High and low pressure systems on a weather map

High and low pressure systems: what they are and how they differ

A low-pressure system, or cyclone, is an area where pressure falls toward the centre. A high-pressure system, or anticyclone, is the reverse: pressure rises toward the centre. On weather maps the centres are marked with the letters L and H, and closed lines of equal pressure circle each one.

In the simulator the tag “Low · 996” sits at the centre of a cyclone and “High · 1026” at the centre of an anticyclone. The background pressure is 1013 hPa, and each system is a dome about 700 km in radius.

Cyclone vs anticyclone in one line: a cyclone has low pressure at its centre and often brings unsettled weather, an anticyclone has high pressure and is often linked with clear skies. Clear usually means heat in summer and frequent frost in winter.

How to read isobars and where wind strength comes from

An isobar is a line along which pressure has the same value. To read a weather map, find the H and L centres first, then see where the lines crowd together. The closer the isobars, the faster pressure changes across that stretch. That rate of change is called the pressure gradient.

Wind speed is directly proportional to the pressure gradient: crowded isobars mean stronger wind, widely spaced ones mean weaker wind (NOAA, origin of wind). The simulator draws an isobar every 4 hPa, so you can watch the rule work: push the systems closer and the lines between them tighten.

Wind is named for the direction it blows from. A west wind blows from the west, and the city readout follows that: “Wind from: west”.

Which way the wind turns: Coriolis, friction and the southern hemisphere

Three forces act on the wind at the same time. The pressure gradient pushes air from high toward low pressure. Earth’s rotation adds the Coriolis force, which turns moving air to the right in the northern hemisphere and to the left in the southern. The third force is friction with the rough surface, which slows the wind.

Gradient and Coriolis together would send the wind along straight isobars. Friction turns it slightly across the lines: away from the centre of a high and toward the centre of a low. In the northern hemisphere the wind runs clockwise around a high and counter-clockwise around a low. In the model, near the ground it is weakened and turned 20° toward the low.

In the southern hemisphere the Coriolis force turns air to the left, so the picture is mirrored: clockwise around a low, counter-clockwise around a high. Switch the hemisphere in the simulator and check.

Why lows bring cloud and rain and highs bring clear skies

Air cannot pile up at one spot near the ground. Around a low the winds converge on the centre, and the air can only rise. It cools, the vapour condenses, clouds form and then precipitation. That is why inclement weather is common near areas of low pressure.

Around a high it goes the other way. Air spreads out from the centre, and air from above sinks to replace it. Sinking air warms, evaporation starts to outpace condensation, and clouds fade. That is why fair weather is often associated with high pressure.

The word “often” matters: this is a tendency, not a rule for every day. In the simulator cloud cover grows toward the low and thins toward the high, and the city readout shows the result: clear, cloudy or rain.

How to read a weather map: what this simulator leaves out

The domes are round, and there are no mountains, sea or land-and-water contrast under them. There are no weather fronts in the model: on real maps the boundaries between air masses are drawn as fronts, and fronts bring the sharp changes. Real cyclones are lopsided. So the simulator explains a principle; it is not a forecast.

For the pressure over your city in the coming hours, see pressure today. For what is known about pressure swings and headache, read barometric pressure and headache.