Why are pressure values often adjusted to a common reference level on isobar maps used for large-area comparison?

Concept overview / definitionIsobars are lines joining places with equal atmospheric pressure on a map. To compare pressure patterns across large areas, pressure values are often adjusted to a common reference so that places at different elevations can be meaningfully compared.

Why the correct option is correctIf elevation were left unadjusted, high-altitude stations would naturally show lower pressure mainly due to height, not because of weather systems. So for surface charts, pressures are commonly reduced to a common reference level to show true horizontal pressure patterns.

Why the other options are incorrectTreating plotted pressure as raw station pressure would distort patterns over mountains. Saying height is “fully included” in isobar placement defeats the purpose of comparing surface systems. Also, isobars do not represent temperature or rainfall; they are specifically pressure contours.

UPSC exam tip / common confusionIn Prelims, watch the wording: “station pressure” versus “mean sea level pressure.” Most synoptic surface charts use sea level reduction so that you can read highs and lows without being misled by topography.