Facts About Air Pressure



  • Although air is light, there is so much of it that air can exert huge pressure at ground level. Air pressure is the constant bombardment of billions of air molecules as they zoom about.
  • Air pushes in all directions at ground level with a force of over 1 kg per sq cm – that is the equivalent of an elephant standing on a coffee table.
  • Air pressure varies constantly from place to place and from time to time as the Sun’s heat varies.
  • Air pressure is measured with a device called a barometer in millibars.
  • Normal air pressure at sea level is 1013 mb, but it can vary from between 800 mb and 1050 mb.
  • Barometers are used to detect changes in air pressure. The first barometer was invented by Evangelista Toricelli in 1644.
  • In this satellite picture, a spiral of clouds indicates that stormy weather in a depression is heading for California, USA.
  • Air pressure is shown on weather maps with lines called isobars, which join together places of equal pressure.
  • High-pressure zones are called anticyclones; low-pressure zones are called cyclones, or depressions.
  • Barometers help us to forecast weather because changes in air pressure are linked to changes in weather.
  • A fall in air pressure warns that stormy weather is on its way, because depressions are linked to storms.
  • Steady high pressure indicates clear weather, because sinking air in a high means that clouds cannot form.