CDS (I) 2022 General Knowledge · Question 58 of 120

Correct — D, salinity. The prefix iso means equal and the rest of the word names the quantity, so the whole family of isolines can be read off if the root is known. Isohaline comes from the Greek hals, meaning salt, the same root behind halite for rock salt and halophyte for a salt-tolerant plant. An isohaline therefore joins points of equal salinity, and oceanographers draw them to map the saltiness of surface water — high values in the subtropical belts where evaporation exceeds rainfall, low values near the mouths of great rivers and in the polar melt zone.

• (a)temperature. — Equal temperature is an isotherm, from the Greek therme for heat. It is the line drawn on every weather map of temperature distribution, and the one behind terms such as isotherm shift and thermal equator.
• (b)pressure. — Equal pressure is an isobar, from baros for weight. Isobars are what define highs, lows and the pressure gradient on a synoptic chart, and their spacing gives the wind speed.
• (c)rainfall. — Equal rainfall is an isohyet, from hyetos for rain. Isohyets are used to draw rainfall distribution maps, including the familiar Indian maps of the 100 cm and 200 cm lines.

An isoline joins points of equal value of some quantity on a map. The common ones in physical geography are the isotherm for temperature, the isobar for pressure, the isohyet for rainfall, the isohaline for salinity, the isohel for sunshine, the isonif for snowfall and the isohypse or contour for elevation. Ocean salinity is usually expressed in parts per thousand; the world ocean averages about 35 parts per thousand, with the open subtropical oceans higher and estuarine and polar waters lower.

This is a vocabulary item, and the reliable method is to learn the second half of each word rather than the whole term. Therme heat, baros weight, hyetos rain, hals salt, helios sun, nix snow, hypsos height. Once those roots are in place a candidate can decode an isoline never met before, which is exactly what these questions test — papers rotate through the less common members of the family, isohel and isonif in particular, on the assumption that only the first three are widely known. Salinity itself repays a little attention beyond the definition: the pattern of isohalines in the Indian Ocean explains why the Arabian Sea is saltier than the Bay of Bengal, since the Bay receives the Ganga, Brahmaputra and Irrawaddy while the Arabian Sea loses more to evaporation.

• Isohaline joins points of equal salinity; the root is the Greek hals, salt.
• Isotherm joins points of equal temperature, isobar of equal pressure and isohyet of equal rainfall.
• Isohel joins points of equal sunshine and isonif points of equal snowfall.
• Isohypse is the line of equal elevation, more familiar as the contour.
• Average ocean salinity is about 35 parts per thousand, higher in the evaporation-dominated subtropics and lower near large river mouths and melting ice.

• Reaching for isotherm out of habit because it is the isoline most often seen.
• Mixing isohyet with isohel — rain against sunshine.
• Forgetting that a contour is itself an isoline, the isohypse of elevation.

As a single-line definition item, or as a match-the-pairs question listing four isolines against four properties.

• practice — not a real PYQ

  Lines joining places of equal sunshine are called

  • (a)isohyets
  • (b)isohels
  • (c)isonifs
  • (d)isohalines

  Answer(b) Isohels — from the Greek helios, sun; isonifs join places of equal snowfall.

• practice — not a real PYQ

  Which one of the following is another name for a contour line?

  • (a)Isobath
  • (b)Isohypse
  • (c)Isobar
  • (d)Isohaline

  Answer(b) Isohypse — the line of equal elevation above a datum; an isobath is the equivalent for depth below water.