Although air molecules are invisible, they still have weight and take up space.
Weather forecasters measure air pressure with a barometer .
A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars. Air pressure can tell us about what kind of weather to expect as well. If a high pressure system is on its way, often you can expect cooler temperatures and clear skies. If a low pressure system is coming, then look for warmer weather, storms and rain. Use a special JavaScript barometer designed to demonstrate how air pressure and weather conditions are linked.