Which one among the following is not an atmospheric natural disaster?

Correct — D, Avalanche. Disaster taxonomies sort hazards by the sphere in which the event happens, not by what triggered it. Drought is an atmospheric hazard because it is a prolonged deficiency of precipitation; hailstorm is atmospheric because hail is grown by repeated circulation of ice pellets in the strong updraughts of a cumulonimbus cloud; frost is atmospheric because it forms when the air layer near the ground radiates its heat away on a clear, still night and water vapour deposits as ice. In all three the atmosphere is where the event occurs. An avalanche is a mass movement — a slab or mass of snow already lying on a slope loses its grip and accelerates downhill under gravity. Heavy snowfall or a rapid thaw may set it off, but so may a skier or an earth tremor, and the disaster itself is the movement of material on the land surface. That places it with landslides, rockfalls and slumps in the terrestrial group.

• (a)Drought — Atmospheric, and the one most often misfiled. A drought is defined by rainfall failing over a season or longer; the soil-moisture and hydrological shortages that follow are consequences, not the hazard itself.
• (b)Hailstorm — Atmospheric. Hail forms inside a cumulonimbus, where an ice pellet is carried up and down through the freezing level repeatedly, gaining a layer of ice each time until the updraught can no longer hold it.
• (c)Frost — Atmospheric. Radiation frost forms on clear, calm nights when the ground and the air immediately above it cool below freezing and vapour deposits directly as ice. It is a serious hazard for horticulture in northern India in December and January.

Natural hazards are conventionally grouped by origin: atmospheric (cyclones, hailstorms, droughts, frost, lightning, cloudbursts), terrestrial (earthquakes, volcanoes, landslides, avalanches, subsidence), aquatic (floods, tsunamis, storm surges) and biological (epidemics, locust swarms). The group tells you which agency forecasts it and which measures reduce it.

The grouping is a classification convenience rather than a statement about causation, and avalanches show why. Almost every avalanche is preceded by weather — a heavy snowfall loading a slope, a warm spell weakening a bonded layer, wind building a cornice — so the meteorological services do issue avalanche warnings. In India that work is done by the Defence Research and Development Organisation's Defence Geoinformatics Research Establishment at Chandigarh, which serves the Himalayan sectors where the Army operates. But the hazard itself is snow moving downhill, which is why it is counted with landslides. The same reasoning puts a rain-triggered landslide in the terrestrial group even though rain caused it.

• Atmospheric hazards include cyclones, hailstorms, droughts, frost, lightning and cloudbursts.
• Terrestrial hazards include earthquakes, volcanic eruptions, landslides, avalanches and subsidence.
• Hail grows by repeated ascent and descent through the freezing level in a cumulonimbus updraught, adding a layer of ice each cycle.
• Radiation frost needs clear skies and calm air, which allow the ground to lose heat rapidly after sunset.
• Avalanche forecasting for the Indian Himalaya is handled by DRDO's Defence Geoinformatics Research Establishment (DGRE), Chandigarh.

• Classifying by trigger rather than by event. Heavy snow causes many avalanches, but the avalanche is still a mass movement.
• Treating drought as a hydrological or agricultural hazard first. Meteorological drought — the rainfall deficit — is the atmospheric origin.
• Assuming frost is too mild to count as a disaster. Frost damage to horticulture is a recognised hazard in northern India.

As an odd-one-out on hazard classification, or as a match-the-list pairing hazards with their sphere of origin. Occasionally reversed, asking which of four IS atmospheric.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which one of the following is classified as a mass movement rather than an atmospheric hazard?

  • (a)Cloudburst
  • (b)Landslide
  • (c)Lightning
  • (d)Hailstorm

  Answer(b) Landslide — material moving downslope under gravity, like an avalanche, whatever the trigger.

• practice — not a real PYQ

  Hail is formed mainly in which one of the following cloud types?

  • (a)Cirrus
  • (b)Stratus
  • (c)Cumulonimbus
  • (d)Altostratus

  Answer(c) Cumulonimbus — only its deep, strong updraughts can cycle a pellet through the freezing level repeatedly.