The barometer is the most important instrument used in interpreting and forecasting the weather, but surprisingly few people are completely certain about how to use a barometer.
This is not surprising when long range forecasts are considered, because air pressure values, which the barometer records, change from place to place, with elevation, and over time. Putting all available information together and arriving at a weather forecast 5 or 10 days ahead needs lots of expensive computer time on very powerful computers, and is far too complex a subject for these pages.
So that leaves us with that elegant instrument on the wall, or standing on the mantel piece. Or perhaps the digital barometer in the receiver of a home weather station. Either way, you have probably calibrated it to the right air pressure for your home, and corrected the pressure to a sea level equivalent. Information on how to do this is usually contained in the instruction sheet or booklet, but you can also find out how to do this by visiting the page on Setting A Barometer. You should only need to do it once.
This page has grown rather larger than I first planned, so here are the main conclusions and recommendations. Read on if you want to know more.
The Very Short Guide To The Barometer
Set the pointer over the needle when you take a reading. This will help you work out the pressure change next time you check the barometer. In general it is the direction of change that is important, not the actual air pressure value.
Unless a major storm or hurricane is approaching, it is only necessary to read the barometer once or twice a day. Around 7-9am or 8-9pm are the best times.
When the needle sits over the words inscribed on the dial of a barometer, it is indicating more about what is likely to happen in a day or two than what the current weather is. But keeping a record of pressure changes against current weather and weather that arrives over the next few days can turn your barometer into a useful local forecasting tool
Unless the weather is really getting bad, frequent checking of your barometer can give a misleading impression of weather trends. During the day air pressure rises and falls in a way that is independent of broader pressure and weather changes. This is due to heating by the sun and local factors such as sea breezes and air drainage.
That's the story in a nutshell. Now let's examine the situation further.
What's On The Dial?
If you look at a conventional barometer, you'll notice a round dial with figures to indicate the air pressure, usually in inches of mercury or the metric equivalent millibars. There will also be a needle, which moves with pressure changes, and a pointer, which you can move. You'll also notice words such as Rain, Change and Fair inscribed on the dial, and it is these which are the main troublemakers.
We'll get back to them, but at this stage it's worth keeping in mind that when the needle points to one of them it's indicating what may happen in 24 hours, not what's happening now.
The dial on most barometers covers a range of three or four inches of mercury, from 27.5 to 31.5" Hg for a four inch range, and from 28 to 31" for the three inch alternative. Don't worry if yours is slightly different, because almost all the action you will see takes place between 29 and 31 inches.
The air pressure is unlikely to go higher, but in a hurricane, or severe storm it may go much lower. And here I don't mean a thunderstorm, although air pressure can vary in interesting ways then. But I'm referring to major weather associated with low pressure cells, such as Nor'easters, accompanied by strong winds, rain or snow.
On those occasions the air pressure may vary quite rapidly, but in general weather systems move quite slowly, and air pressure changes are equally slow and unspectacular. So for the most part there is no need to keep an eagle eye on the barometer.
When And How To Read A Barometer
In fact, a barometer is most useful if it is read only once a day, preferably at the same time - 7-8am or 8-9pm are good times for reasons which we'll get to later. Twice a day is also fine, but it's really the 24 hour changes which are most significant.
Unless the weather is really spectacular, it is not the actual value of the air pressure that is important, it is the change from the previous day - whether it is rising, falling or steady.
The best way to do this is to lightly tap the case of the barometer in case the needle is slightly stuck, record the air pressure if you wish, then move the pointer over the needle. The next time you read it, it will be easy to compare the change from the previous day.
"Why Is it Sunny When The Barometer Says Rain?"
The weather we experience is caused by the interaction of air masses and pressure cells, which mostly move fairly slowly over the earth's surface. Considering only air pressure variations, the extremes are high pressure cells and low pressure areas or fronts, the boundaries between different air masses.
Between these extremes air pressure will vary fairly consisitently, and we have a pressure gradient between the two extremes. So once the center of a low has passed, the pressure will gradually rise until the center of the following high passes through. So on your barometer the needle will gradually rise up the scale from day to day.
In most cases, a high pressure cell means calm, clear, sunny weather, and as the needle moves up the scale, the increase in pressure is suggesting fine, or "fair" weather is on the way.
Similarly, as the high passes over the daily change in the needle's position will show a decrease in the air pressure, and suggests that if it continues you will experience a period of low pressure.
Much of what we see as weather is the result of the atmosphere trying to equalise pressure over wide areas. The only way to do this is to move air from areas of high pressure to low pressure zones, and moving air equals winds. So winds blow towards low pressure areas, and if they carry moist air the chances of rain or snow are strong. So a persistent downward movement of the needle in the barometer suggests that rain may be on the way.
Of course, as in most things to do with weather and life, it's not quite that simple, which means that although the pressure changes recorded by your barometer have some predictive value, they won't always be right. For example, if the center of a high passes through some distance away from you, you may not experience the fair weather that your barometer predicted before the pressure, and the needle begins to fall.
Another rule of thumb is that very high pressure readings may indicate windy conditions to follow. The reasining behind this is that an extreme reading usually means a high pressure gradient to the next low, and high pressure gradients mean strong winds. This rule doesn't apply in quite the same way to very low pressures. While the pressure gradient will certainly be high, the low is actually the focus of the winds, and unless you happen to be under the eye of a hurricane, the strong winds have already arrived.
But you can, over time, turn your barometer into a very useful forecasting tool, but you will need to keep records of the weather to do so. Apart from pressure changes, you can record wind direction, wind direction changes, cloud cover and type of cloud, and the occurrence of rain, snow or thunderstorms. Comparing your daily weather against the background of the seasons will allow you to develop a record of significant weather events which will help you become a pretty useful forecaster. And all with just a barometer and your observational abilities.
The Perils Of Too Many Pressure Readings
To develop reliable weather forecasts, we rely on general, relatively long term trends in air pressure changes. That's why it's really only necessary to record the air pressure at your home once or twice a day.
There are other reasons for not checking air pressure more often during normal weather patterns, and these are readily seen if you have a home weather station and can record your data on a continuous 24 hour or longer chart. You can also check continuous pressure and other data on online weather stations - try WeatherMatrix for online weather stations in your area.
Unless you live near the poles, most times you will see regular up and down movements in air pressure, commonly with two highs and lows each day. In other words, air pressure changes from day to day are not smooth and regular.
These daily variations are the result of atmospheric tides. Although the sun and moon exert a gravitational
force on the atmosphere, just like they do on the oceans, these sort of atmospheric tides are hard to detect, although they do exist.
Far more significant are changes caused by the sun's heat. In simplest terms, the sun heats the air, which expands and becomes less dense, and the air pressure drops. And at night, when the sun has set, the air cools, becomes more dense, and the air pressure rises.
It all sounds pretty simple - perhaps too simple. Because other factors come into it, some of which affect air pressure everywhere, and others which are local.
The easiest to explain are the air pressure drops which start around noon and continue to fall to early evening, and are followed by a rise in pressure until around midnight. The start of the pressure drop usually coincides with the atmosphere having warmed up to near its maximum for the day. Often this coincides with increases in wind speed, due either to convection as the warm air begins to rise, or maybe to a sea or valley breeze which also begins once warm air starts rising.
Once this begins the air pressure begins to drop, and continues to do so to around sunset, when the temperature begins to fall more quickly and the winds drop. At this stage the pressure begins to climb again.
Less easy to explain are the small pressure drops in the early hours of the morning, and the rise in pressure through to late morning. They are part of the warming/cooling cycle, but may have local influences such as cold air drainage from uplands into a valley during the night. This is an interesting subject which I am doing more research on. If you have any information or ideas about these less obvious daily pressure changes, please them to me through the Contact Us page.
These daily changes are most obvious when the overall air pressure is stable, but they are also recognizable as a sort of overprint even when the pressure is rising or falling fast.
Other minor fluctuations in air pressure sometimes when it's raining, when the pressure drops as rain cools the air. Thunderstorms can cause some odd jumps and dips too - apart from the rainfall effect, updrafts and downdrafts can cause brief pressure changes.
Observation of these short term variations in air pressure, and their relationships to temperature, rainfall and wind speed and direction can all be added to your store of knowledge of local weather, and give your barometer greater usefulness as a forecasting tool.
But for overall trends, one or two readings a day, at the same time each day, are all that's needed during normal weather. Earlier I suggested recording the pressure around breakfast time or in the early evening. At these times the air pressure tends to be more stable, away from major temperature changes and before the winds associated with atmospheric heating are active.
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Barometers At eBay
Lots of Barometers are for sale on eBay every day - new, used and antique, good and not so good. It's always worth checking what's available. Even if you aren't in a buying mood, the list below shows you what's on sale at this very minute, and eBay can be a great way to learn about specific items and barometers in general.