CDS (II) 2023 General Knowledge · Question 97 of 120

Correct — A, 1, 2 and 3. Three of the four pairs hold and the fourth swaps two very similar-looking words. An isonif joins places with the same snowfall, the root being the Latin nix, nivis for snow. An isohel joins places receiving the same amount of sunshine — Greek helios, the sun — and appears on solar-radiation and sunshine-duration maps. An isohaline joins points of equal salinity, from Greek hals, salt, and is the everyday tool of oceanographers mapping how far a river's freshwater plume pushes into the sea. The pair that fails is the fourth. An isohypse is a line of equal height — Greek hypsos — which on a topographic sheet is an ordinary contour and on a weather chart is the geopotential-height line drawn on a constant-pressure surface such as 500 hPa. Cloudiness has its own line, the isoneph, from Greek nephos, cloud. Swap isohypse for isoneph in pair 4 and all four would be right; as printed, only 1, 2 and 3 are.

• (b)2, 3 and 4 — Keeps the one broken pair, isohypse with cloudiness, and throws away the sound snowfall pair.
• (c)1 and 3 only — Rejects isohel for sunshine, which is right — helios is the sun, and sunshine-duration maps are drawn with isohels.
• (d)1, 2 and 4 — Drops the salinity pair, the least disputable of the four, and retains the height-for-cloudiness error.

Any line joining points of equal value for some quantity is an isoline, and a map built out of such lines is an isopleth map. The family is named by Greek or Latin roots welded to the prefix iso-, equal: isobar for pressure, isotherm for temperature, isohyet for rainfall, isobath for depth below water, isohypse for height, isohel for sunshine, isohaline for salinity, isoneph for cloud cover, isonif for snowfall. Learning the root rather than the whole word is what makes an unseen member of the family readable.

The item is decided by pair 4, and the trap is that isohypse and isoneph both sound like weather words. Hypsos is height, so an isohypse belongs with contours and with the 500 hPa charts forecasters read; nephos is cloud, so cloud cover is the isoneph. Once pair 4 is out, options (b) and (d) go with it, and the choice narrows to (a) against (c) — that is, to whether isohel really means sunshine. It does, and standard isoline glossaries carry it, so the key's (a) follows. One honest note on sourcing: isonif is much the rarest of the four terms and does not appear in the general dictionaries or in the standard isoline list this card was checked against, though it is common in Indian geography texts. The answer does not rest on it, because isohel alone separates (a) from (c).

• An isohypse is a line of equal elevation or geopotential height — the ordinary contour on a topographic sheet, and the height line on a constant-pressure weather chart.
• Cloud cover is mapped by isonephs, from the Greek nephos for cloud.
• An isohel is a line of equal sunshine or solar radiation, from Greek helios.
• An isohaline joins points of equal salinity in the ocean, from Greek hals, salt.
• A map drawn with isolines of any kind is an isopleth map, as against choropleth maps, which shade whole areas instead.

• Reading isohypse as a cloud word because it sits next to weather terms in the list — it is the height line.
• Confusing isohyet, for rainfall, with isohypse, for height; one letter separates them.
• Assuming every iso- word is meteorological. Isobath, isohaline and isohypse are oceanographic or topographic.

Usually as a matching item pairing isoline names with the quantities they map, or as a one-line question asking what a named isoline joins.

• practice — not a real PYQ

  Lines drawn on a map to join places having equal cloud cover are called

  • (a)Isohels
  • (b)Isonephs
  • (c)Isohypses
  • (d)Isohalines

  Answer(b) Isonephs — from the Greek nephos, cloud. Isohels map sunshine, isohypses map height and isohalines map salinity.

• practice — not a real PYQ

  On a 500 hPa upper-air chart, the contours drawn by a forecaster are known as

  • (a)Isobars
  • (b)Isotherms
  • (c)Isohypses
  • (d)Isohyets

  Answer(c) Isohypses — on a constant-pressure surface the contour drawn is a line of equal geopotential height, which is what isohypse means.