NDA (II) 2017 General Ability Test · Question 149 of 150

Correct — A, the formation and initial stage. All three characteristics listed are preconditions, the things that must already be present before a cyclone can come into being at all. A sea surface warmer than about 26 degrees Celsius supplies the water vapour and latent heat that fuel the system — this is the classic threshold, established by Palmen in 1948, below which tropical cyclones do not form. Deep moist air lets the rising vapour condense into towering cumulus rather than evaporating away. Atmospheric instability allows that air to keep rising once it starts. Together they describe genesis, not what the storm is like once it has formed. The later stages are defined by the storm's own behaviour — falling central pressure and rising wind speeds as it intensifies, a fully developed eye and spiral rainbands at maturity, and a rising central pressure with weakening winds as it decays after losing its warm moist supply.

• (b)Modification stage — Modification describes changes the storm undergoes once it is already established, typically as it moves over cooler water or over land and its structure begins to alter. That is a stage about the storm, not about the environment that had to exist before it.
• (c)Full maturity — Maturity is when the storm reaches maximum intensity — central pressure at its minimum, surface winds at their maximum, spiral bands of giant cumulus wrapped around a relatively calm and cloud-free eye. It is characterised by the cyclone's own features, not by ambient sea temperature and humidity.
• (d)Decay — The exact reverse of the listed conditions. A cyclone decays precisely when the supply of warm, moist air is cut off — on landfall or over cooler sea — so a warm ocean and deep humid air describe the beginning of its life, not the end.

A tropical cyclone is a warm-core low-pressure system that draws its energy from the latent heat released when water vapour evaporated from a warm ocean condenses in rising air. Standard accounts divide its life into stages that begin with formation and initial development, run through intensification to full maturity, and end in modification and decay. The conditions needed for genesis are a sea surface temperature above about 26 degrees Celsius over a sufficient depth, abundant moisture through a deep layer of the atmosphere, an unstable atmosphere, enough Coriolis force to set a vortex spinning, weak vertical wind shear and a pre-existing low-level disturbance.

Two points about how this item is printed are worth noting, neither of which changes the answer. The correct option reads 'Formulation and initial stage', where standard descriptions of the cyclone life cycle call it the formation and initial development stage. And the humidity condition is printed as a height of more than 700 metres, whereas the usual textbook statement puts the requirement of high relative humidity through a much deeper layer, above about 7,000 metres, or equivalently up to the 700-millibar level. The physical content is the same either way — the air must be moist well above the surface — and the answer turns on recognising that all three items are pre-conditions rather than on the exact figure.

• Tropical cyclones need a sea surface temperature above about 26 degrees Celsius; Palmen showed in 1948 that they do not form below this threshold.
• High relative humidity through the lower and middle troposphere is required, because it maximises latent heat release.
• Atmospheric instability, weak vertical wind shear and a pre-existing low-level disturbance are also genesis conditions.
• The Coriolis force must be strong enough to set a vortex spinning, which is why cyclones do not form within a few degrees of the equator.
• At full maturity the central pressure is lowest, winds are strongest, and a calm eye is surrounded by spiral bands of cumulus.
• Decay begins when the supply of warm moist air is cut off, on landfall or over cooler water.

• Reading conditions of the environment as descriptions of the storm's own structure.
• Assuming a warm ocean alone is enough; instability, moisture depth, weak shear and Coriolis force all matter.
• Confusing the eye, which is calm, with the eyewall, where the winds are strongest.

Cyclones are asked as a stage-identification item like this one, as a list of formation conditions to be filtered, or as a naming question across ocean basins.

• practice — not a real PYQ

  Tropical cyclones rarely form within about five degrees of the equator because

  • (a)the sea surface is too cool there
  • (b)the Coriolis force is too weak to set up a vortex
  • (c)the air is too dry
  • (d)vertical wind shear is very strong

  Answer(b) the Coriolis force is too weak to set up a vortex — it falls to zero at the equator.

• practice — not a real PYQ

  A tropical cyclone weakens rapidly after landfall mainly because

  • (a)friction with the land increases its wind speed
  • (b)its supply of warm moist air from the sea is cut off
  • (c)the Coriolis force disappears over land
  • (d)the atmosphere becomes unstable

  Answer(b) its supply of warm moist air from the sea is cut off — the latent heat that fuels it is no longer available.