How to Read a Barometer

Use Rising and Falling Air Pressure to Predict the Weather

aneroid barometer high pressure
An aneroid barometer shows a high pressure (fair weather) reading. Peter Dazeley/Photographer's Choice RF/Getty Images

barometer is a device that reads atmospheric pressure. It is used to predict the weather as atmospheric pressure changes due to warmer and colder weather systems. If you are using an analog barometer at home or a digital barometer on your cell phone or other electronic device, you may see the barometric reading reported in inches of mercury (inHg) in the U.S. Meteorologists use the unit millibars (mb) and the SI unit used worldwide is Pascals (Pa).

Learn how to read a barometer and how changes in air pressure predict the weather.

Atmospheric Pressure

The air that surrounds the Earth creates atmospheric pressure. As you go up into mountains or fly high in an airplane, the air is thinner and the pressure is less. Air pressure is also known as barometric pressure and is measured using a device called a barometer. A rising barometer indicates increasing air pressure; a falling barometer indicates decreasing air pressure. The air pressure at sea level at a temperature of 59 F (15 C) is one atmosphere (Atm).

How Air Pressure Changes

Changes in air pressure are also caused by the difference in air temperature above the Earth. Continental landmasses and ocean waters change the temperature of the air above them. These changes create wind and cause pressure systems to develop. The wind moves these pressure systems that change as they pass over mountains, oceans, and other areas.

The Relationship Between Air Pressure and Weather

Years ago the French scientist and philosopher Blaise Pascal, discovered that air pressure decreases with height, and pressure changes at ground level at any one place can be related to daily weather changes. Often, weather forecasters refer to a storm or low-pressure area moving toward your region. As air rises, it cools and often condenses into clouds and precipitation. In high-pressure systems the air sinks toward the Earth and warms up, leading to dry and fair weather.

Changes in Barometric Pressure

  • When the air is dry, cool, and pleasant, the mercury or barometer reading rises.
  • When the air is warm and wet, the barometer reading falls.
  • When the air pressure falls, it usually indicates some type of storm or wet weather is coming.
  • When it rises, it often means clear weather.
  • If the barometer remains steady, there will be no immediate change in the weather.

Predicting the Weather With the Barometer

Checking a barometer with readings in inches of mercury (inHg), this is how you may interpret them:

Over 30.20:

  • Rising or steady pressure means continued fair weather.
  • Slowly falling pressure means fair weather.
  • Rapidly falling pressure means cloudy and warmer conditions.

29.80 to 30.20:

  • Rising or steady pressure means present conditions will continue.
  • Slowly falling pressure means little change in the weather.
  • Rapidly falling pressure means that rain is likely, or snow if it is cold enough.

Under 29.80:

  • Rising or steady pressure indicates clearing and cooler weather.
  • Slowly falling pressure indicates rain
  • Rapidly falling pressure indicates a storm is coming.

Isobars on Weather Maps

Meteorologists use a metric unit for pressure called a millibar and the average pressure at sea level is 1013.25 millibars. A line on a weather map connecting points of equal atmospheric pressure is called an isobar. For example, a weather map will show a line connecting all points where the pressure is 996 mb (millibars) and a line below it where the pressure is 1000 mb. Points above the 1000 mb isobar have a lower pressure and points below that isobar have a higher pressure.