SSC
An imaginary line on a map that joins places having equal atmospheric (barometric) pressure is called an:
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Answer: Option A — Isobar
Detailed Solution
An isobar is a line drawn on a weather map joining places that have the same atmospheric pressure at a given time, usually after the pressure readings have been reduced to sea level.
Meaning of the word:
"Iso" means equal and "baros" means weight or pressure in Greek.
Why pressure is reduced to sea level:
Atmospheric pressure decreases with height. To compare pressures at places with different altitudes, the readings are adjusted to what they would be at sea level.
How isobars are used:
- Isobars show the pattern of high-pressure and low-pressure areas.
- A low-pressure area (depression or cyclone) is shown by a set of closed isobars with the lowest pressure at the centre.
- A high-pressure area (anticyclone) is shown by closed isobars with the highest pressure at the centre.
Pressure gradient and wind:
- The spacing between isobars shows the pressure gradient, the rate at which pressure changes over distance.
- Closely spaced isobars indicate a steep pressure gradient and strong winds.
- Widely spaced isobars indicate a gentle gradient and light winds.
- Winds blow from high pressure towards low pressure, but are deflected by the Earth's rotation (the Coriolis force), so they blow almost parallel to the isobars at higher levels.
Units of pressure:
- Atmospheric pressure is usually measured in millibars (mb) or hectopascals (hPa); 1 mb = 1 hPa.
- The average sea-level pressure is about 1,013 mb.
- The instrument used to measure pressure is the barometer, such as the mercury barometer or the aneroid barometer.
Other isolines (lines of equal value):
- Isotherm: equal temperature.
- Isohume: equal humidity.
- Isoneph: equal cloud cover.
- Isoseismal line: equal intensity of an earthquake.
- Contour line: equal height above sea level.
- Isogonic line: equal magnetic declination.
Why not the other options?
- ✗An isotherm joins places of equal temperature.
- ✗An isohume joins places of equal humidity.
- ✗An isoneph joins places of equal cloudiness.
Quick tip
Remember: Isobar = equal pressure; closely spaced isobars = strong winds; average sea-level pressure = about 1,013 mb.
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Atmosphere, Weather & Climate — Preparation
This question tests your understanding of Atmosphere, Weather & Climate, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers the composition and layers of the atmosphere, insolation and temperature, atmospheric pressure and the planetary winds, local winds, humidity, clouds and rainfall, cyclones and anticyclones, and the climatic regions of the world. Exam questions often test the layers of the atmosphere in order (troposphere, stratosphere, mesosphere, thermosphere and exosphere), the ozone layer in the stratosphere, and local winds such as the Loo (India), Chinook (North America) and Foehn (Alps). Learn the names given to tropical cyclones in different regions: hurricanes, typhoons and willy-willies. Practise the atmosphere, weather and climate questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.