How to read a barometer
A barometer measures the weight of the air above it. One reading tells you little. The direction and speed of the change tell you much more: falling pressure usually means unsettled weather is on the way, and rising pressure usually means it is clearing.
What the number means
Most sea-level readings lie between about 980 and 1040 hPa, or 28.9 to 30.7 inHg. The standard reference is 1013.25 hPa, which is 29.92 inHg or 760 mmHg. That is only a reference point. A reading of 1005 hPa isn't bad and 1025 hPa isn't good. The units are different names for the same thing, and the converter moves between them.
Check which pressure you are reading
A barometer measures the pressure where it is. That is station pressure, and it falls by about 1 hPa for every 8 m you climb near sea level. Weather reports use sea-level pressure, an adjusted figure that lets places at different heights be compared. If your reading looks far from the weather report, this is usually why. The sea-level calculator shows the size of the gap.
Station pressure also changes when you move. An elevator or a drive up a hill looks exactly like a change in the weather, so compare readings taken in the same place.
Read the trend
Weather services often describe the change over 3 hours. BaroLog does the same and uses these words:
| Change in 3 hours | In inHg | BaroLog says |
|---|---|---|
| +3.6 hPa or more | +0.11 or more | Rising fast |
| +0.8 to +3.6 hPa | +0.02 to +0.11 | Rising |
| Less than 0.8 hPa either way | Less than 0.02 | Steady |
| −0.8 to −3.6 hPa | −0.02 to −0.11 | Falling |
| −3.6 hPa or more | −0.11 or more | Falling fast |
The words and cut-offs differ between services. These are the ones the app uses.
What falling pressure means for the weather
Falling pressure usually means a low-pressure system or a front is getting closer. As a rough guide:
- A slow fall over many hours often comes with thickening cloud, and rain or snow may follow.
- A fast fall often means stronger wind is coming, and the faster the fall, the stronger the wind tends to be.
- A fall that stops and turns into a rise often marks a front or the centre of a low passing. The wind often shifts, and showers and gusts can continue for a while.
Rising and steady pressure
Rising pressure usually means drier, more settled weather is coming, although a fast rise can bring strong, gusty wind. Steady high pressure often means settled conditions, which can be sunny, or foggy and grey, especially in winter. Steady low pressure often means the unsettled weather is staying around.
Reading a dial barometer
A classic aneroid barometer usually has two needles. One, often black, shows the pressure. The other, often brass, is a marker that you set by hand with the knob on the glass. Tap the glass gently, then move the marker over the pressure needle. When you look again hours later, the gap between them shows how far the pressure has moved, and in which direction.
Words on the dial, such as "Rain", "Change" and "Fair", are only rough labels for pressure values. Trust the movement of the needle more than the word it points to.
What a barometer can't tell you
- It reads one place. It can't show where a storm is, or how much rain it holds.
- Rain can come with steady pressure, for example from summer showers. A fall can also pass with no rain at all.
- In the tropics, pressure rises and falls by a small amount twice a day, as a regular rhythm. Don't read that as a change in the weather.
- It adds to a forecast and doesn't replace one. For a trip on the water or in the mountains, check the official forecast.