Isobars are lines on a weather map joining together places of equal atmospheric pressure.
. The numbers measure the atmospheric pressure in millibars.
. Usually isobars are drawn at intervals of two or four millibars (one thousandth of a bar).
. The closer the isobars are together the windier it is.
. If the lowest number is in the middle circle this is a low pressure or depression. Often low pressures mean wet and windy weather.
. If the highest number is in the middle circle, this is a high pressure or anticyclone. Often high pressures mean dry, sunny weather.
. Air pressure tends to range from 890 mb (a hurricane) to 1060 mb (an anticyclone).