Geographical Terms and Isolines: Isobar to Isohyet

Geography has a vocabulary of its own, and a large part of the general knowledge paper is simply a test of that vocabulary. One line of a question gives a definition and the four options give four look-alike words: isohyet and isohel, isobar and isobath, isthmus and strait. The words are of two kinds. The first are the isolines, the lines drawn on a map through places having the same value of something, and the second are the names of landforms, of parts of the atmosphere and the ocean, and of features of the earth's interior. Learn the first by the quantity each line measures and the second by a one-line definition, and this chapter becomes two or three certain marks.

What an isoline is

An isoline, also called an isopleth, is a line on a map joining all points at which a quantity has the same value. The Greek prefix iso means equal, and the second half of the word names the quantity: therme for heat, baros for weight or pressure, hyetos for rain, helios for the sun, bathos for depth. A contour on a survey map is an isoline of height; the closely spaced contours of a steep slope and the widely spaced contours of a gentle slope are the first thing a map-reading question tests. On a weather map, isobars drawn at intervals of a few millibars show the pressure pattern, and their spacing gives the pressure gradient, which decides the strength of the wind.

The isolines asked in examinations

Terms of land and water bodies

• Isthmus: a narrow neck of land joining two larger landmasses, such as the Isthmus of Panama; a strait is the water counterpart, a narrow channel joining two seas, such as the Palk Strait.
• Peninsula: land surrounded by water on three sides; a cape is a pointed piece of land projecting into the sea, and an archipelago is a group or chain of islands.
• Atoll: a ring-shaped coral reef enclosing a lagoon; a fjord is a deep, steep-sided inlet of the sea in a glaciated valley.
• Delta: the depositional plain at a river's mouth; an estuary is a funnel-shaped mouth where no delta forms, as with the Narmada and the Tapi.
• Doab: the land between two rivers; the confluence is where two rivers meet, and a watershed or divide is the high ground separating two drainage basins.
• Canyon and gorge are deep, narrow valleys with steep walls; a mesa is a flat-topped highland with steep sides and a butte is the small remnant left when a mesa is eroded further.

Glacial, desert and karst vocabulary

• Glacial: a cirque is the armchair hollow at a glacier's head, an arête the knife-edged ridge between two cirques, a tarn the lake in a cirque, a moraine the ridge of glacial debris, a drumlin an inverted-boat-shaped mound of till and an esker a winding ridge of sand and gravel left by a sub-glacial stream.
• Desert: a barchan is a crescent-shaped sand dune, a yardang a wind-sculpted ridge, a playa the shallow salty lake of an inland basin, a wadi a dry desert valley and loess the fine wind-blown silt deposited far from the desert.
• Karst: stalactites hang from the roof of a limestone cave and stalagmites rise from its floor; a sinkhole or doline is a surface depression, lapies the grooved limestone surface and a polje a large flat-floored karst basin.

Terms of the atmosphere, the oceans and the interior

The earth is nearest the sun at perihelion, about 3 January, and farthest at aphelion, about 4 July. The equinoxes fall about 21 March and 23 September, when day and night are equal everywhere, and the solstices about 21 June and 22 December. A great circle is any circle whose plane passes through the centre of the earth, so the equator is a great circle and the shortest air route between two places lies along one. In the troposphere the normal lapse rate is about 6.5 degrees Celsius for every 1,000 metres of ascent; when the order is reversed and the air above is warmer, it is called a temperature inversion. Albedo is the proportion of insolation reflected back by a surface, and the Coriolis force is the deflecting force of the earth's rotation, right of the direction of motion in the northern hemisphere. In the ocean, the continental shelf is the shallow margin where the richest fishing grounds lie, a trench is the deepest ocean floor feature, a guyot a flat-topped submerged volcanic peak and upwelling the rise of cold nutrient-rich water along a coast. Inside the earth, the focus is where an earthquake originates and the epicentre the point on the surface above it; the Mohorovicic discontinuity separates crust from mantle and the Gutenberg discontinuity mantle from core, while isostasy is the balance between the standing blocks of the crust.

The look-alikes that decide the mark

• Isohyet is rainfall, isohel is sunshine; both begin with the same two letters.
• Isobar is pressure in the air, isobath is depth in the water.
• The agonic line has zero declination; the aclinic line has zero dip and is the magnetic equator.
• An isthmus is land between two seas; a strait is water between two lands.