On a synoptic weather map, what does an isobar represent and how is it defined?

Introduction / Context:
Weather maps use several types of lines to show spatial patterns of different variables. Understanding what each type of line represents is essential for interpreting synoptic charts. Isobars are among the most frequently used lines on surface weather maps, and they are directly related to wind and storm patterns. This question asks for the correct definition of an isobar.

Given Data / Assumptions:

• We are dealing with synoptic weather maps that show surface observations from many stations.
• Various meteorological elements such as pressure, temperature, rainfall, and wind speed can be contoured.
• The term isobar must be distinguished from other terms like isotherm or isohyet.
• Pressure values are usually adjusted to a common reference level such as mean sea level.

Concept / Approach:
In meteorology, the prefix iso means equal. Different suffixes specify the measured quantity. For example, isotherms connect points of equal temperature, and isohyets connect points of equal rainfall. An isobar specifically connects points with equal atmospheric pressure. To make fair comparisons between stations at different altitudes, pressures are often reduced to a common level, usually mean sea level, before being plotted and joined by isobars.

Step-by-Step Solution:

Verification / Alternative check:
Any standard meteorology textbook defines isobars as lines of equal pressure on a weather map, usually corrected to sea level. Weather forecasters use isobars to identify high and low pressure centers and to estimate wind strength based on spacing. This matches option A. Isotherms, isohyets, and lines of constant wind speed or humidity have their own names and are not called isobars.

Why Other Options Are Wrong:
Option B describes isotherms, not isobars. Option C describes isohyets, lines of equal rainfall. Option D refers to a hypothetical contour of wind speed sometimes called isotachs, not isobars. Option E refers to humidity patterns, which are not represented by isobars.

Common Pitfalls:
A common mistake is to confuse the various iso terms. Remember that bar refers to pressure, therm refers to heat or temperature, and hyet refers to rain. Another pitfall is ignoring the phrase reduced to a common level; without this reduction, stations at different elevations could not be compared properly.

Final Answer:
An isobar is a line joining places having the same atmospheric pressure reduced to a common level, usually mean sea level, on a synoptic weather map.