Bernard Redshaw, of Alexandra, asks :-

Denis Savin, of Porirua, asked:-

If winds are due to air moving from areas of high pressure to areas of low pressure, then why do weather maps show the wind direction as parallel to the isobar lines and not at right angles to them?

Tony Trewinnard, of Blue Skies Weather and Climate Services, responded.

If the world was just a football sitting on a laboratory desk, and if the middle of the ball were warm (equator) and the ends cold (poles) then air would indeed flow directly from the equator to the poles. This is because warm air is less dense than cold air, so warm air tends to rise. This creates areas of low pressure over the warm equator. Colder air from the poles flows towards the low pressure to try to equalise things out, making the air travel directly from high to low pressure.

This happens in real life on the planet Earth too, but with one important difference. The earth is rotating – so as the air moves from the poles towards the equator the earth is moving underneath it. This means that the air appears to move partly sideways. In the Northern Hemisphere the air is deflected to the right, and in the Southern Hemisphere to the left, as the earth turns underneath. In this way, air rotates clockwise around depressions (areas of low pressure) in the Southerrn Hemisphere (and anticlockwise in the Northerrn Hemisphere). So, as a depression approaches New Zealand from the Tasman Sea winds are usually from a northerly direction, and as it moves away past the Chatham Islands winds are usually southerly.

Isobars are lines on a weather map showing locations with the same air pressure. Winds blow almost along the isobars, but not quite, as friction from the earth’s surface causes them to deflect slightly sideways. Hence although isobars might show a true southerly airflow over New Zealand, the winds affecting the country will be more southwest than southerly. The closer the isobars are together, the stronger the pressure gradient is, and therefore the stronger the winds. The curvature of the isobars is also important – winds will be stronger where the isobars curve tightly around an anticyclone, and lighter where they curve around a depression.