An isobar is a line on a weather map that runs through every place where the air is pressing down with the same strength at that time. Follow the line and the pressure stays the same; cross it and you are moving into higher or lower pressure. Where the lines sit close together, pressure is changing fast over a short distance and the wind is usually stronger. Where they sit far apart, the air is quieter. Weather people use rings of isobars to mark highs and lows. Though actually, outside a forecast chart or a geography class you will almost never hear the word — it is specialists' English, not café English. People sometimes glance at the bar in the middle and think of a drinking bar; it is not that. (Chemists have a rarer second use of the same spelling, for atoms with the same mass number. On a school weather map, isobar always means the equal-pressure line.)

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