How is strong pressure gradient represented on a weather map?
- (a)Closely spaced isobars
- (b)Widely spaced isobars
- (c)Closely spaced isobars at the periphery
- (d)A set of two widely spaced isobars followed by two closely spaced isobars
Correct — A, Closely spaced isobars. An isobar joins places of equal atmospheric pressure, reduced to sea level so that altitude does not distort the map. The pressure gradient is the rate at which pressure changes with distance, and NCERT states the rule in one line: the pressure gradient is strong where the isobars are close to each other and weak where the isobars are apart. The reasoning is the same as with contour lines on a topographic map, where crowded contours mean a steep slope. If pressure falls by the same amount between successive isobars, then the shorter the distance in which that fall occurs, the steeper the change and the stronger the force pushing air across it. Since the pressure gradient force is what sets wind moving in the first place, tightly packed isobars on a weather chart are also the sign of strong winds — which is why the isobars around a deep cyclone are drawn almost on top of one another.
- (b)Widely spaced isobars — The opposite case. Isobars far apart mean pressure changing slowly over a long distance, a weak gradient and light winds.
- (c)Closely spaced isobars at the periphery — Adds a condition the definition does not have. The gradient is strong wherever the isobars crowd, at the edge of a system or at its centre.
- (d)A set of two widely spaced isobars followed by two closely spaced isobars — Describes a chart on which the gradient changes from weak to strong across the map, not a representation of a strong gradient as such.
Horizontal pressure differences are what make wind. Three forces then act on air moving near the surface: the pressure gradient force, which pushes air from high pressure towards low; the Coriolis force, which deflects it right in the northern hemisphere and left in the southern; and friction, which is greatest at the ground and fades out by about one to three kilometres up. Above the friction layer the pressure gradient and Coriolis forces come into balance and the wind blows parallel to the isobars as the geostrophic wind. Near the surface friction drags the wind across the isobars at an angle, which is why air spirals inward around a low and outward around a high.
The map-reading analogy is the whole answer. Contours crowd where the ground is steep; isobars crowd where the pressure falls steeply. Options that add qualifications about the periphery or about alternating spacing are there for the candidate who suspects that a simple answer must be a trap. It is not. One further use for the same picture: the vertical pressure gradient is far larger than the horizontal one, but it is almost exactly cancelled by gravity, which is why we feel horizontal winds and not violent updraughts.
- Isobars are lines joining places of equal pressure, drawn after pressure has been reduced to sea level.
- NCERT states that the pressure gradient is strong where isobars are close to each other and weak where they are apart.
- Surface winds respond to three forces — the pressure gradient force, the frictional force and the Coriolis force.
- Friction is greatest at the surface and its influence extends up to about one to three kilometres; over the sea it is minimal.
- The vertical pressure gradient force is much larger than the horizontal one but is balanced by an almost equal and opposite gravitational force.
The same logic as contour lines: crowded lines mean a steep change.
- Confusing the pressure gradient with pressure itself; a very low pressure at the centre does not by itself mean a strong gradient.
- Assuming crowded isobars matter only at the edge of a system.
- Forgetting that wind blows along the isobars aloft and across them near the ground, because of friction.
As a one-line reading of a weather map, or as an assertion-and-reason pair on geostrophic wind and the balance of forces.
Daily weather map showing isobars is an example of
- (a) Choropleth map
- (b) Isopleth map
- (c) Chorochromatic map
- (d) Choroschematic map
Answer(b) Isopleth map
The cartographic side of the same chart. An isobar map is an isopleth map, in which lines join points of equal value — the same construction that makes spacing readable as gradient.
Directions : The following six (6) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the correct answer using the code given below. Statement I : Geostrophic wind blows above a height of 600 metres, parallel to the isobars. Statement II : Geostrophic wind is the horizontal wind velocity, in which the Coriolis force balances the horizontal pressure force.
- (a) Both the statements are individually true and Statement II is the correct explanation of Statement I
- (b) Both the statements are individually true but Statement II is not the correct explanation of Statement I
- (c) Statement I is true but Statement II is false
- (d) Statement I is false but Statement II is true
Answer(a) Both the statements are individually true and Statement II is the correct explanation of Statement I
Carries the isobar picture one step further. Once you can read spacing as gradient strength, the next question is what the resulting force does once the Coriolis deflection balances it — and the answer is a wind running along the isobars.
- practice — not a real PYQ
Lines on a weather map joining places of equal atmospheric pressure are called which of the following?
- (a)Isotherms
- (b)Isobars
- (c)Isohyets
- (d)Isohalines
Answer(b) Isobars — isotherms join equal temperatures, isohyets equal rainfall and isohalines equal salinity.
- practice — not a real PYQ
Above about one kilometre, where friction is negligible, the wind blows parallel to the isobars. This wind is called which of the following?
- (a)Geostrophic wind
- (b)Katabatic wind
- (c)Anabatic wind
- (d)Chinook
Answer(a) Geostrophic wind — the state in which the Coriolis force balances the horizontal pressure gradient force.