CSIR NET Dec 2015 Q119: Meteorology

Planetary albedo is the fraction of incoming solar radiation reflected back to space by the Earth-atmosphere system as a whole. While snow, ice, and ocean surfaces each reflect varying amounts of sunlight (snow/ice having high albedo but limited areal coverage; oceans having very low albedo and covering most of the surface but reflecting little), clouds cover roughly two-thirds of the Earth's surface at any given time and have comparatively high reflectivity, making them the single largest contributor to the Earth's overall (top-of-atmosphere) planetary albedo, accounting for roughly half of the total reflected shortwave radiation. This matches option 1.

Option 1 is correct: clouds, due to their high reflectivity and extensive average coverage of the globe, contribute the largest single share of Earth's planetary albedo. Option 2 (snow) has high reflectivity but covers only a limited and seasonally variable fraction of the Earth's surface, so its overall contribution to planetary albedo is much smaller than that of clouds. Option 3 (oceans) cover the majority of Earth's surface but have very low albedo (dark, absorbing most incident sunlight), so despite their large area they contribute comparatively little to total reflected radiation. Option 4 (volcanic dust in the stratosphere) can transiently increase albedo after major eruptions, but it is episodic and not a persistent, large-magnitude contributor compared to the continuously present cloud cover.