Predicting the Weather With a Barometer: What Pressure Means
In short: a weather barometer measures atmospheric pressure, and it is the change in pressure, more than its value, that signals the weather for the next 12 to 24 hours. Here is what a weather barometer is for and how barometer weather prediction works: the 1013 hPa threshold, a rise, a slow fall or a sudden fall.
The essentials
| Needle movement | Pressure | Forecast |
|---|---|---|
| Above 1013 hPa, moving right | Rising | Calm, stable weather |
| Below 1013 hPa, moving right | Rising | Improvement |
| Above 1013 hPa, slowly moving left | Gradual fall | Little change, calm weather |
| Below 1013 hPa, slowly moving left | Gradual fall | Often a sign of rain |
| Above 1013 hPa, sharply moving left | Sudden fall | Cloudy sky, chance of rain |
| Below 1013 hPa, suddenly moving left | Sudden fall | Strong wind and possible storm |
| Needle not moving | Stable | No change in the next few hours |
In summer, the weather plays an important role in planning your holidays and outings. What could be cooler than having the power to predict the weather! Never get caught out by the rain again thanks to the barometer, the ideal tool for knowing the trend of the next few hours. In this article, we explain what a weather barometer is used for and how to predict the weather from pressure changes.
What is a weather barometer used for?
A barometer measures atmospheric pressure: its variations help you anticipate a change in the weather, showers, a storm or stable fine weather.
The weather barometer is an essential tool for understanding short-term weather changes. By observing variations in atmospheric pressure, it is possible to predict upcoming weather conditions: changes in the weather, showers, storms or high-pressure conditions.
In 1643, the Italian physicist Torricelli devised the very first barometer in history. This instrument made it possible to observe atmospheric pressure using a column of mercury sealed in a tube. Atmospheric pressure is the force exerted by the air on a surface. It varies with the mass of air above a given place.
Over the decades and centuries, the mercury barometer (in French) has been replaced by other, less dangerous instruments: aneroid barometers, gas barometers and water barometers.
The aneroid barometer
How does a needle barometer work? This type of barometer works without batteries, thanks to a capsule partially emptied of air, called a Vidie capsule. A change in atmospheric pressure deforms this casing, which triggers a mechanism and moves the needle across a dial. Reliable and simple to use, it is one of the most common instruments for following atmospheric pressure at home.
The gas barometer
It measures atmospheric pressure using an enclosed volume of gas, which expands or compresses according to the pressure. This instrument needs a correction, because the volume of gas also depends on the temperature.
The water barometer
This barometer uses the variation of the water level to measure atmospheric pressure. It relies on the principle that water rises or falls in a tube depending on the pressure. Less accurate than the others, it is sometimes used for educational or decorative purposes.
Despite these many measuring techniques, the aneroid barometer remains the reference tool for forecasting the weather at home. Note that there is also the electronic weather station, which gives an indication of atmospheric pressure.
Predicting the weather from pressure changes
It is not the pressure value that matters but how it changes: watch the direction and speed of the needle relative to the reference needle.
After the history and role of the barometer, here is how to interpret pressure variations to guess the colour of the sky over the next 12 to 24 hours.
First, remember that the aneroid barometer shows atmospheric pressure using a dial and a needle connected to its mechanism. When you predict the weather with a barometer, it is not the pressure value that matters, but its variation. It is this that gives clues about the weather changes to come.
This is why the aneroid barometer has a reference needle. Represented by a wooden lightning bolt on the original Ocean Clock barometer, it lets you quickly see variations in atmospheric pressure.
Before you start: the reference needle
Place the reference needle over the pressure needle, then tap the glass lightly to free the mechanism. For a step-by-step guide to the dial, the scale and the settings, see our page on how to read a barometer.
Barometer weather prediction then comes down to how the pressure changes. The forecast depends on whether atmospheric pressure rises or falls: so watch how the needle behaves, that is, the speed and direction of its movement.
When atmospheric pressure rises
- If the needle is above 1013 hPa, the sea-level pressure, and moves to the right of the dial, the weather will stay calm and stable.
- If the needle is below 1013 hPa and moves to the right of the dial, it signals an improvement.
When pressure falls gradually
- If the needle is above 1013 hPa and moves gradually to the left, the weather will change very little and stay calm.
- If the needle is below 1013 hPa and moves slowly to the left, it is often a sign of rain.
When pressure falls suddenly
- If the needle is above 1013 hPa and moves sharply to the left, the sky will turn cloudy and there is a risk of rain.
- If the needle is below 1013 hPa and suddenly moves to the left, it signals strong wind and a possible storm.
When pressure stays stable
Sometimes the needle does not move for several days, even when you tap the glass of the barometer. This means atmospheric pressure is stable and the weather will not change in the next few hours.
The speed of pressure changes gives an extra indication of how long the new weather conditions will last. Rapid changes often point to turbulent weather and sudden shifts, while slower changes suggest a gradual evolution.
A barometer gives you an idea of how the weather will develop. For fuller forecasts, also take temperature, wind and humidity into account: you will become a true meteorologist.
Keep an eye on the weather
Frequently asked questions
What is a weather barometer used for?
It measures atmospheric pressure. Its variations let you anticipate conditions over the next 12 to 24 hours: a change in the weather, showers, a storm or stable fine weather.
How does a needle barometer work?
It works without batteries, thanks to a capsule partially emptied of air, the Vidie capsule. The change in pressure deforms this casing, which triggers a mechanism and moves the needle across the dial.
What does 1013 hPa mean on a barometer?
It is the pressure at sea level. Depending on whether the needle is above or below this mark, and on the direction it moves, the forecast changes.
Can a barometer predict rain?
Yes, as a trend for the next 12 to 24 hours: a slow fall in pressure below 1013 hPa is often a sign of rain, and a sudden fall announces strong wind or a possible storm.
What does a needle that does not move mean?
Atmospheric pressure is stable: the weather should not change in the next few hours.
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