Reading isobars: pressure and the path of the wind

Follow the pressure

A chart filled with curved lines becomes easier to read when you start with the values. An isobar connects places with equal air pressure. Following the same contour therefore keeps the value unchanged; moving to another contour may change it. Around a high, pressure increases towards the centre, whereas it decreases towards the centre of a low. [s1]

The gaps between contours also matter. Closely packed isobars show that pressure changes more rapidly across the map: the pressure gradient is stronger and winds are generally stronger. This is a qualitative relationship. The spacing drawn on a chart does not, by itself, supply a measurement of the actual wind speed at an individual weather station. [s1]

Understand the movement

A pressure difference pushes air towards lower pressure, but that force does not act alone. Earth’s rotation changes the direction of movement through the Coriolis effect, while friction near the surface slows the flow. Drawing a straight arrow from a high towards a low therefore represents only part of the physical process that determines the wind. [s1]

Near the ground in the Northern Hemisphere, the general circulation turns anticlockwise around lows and clockwise around highs. Those directions reverse in the Southern Hemisphere. Surface wind also crosses the contours with a component towards lower pressure. Isobars are therefore useful guides to the pressure pattern, rather than tracks that every moving parcel of air follows exactly. [s2]

Keep the direction clear

A hypothetical example clarifies another distinction: a west wind arrives from the west and carries air eastwards. The meteorological direction names the origin, whereas an animated trail shows the movement. Before comparing a written wind report with moving marks on a map, establish which of these different descriptions you are reading so that the flow is not reversed. [s1]

For a reading exercise, imagine a chart that represents no particular day. First follow the pressure values, then compare areas where the lines are close together or far apart. This reveals the pressure pattern and its gradient. To learn the wind direction and speed actually observed at a place, read the relevant station report separately from your interpretation of the sketch. [s1] [s2]

Sources consulted

1. Origin of Wind — NOAA JetStream — Source: NOAA / National Weather Service
2. Basic Discussion on Pressure — National Weather Service — Source: NOAA / National Weather Service