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Barometers for Weather Watching
One of the standard tools for weather watching and for weather forecasting is the barometer. A good quality barometer should be in the toolbox of all avid watchers/forecasters. Today, barometers are relatively inexpensive and can sometimes be found at garage sales and consignment/thrift shops.
The three types of barometers readily available today are: the liquid barometer, the aneroid barometer and the electronic barometer.
The liquid barometer is for all practical applications, a mercury barometer. Although any liquid could be used, only liquid mercury is dense enough to make a practical liquid barometer for atmospheric conditions. Liquid barometers made with water or other liquids having a similar density as water would require a barometer tube around 9-10 metres tall to properly measure air pressure. Water barometers can, however, be designed to show pressure changes over short time periods.
Mercury barometers are mostly found in laboratories and weather stations where very exact measurements of air pressure are needed. Although you could purchase one for home use, I don't recommend it, mostly because they contain substantial quantities of mercury which could pose a threat the health of children and pets (as well as careless adults).
An interesting genre of liquid barometers designed for home use are the weatherglasses. Weatherglasses are usually glass vessels shaped like long-spout pitchers or watering cans that react to changes in atmospheric pressure using water as the sensing liquid.
The weatherglass indicates changing pressure through changes in the water level in the barometer's spout. A high or rising level indicates stormy weather conditions present or forthcoming due to a lowering of the air pressure. In extreme situations, the rise may be sufficient to cause the water to overflow the spout. With approaching high pressure, the water level retreats into the vessel bowl, indicating improving weather.
The weatherglass -- known also as the storm glass -- dates back to early 17th-Century Netherlands and likely reached North America with the Pilgrim/Puritan colonists. It was among the first weather forecasting tools available to non-scientists to be based on scientific principles.
While the weatherglass reacts to temporal changes in atmospheric pressure, most units do not allow reading of pressure changes in standard units nor have indicators to judge smaller pressure trends over short time periods. Therefore, the weatherglass is of minimal aid to the serious weather watcher/forecaster. They are also not as accurate as other methods of measuring air pressure. They are, however, often beautifully handcrafted glasswork that can be a conversation piece or object of decor. If you buy one, do so for its beauty rather than its scientific value.
Another version of the weatherglass has been attributed to Admiral Robert Fitzroy of HMS Beagle fame and father of the British Meteorological Office. In Fitzroy's instrument, changes in pressure are sensed by changes in the cloudiness of a water solution within the barometer. I leave further discussion of this interesting barometer for future writings. This weatherglass is also more a conversation piece today than a weather instrument.
Aneroid barometers -- the word aneroid means "without liquid" to contrast the genre against the liquid barometers -- are likely still the most common home barometer type. In the last decade or so, electronic barometers have become very affordable, and they may eventually become the most commonly used, although aneroid barometers make useful home decor items as well as being of practical use.
In the aneroid barometer, the pressure sensing unit is a small, flexible metal compartment or diaphragm that has been partially evacuated of air and then tightly sealed. When the external ambient pressure changes, it creates a pressure differential between the ambient air and the air sealed within the aneroid cell. The pressure differential causes the cell to expand or contract in response. The distortion of the cell is "read" though a set of springs or levers (depending on the make) and transmitted to an indicator dial or pointer.
Aneroid barometers are ideal for the weather enthusiast as they are often very inexpensive, portable and sufficiently accurate for most uses. Aneroid barometers are often combined with thermometers and hygrometers into a "home weather station" to make interesting pieces for home or office decor. But don't just let them sit there on a desk or wall looking pretty; use them!
Only the barometer, however, will actually sense the weather unless they are hung or placed in an outdoor location protected from the elements, or have some form of remote connection to an outdoor thermometer and hygrometer sensing element. Otherwise, the thermometer/hygrometer sense the indoor conditions of the desk or wall where they are located. (In summer when windows are wide open and heating/air conditioning off, they may give some indication of general outside conditions.)
Aneroid barometers travel well -- they have been used by mariners for over a century -- and can thus be easily carried out into the field. However, their mechanisms can be shaken out of alignment with rough handling, such as driving down a washboard road or over rough terrain, so take some care when transporting them.
If you decide to purchase an aneroid barometer for weather watching or weather analysis and forecasting purposes, look for one with the most detailed scale available and a thin indicator needle. You will be mostly interested in pressure changes, likely over short time periods, so finding one with the most precise scale will provide the most enjoyment.
I have two aneroid barometers, a wall-mounted model and a desktop model. The former is graduated in 0.05 inch increments (they are quite old) and can be read to within 0.02 inches or so. The desk model only has 0.10 inch increment markings and its thick indicator covers about 0.04 scale inches in width, its design more for aesthetics than practical use.
There are two common variations on the aneroid barometer also available: the altimeter and the barograph.
The altimeter is constructed similarly to the barometer except that the scale reads in height -- metres or feet -- above a datum. The height reading increases in value when the pressure decreases. The basic altimeter has a knob or similar device that allows the altimeter to be easily reset to the reference datum or reference pressure. In an aircraft, the aneroid altimeter is set to read zero when the aircraft is on the ground so that when the aircraft ascends, the dial or display reads the altitude above the ground reference datum, increasing by 100 m for every 10 hPa decrease in pressure.
The barograph is a recording barometer. Its indicator arm is usually tipped with a pen which records a continuous trace on a chart that moves steadily with time using some form of clock mechanism. Charts are usually run for one day, one week or, at times, one month, depending on the application and the degree of temporal resolution desired. As we shall discuss elsewhere, the time change of pressure is the most important aspect of atmospheric pressure to the weather watcher and forecaster.
Electronic barometers use detection cells whose resistance or capacitance level changes sufficiently with air pressure to react to the small changes found in the atmosphere. The detection cell can be connected to electronic circuits or microprocessors in various ways to provide a wide range of possible applications. The reading outputs on some units can be downloaded to a computer for storage and analyses.
Electronic barometers can be configured to produce altimeters and barographs, or all three at once, stepping between functions with the push of a button. These barometers are usually fairly rugged, very portable and reliable for most uses, and well within the budget of most weather enthusiasts. Several have been designed for outdoor recreational usage and are quite compact in size. As I write this, I have an electric barometer/weather station (the Kestrel 4000) in my jacket pocket; its size is comparable to that of a cell phone.
I have reviewed a couple electronic barometers in the past years. Here are the links to those reviews:
Considering the broad application of the electronic barometer, it is the type I recommend highest as an aid to weather watching, particularly if you do so away from your home. I have two such units. One, a desk/wall unit, not only gives me the current pressure, it displays a running chart of the past pressure values over the past day. It also allows me to step through the hourly readings for the past 24 hours and provides a simple weather forecast based on the barometric history over the past hours. The other unit, a portable barometer/altimeter, is about the size of a wallet and can give me displays similar to my desk/wall electronic barometer or can be switched to altimeter mode with several other functions at the press of a few buttons.
Before I conclude, a few words on units of pressure measurement and the setting of barometers to sea-level pressure.
Measurement Units for Pressure
If you thought the Celsius/Fahrenheit thing was confusing, a handful or two of air pressure units have been in common use over the last century, many of which are still used today. Many aneroid barometers continue to have their scales marked in the traditional inches or millimetres of mercury, mostly for their nostalgic value. Living along the US-Canada border, I can hear inches (of mercury), millibars and kilopascals/hectopascals all in use when I flip around my television dial to get various weather reports. Note that when I speak of the various units in use, I refer to public usage and not the scientific community who all adhere to international conventions.
We begin the discussion with Blaise Pascal's research on pressure in the 17th Century. Pascal was the first to show that the height of the mercury column varied with changes in atmospheric pressure and with changes in elevation and weather. Thus, the initial unit of pressure became the height of that column, expressed as millimetres of mercury (mm Hg) in the metric system and inches of mercury (inches Hg) in the Imperial system.
However, since pressure is correctly defined as a force acting over a unit area, the pressure unit, the bar (defined as 1,000,000 dynes of force per square centimetre) was adopted some year back for standard use in international weather observations. (The United States was slow in adopting the millibar sticking with inches of mercury for many years as the publicly reported pressure measurement.) Average sea-level pressure is 1.01325 bars. For ease of plotting weather maps and in their analysis, the millibar (mb) became standard usage. Thus, 1013.25 mb denoted one standard, sea-level atmosphere of pressure.
The bar was then replaced as the accepted standard unit for pressure by the pascal (Pa), as the international science community moved to rename many derived measurement units in honour of those scientists who made important early discoveries.
One sea-level atmosphere of pressure, which had been expressed as 1.01325 bars or 1013.25 mb, thus became 101,325 Pa. This basic unit is too clumsy for general usage in meteorology since most atmospheric pressure changes occur in the middle two digits (those in bold) and variations of the last two digits are not considered important for most synoptic applications. Therefore, atmospheric pressure was reported as the kilopascal (kPa) which has again morphed further to the hectopascal (hPa) which brings us back to the same numeric value for one atmosphere of pressure as with the millibar.
Confused? Well, hopefully the line below will help you see one atmosphere of sea-level pressure morph through those various units:
1 atmosphere => 29.92 in Hg /760 mm Hg => 1.01325 bar => 1013.25 mb => 101.325 kPa => 1013.25 hPa
Needless to say, when you read various weather or science books or surf the internet or listen to weathercasts, you will likely see/hear atmospheric pressure reported as inches of mercury -- or just inches -- millibars, kilopascals and perhaps hectopascals. Even text books written in the past few years and published in the US continue the use of inches and/or millibars.
[The prefix hecto, denoted by h, has a value of 100, so a hectopascal is 100 pascals. Hecto and deka (with a value of 10) have not entered the common jargon of most scientists until quite recently -- I can't recall hearing them used until hPa became common in meteorology in Canada -- and are not at all common in general language usage. Another rarely heard prefix is deci for one tenth.]
I hope I haven't totally confused you with this plethora of atmospheric pressure units (and I did not even mention the many others which have been used in engineering applications). Units are a real problem when writing for an international audience because Americans, who make up a large part of that audience, continue to stick mainly to the old Imperial system. Canadians straddle the fence, having only been metric for a few decades or so, and frequently mix units from both systems in general talk and writing.
Feeling rebellious today and scornful of the "units police," I settled on millibars (mb) for my forthcoming discussion on using barometers for weather watching. If you must convert to any of the other common pressure units, here are the conversion factors:
1 mb = 100 Pa = 0.1 kPa = 1 hPa = 0.0295 inches Hg = 0.75 mm Hg.
I chose the millibar because weather maps are usually drawn with pressure contours (isobars) every 4 mb and pressure drops with elevation 10 mb per 100 m increase in altitude, so using millibars avoids using decimals whose point can often be "lost" on monitor screens. It also avoids the use of the unfamiliar hectopascal at this time.
Setting the Barometer
Now that you have obtained a barometer, many of you will wish to set your barometer so that they correspond to the local sea-level pressure equivalent. Doing so places you on equivalent footing within the weather observation network around you. However, unless you are entering the pressure reading into weather observation records or journal notes, or wish to compare your reading against other locations, it isn't absolutely necessary. You will, for the most part, be most interested in changes in pressure over time or vertical space and therefore, the absolute or sea-level corrected value is not really that important. (That is why the weatherglass can be of some use.)
If you wish to set it to local sea-level pressure, there are a few ways to do so, though one is strictly applicable only to mercury barometers, since it is sure to give a proper local absolute pressure reading unless damaged.
Unless you live in the mountains or a long distance from a weather-reporting station (such as a local airport or weather office), you can set the barometer to coincide with the latest sea-level pressure reported from the nearest weather station on the broadcast media or internet. Try to do so on a day when the weather is consistent over a number of hours such as when under the influence of a high-pressure cell. When winds are light, it is usually a good indication the pressure is fairly constant over a large area and conditions are changing only slowly.
An alternate method is to download the most recent surface weather map from the internet and estimate your local pressure from the regional isobar pattern. Again, pick a day when the weather conditions are constant so that the pressure pattern is changing slowly over your location. Avoid newspaper weather maps as they are either quite old or are forecast maps rather than observation maps.
In either case, check the pressure readings against the same source a few times over a period of days to insure you have a close match.
You can also calculate the sea-level pressure by adding the pressure of a fictitious column of air stretching from your elevation down to sea level to the absolute pressure reading on your barometer. This method is only totally reliable with a mercury barometer. An aneroid or electronic barometer was likely set at the factory and you have no way to know what that pressure was, nor the elevation of the factory above sea level.
To make this calculation, you need to know your elevation above sea-level (the more accurate, the better) and the 12-hour mean temperature, the average calculated from the present ambient temperature and the ambient temperature 12 hours ago. If you wish to try this method, see Reduction of sea level pressure for full details (see bottom of page). For a very accurate method, see MEAN SEA LEVEL PRESSURE.
For many applications, knowing the sea-level pressure is not necessary. In fact, when working on chemical or engineering applications to gases, we require the absolute pressure and might have to reverse the above calculation to remove the sea-level correction. So you ask, why do we care about the sea-level pressure at all?
Sea-level corrections to pressure readings only become important for spatial applications such as drawing a surface pressure/weather map. For large-scale analyses such as the North American surface map, there are many reporting stations very close to sea-level and other with elevations well above 1000 m (3280 ft). If we did not correct for elevation differences (sea-level is a convenient reference elevation), the weather map would look very odd. Over high elevation regions such as the "Mile High City" of Denver, there would be deep, permanent low-pressure areas relative to sea-level cities such as Los Angeles, Miami, New York City or Vancouver (BC). An uncorrected surface pressure chart would, in fact, resemble an inverted relief map with high pressure hugging the coast and low pressure covering the mountains, particularly the high Rockies. The chart would be of limited use in tracking high and low pressure systems for weather analysis or forecasting.
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