In synoptic weather charts, what does it mean when pressure values are shown as reduced to sea level?

Concept overview / definition On synoptic weather charts the pressure values drawn as isobars are often described as reduced to sea level. This means the values do not show the raw station pressure at the actual height of a hill, plateau or mountain. Instead, all readings are mathematically converted to what they would be if the land surface everywhere was at sea level, so that differences on the map mainly reflect true horizontal pressure contrasts rather than simple height differences.

Why the correct option is correct The correct option states that pressures are adjusted to a common reference by neglecting actual terrain height and assuming stations are at sea level. This matches the explanation in the lecture that while drawing isobars we conceptually remove the effect of Himalayas, plateaus and other relief. By treating continents as if they lie at sea level, the map becomes a simplified tool for understanding pressure patterns that would otherwise be very complex if every altitude difference was included.

Why the other options are incorrect Option B is wrong because high level stations are not ignored; their readings are transformed to sea level equivalents and still used in analysis. Option C is incorrect since pressures over continents are not omitted in favour of only ocean data; in fact continental stations are very important. Option D is also wrong because the reduction is primarily about height rather than only temperature corrections. None of these alternatives reflects the idea of removing relief and bringing everything to the same reference level.

UPSC exam tip / common confusion Students often see the phrase reduced to sea level on maps but never think about its meaning. In an exam question, whenever you read this phrase, immediately recall that it implies neglect of actual topography for simplicity. This helps you handle questions that ask why we can compare pressure over the Himalayas and nearby plains on a single chart. Remember that the Survey of India and similar agencies explicitly use this convention, so it is a favourite line for conceptual prelims questions.