How to read a barometer

The most important instrument used for weather forecasting is the barometer, but surprisingly only a handful of people know exactly how to read a barometer and interpret its measurements. A barometer does not only  indicate present weather conditions but forecasts probable weather conditions 12 to 24 hours in advance.However it’s not that simple to read a barometer, taking into consideration the fact that the wind changes its direction constantly. A “rise” in barometric pressure must be considered in relation to the former reading and the same principle applies to a “fall”. In fact, a barometer is most useful if it is read only once a day, preferably at the same time of the day – 7-8am or 8-9pm are good times to do barometer readings – 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 for weather forecasters, so learning how to read a barometer is of great importance at this point. Sometimes the readings of the barometer can appear inaccurate saying that there is going to be rainy but in fact it’s going to be sunny. That happens because the weather we experience is caused by the interaction of air masses and pressure cells, which move fairly slow over the earth’s surface. Considering only air pressure varies, the extremes are high pressure cells and low pressure areas are the fronts, these two being the boundaries between different air masses. Between these extremes air pressure will vary fairly consistently, 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 when you read a 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 this continues you will experience a period of low pressure. Of course, as in most things to do with weather and life, it’s not quite that simple to read a barometer, 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 at a certain distance from where you’re located, you may not experience the fair weather that your barometer predicted before the pressure, and the needle begins to fall. Another general rule of thumb in “how to read a barometer” is that very high pressure readings may indicate windy conditions to follow. The reasoning 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, if you know how to read a barometer ,turn it 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 clouds, and the occurrences of rain, snow or thunderstorms.

So let’s recap on what you need to know about how to read a barometer:

Get a regular mercury barometer and start measuring the atmospheric pressure around you by simply measuring how high the mercury column goes up or down. The way you’re going to measure it is from bottom to top using a simpler ruler, keeping in mind that under regular atmospheric pressure the vacuum at the top of the barometer is at almost 30 inches (29.92 to be more precise). When the mercury column drops below that point, we’re experiencing a drop in atmospheric pressure and that means we’re going to get bad weather. This thing works viceversa as well so if the mercury column rises you can expect nicer weather. All in all learning how to read a barometer is a good thing if you’re an outdoor enthusiast and want to know how to plan your trips.