How to Read a Barometer

A barometer is a device that reads atmospheric pressure. It uses liquid mercury to predict the weather by tracking atmospheric pressure changes resulting from the movement of warm and cold weather systems.

If you are using an analog barometer at home or a digital barometer on your cell phone in the U.S., the barometric reading will likely be reported in inches of mercury (inHg). However, the SI unit for pressure used worldwide is the pascal (Pa), which is approximately equal to 3386.389 times one inHg. Most often, meteorologists use the more precise millibar (mb), equal to exactly 100,000 Pa, to describe pressure.

Here's how to read a barometer and what those readings mean in terms of air pressure changes and what weather is headed your way.

Atmospheric Pressure

The air that surrounds the Earth creates atmospheric pressure and this pressure is determined by the collective weight of air molecules. Higher air molecules have fewer molecules pressing down on them from above and experience lower pressure, while lower molecules have more force or pressure exerted on them by molecules piled on top of them and are more tightly packed together.

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 equal to one atmosphere (Atm) and this is the baseline reading for determining relative pressure.

Atmospheric pressure is also known as barometric pressure because it is measured using a barometer. A rising barometer indicates increasing atmospheric pressure and a falling barometer indicates decreasing atmospheric pressure.

What Causes Changes in Atmospheric Pressure

Changes in air pressure are caused by differences in air temperature above the earth, and the temperature of an air mass is determined by its location. For example, air masses above oceans are typically cooler than air masses above continents. Air temperature differences create wind and cause pressure systems to develop. The wind moves pressure systems and these systems tend to change as they pass over mountains, oceans, and other areas.

17th-century French scientist and philosopher Blaise Pascal (1623–1662) discovered that air pressure decreases with height and that pressure changes at ground level can be attributed to the daily weather. These discoveries are used to predict the weather today.

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Often, weather forecasters refer to high- or low-pressure areas moving toward particular regions in order to describe predicted conditions for those areas. As air rises in low-pressure systems, it cools and often condenses into clouds and precipitation, resulting in storms. In high-pressure systems, the air sinks toward the Earth and warms upward, leading to dry and fair weather.

How Pressure Changes Affect the Weather

In general, a mercury barometer can let you know if your immediate future will see clearing or stormy skies, or little change at all, based only on atmospheric pressure.

Here are a few examples of how to interpret barometric readings:

• When the air is dry, cool, and pleasant, the barometer reading rises.
• In general, a rising barometer means improving weather.
• In general, a falling barometer means worsening weather.
• When atmospheric pressure drops suddenly, this usually indicates that a storm is on its way.
• When atmospheric pressure remains steady, there will likely be no immediate change in the weather.

Predicting the Weather With the Barometer

Reading a barometer is simple if you know what different atmospheric pressure values indicate. To understand your barometer and how atmospheric pressure is changing, interpret readings as follows (pay attention to units).

High Pressure

A barometric reading over 30.20 inHg is generally considered high, and high pressure is associated with clear skies and calm weather.

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.

Normal Pressure

A barometric reading in the range of 29.80 and 30.20 inHg can be considered normal, and normal pressure is associated with steady weather.

If the reading falls between 29.80 and 30.20 inHg (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.

Low Pressure

A barometric reading below 29.80 inHg is generally considered low, and low pressure is associated with warm air and rainstorms.

If the reading is under 29.80 inHg (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. They define the average pressure of a given point at sea level and 59°F (15°C) as one atmosphere or 1013.25 millibars.

Meteorologists use lines called isobars to connect points of equal atmospheric pressure. For example, a weather map may feature a line connecting all points where the pressure is 996 mb and a line below that where the pressure is 1,000 mb. Points above an isobar are lower pressure and points below are higher pressure. Isobars and weather maps help meteorologists plot the coming changes in weather over a region.