A barometer is a device that reads atmospheric pressure. It is used to predict the weather by tracking atmospheric pressure changes resulting from the presence and movement of 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 devices in the U.S., you may see the barometric reading reported in inches of mercury (inHg). The International System of Units (SI unit) used worldwide is Pascals (Pa, which is approximately equal to 3386.389 times inHg), and meteorologists use the unit millibars (mb, or 33,864 times inHg).
Here's how you read a barometer and what those readings mean in terms of changes in air pressure and what weather is headed your way.
The air that surrounds the Earth creates atmospheric pressure. When you go up into the mountains or fly high in an airplane, the air is thinner and the pressure is lower. The air pressure at sea level at a temperature of 59 F (15 C) is one atmosphere (Atm), and it is the baseline reading for determining your relative pressure.
Air pressure is also known as barometric pressure and it is measured using a device called a barometer. A rising barometer is one that indicates increasing air pressure; a falling barometer indicates decreasing air pressure.
How Air Pressure Changes
Changes in air pressure are also caused by the difference in air temperature above the Earth. Air temperature of masses is affected by what they are above: an air mass above continental landmasses has a different temperature than that above an ocean. Those differences create wind and cause pressure systems to develop. The wind moves those pressure systems, and they in turn change as they pass over mountains, oceans, and other areas.
The French scientist and philosopher Blaise Pascal (1623-1662) discovered in the 17th century that air pressure decreases with height, and measuring air 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 upward, leading to dry and fair weather.
Changes in Barometric Pressure
In general, the barometer can let you know if your immediate future will see clearing or stormy skies, or you are not likely to experience a change.
- When the air is dry, cool, and pleasant, the mercury or barometer reading rises.
- When it rises, it often means clear weather.
- 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.
- If the barometer remains steady, there will be no immediate change in the weather.
Predicting the Weather With the Barometer
More specifically, a barometer with readings in inches of mercury (inHg) can be interpreted in this manner:
If the reading is over 30.20 inHg (102268.9 Pa or 1022.689 mb):
- Rising or steady pressure means continued fair weather.
- Slowly falling pressure means fair weather.
- Rapidly falling pressure means cloudy and warmer conditions.
If it falls between 29.80 and 30.20 (100914.4-102268.9 Pa or 1022.689-1009.144 mb):
- 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.
If the reading is under 29.80 (100914.4 Pa or 1009.144 mb):
- 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
Weather researchers (called meteorologists) use a metric unit for pressure called a millibar and they define the average pressure of a given point at sea level and 59 F (15 C) as one atmosphere, or 1013.25 millibars.
When a meteorologist points to a line on a weather map and refers to it as an isobar, she is referring to a line which connects points of equal atmospheric pressure. 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. That helps the meteorologist plot the coming changes in weather over the region.