GATE CS 2017 Set 2 Q.65 (GA Q.10): Data Interpretation and Graphs

An air pressure contour line joins locations in a region having the same atmospheric pressure. The following is an air pressure contour plot of a geographical region. Contour lines are shown at 0.05 bar intervals in this plot.

If the possibility of a thunderstorm is given by how fast air pressure rises or drops over a region, which of the following regions is most likely to have a thunderstorm?

Air pressure contour plot with contour interval 0.05 bar and a 0 to 2 km scale bar. Labelled contours: 0.65 and 0.7 at the top left; 0.9 and 0.95 around a closed high on the left containing region P; 0.9 and 0.95 around a closed high on the right containing region S; 0.8 at the bottom and between Q and S; 0.75 at the bottom right. Region R at the top left lies where the 0.65 and 0.7 contours are crowded close together; region Q at the bottom centre lies between widely spaced contours.

Official answer and solution

Official answer: C
From: Official GATE 2017 answer key

Solution

Relate the possibility of a thunderstorm to the spatial rate of change of air pressure (pressure gradient), which is geometrically represented by the density of contour lines.

The problem states that the possibility of a thunderstorm is determined by how fast air pressure rises or drops over a region, which is the spatial pressure gradient:

\left| \frac{\Delta P}{\Delta s} \right|

In a contour plot:

Contour lines represent lines of constant air pressure (separated by equal intervals of 0.05 bar).
The closer together the contour lines are in a given geographical region, the larger the change in pressure over a given distance (i.e., the steeper the pressure gradient).
Conversely, widely spaced contour lines indicate gentle or slow pressure variation.

Examine the four candidate circular regions:

Region R: Multiple contour lines (0.65 bar, 0.70 bar, etc.) pass through this small circular area in very close proximity. The contour lines are the most densely packed here, indicating the highest spatial rate of change of air pressure.
Regions P and S: These encompass local centers of pressure (enclosing closed loops of 0.90 and 0.95 bar), where the contours are more broadly spaced within the circular zones.
Region Q: Positioned in an area with widely spaced contours (near 0.80 bar), indicating very low pressure gradient.

Therefore, region R has the steepest pressure gradient and is the most likely to experience a thunderstorm. This corresponds to option C.

Final answer

Common trap

Selecting the region with the highest absolute pressure (P or S) instead of the region with the steepest rate of change (gradient) of pressure (R).

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2017 Computer Science and Information Technology (CS) Session 2
Question: 65 of 65, printed as GA Q.10
Section: General Aptitude

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