SWIR absorption window, and is in fact the operational basis for well-known snow indices used by satellite sensors such as MODIS and Landsat to separate snow from cloud.
Step 5: Rule out the remaining options.
Options (B) 0.45-0.52 μm, (C) 0.52-0.60 μm, and (D) 0.76-0.85 μm all lie in the visible or near-infrared range, where both snow and cloud remain strongly and similarly reflective, so none of them can reliably tell the two apart.
Step 6: Conclude.
The SWIR range around 1.55 μm to 1.75 μm is the range that best distinguishes snow from cloud.
\[ \boxed{1.55\ \mu m\ \text{to}\ 1.75\ \mu m} \]