Pressure by Sander Schimmelpenninck
sander@idirect.com

This article in my how-to series for weather amateurs concerns atmospheric pressure, another basic phenomenon like temperature, humidity, and precipitation, already covered.

You can feel temperature and, to a degree, humidity. You can feel and see rain and snow. But you do not feel atmospheric pressure: the weight of the earth's atmosphere bearing down on you to the tune of about 1 kilogram (Kg) per square centimetre (cm²). That is almost 15 pounds per square inch for Americans and Canadian senior citizens reading this. To feel such pressure, balance two standard-size packages of butter on a fingertip. Heavy, isn't it?

We don't feel atmospheric pressure because we were created with an equal, opposing body pressure. If we were thrown overboard from a spaceship, we would explode. Let that discourage would-be interplanetary stowaways.

Also, we don't think of air as having weight. This is why the cartoon cat Sylvester is lofted by a balloon of bubblegum, until Tweety shoots him down with a blow-dart. But air does weigh. To prove this, blow two glass spheres with a capacity of one cubic metre. Fill one with air at sea level and put a vacuum in the other (Watch your local newspaper for vacuum specials). When weighing the two, you will find that the air-filled sphere weighs 1.2 grams more than the empty one. One gram is not much, but consider that the earth's atmosphere is roughly 1,000 km thick, and the weight of all that air adds up. If you are not a glass blower or do not live at sea level, please take my word for the above or go to the sports section.

You probably know that air pressure varies with height, time, and location. The higher you are (1,609 metres above sea level in Denver, the mile-high city), the lower the pressure, because the air column pushing down on you there is shorter than in Halifax, Nova Scotia. This why they make altimeters, which express air pressure as an approximation of height above mean sea level, the standard zero-elevation reference.

Sea-level pressure variations between one place and another form what meteorologists call highs and lows, H and L on your television or newspaper weather map. Don't ask why those pressure extremes occur, why they strengthen or weaken, or why they move, typically to the east in the Northern Hemisphere. The answers have something to do with irregularities in energy emissions from the sun and with chaos theory, which I will not explain for the price you paid to read this article. Suffice it to say here that pressure differences are caused by differential heating in the Earth's always-restless atmosphere.

If it happens, man will measure it. In 1643, Evangelisto Torricelli (an assistant to, the Earth is round, Galileo) built the first barometer: a glass tube about one metre tall, sealed at the top, partly filled with mercury, its open bottom end submerged in a cistern containing that liquid metal.

The space above the mercury column in the tube is a vacuum, except for some contaminants, including water vapour and a few disconsolate mercury-vapour molecules minding their own business. The Earth's air blanket pushes down on the mercury in the cistern and thus on the open bottom of the mercury column in the tube. Nothing pushes on that column from above, because that is where the vacuum is. Therefore the mercury settles around 76 cm above the mercury level in the cistern, at which height its weight balances the atmospheric pressure. Tack on a scale and you have yourself a barometer. Do that 200 years ago and you have an antique like the one in my 1930s Dutch home, with a scale etched in Rhineland inches, whatever they were.

Three hundred fifty years after Torricelli, mercury barometers remain in use at Canadian airport weather stations. For its standard Canada's Atmospheric Environment Service (comparable to the US National Weather Service) uses a dead-weight gauge: a very accurate balance that measures the pressure on a piston..

However, mercury barometers are big and heavy, and mercury is a poison. Observers now increasingly use other instruments to measure atmospheric pressure. Two major categories exist.

First came the aneroid barometer. Scientists use Greek to cow laymen, but aneroid means airless. An aneroid contains an enclosed hollow, airless cylindrical or quasi-circular metal chamber, connected to an analog pointer or digital readout. Air pressure affects the shape of the chamber and thus the readout. Aneroids, too, are ancient: about 100 years old, but the technology remains good enough for government work. Aneroid-based aircraft altimeters are typically accurate within 30 feet the rough equivalent of 1 millibar.

In recent decades solid-state technology and its attendant miniaturization have spawned small electronic pressure sensors like the piezo-electric ones whose electrical characteristics vary with pressure. Some wrist watches show pressure and altitude. Don't worry if yours indicates 10,000 feet when you're toying listlessly with Air Canada chicken at 34,000 feet over Greenland. Your watch sees cabin altitude. The flight engineer monitors that pressure, except when momentarily distracted by a strikingly handsome flight attendant, called a stewardess before women's liberation.

Atmospheric pressure is expressed in several ways.

First, two main units exist. One is the millibar (mb, 1.0197 kg/cm²), preferred in most of the world since WWII and now called the hectopascal to conform with the metric system. In the International Standard Atmosphere, a United Nations-approved theoretical standard for aviation purposes. The standard pressure is 1,013.2 mb. The other main unit is inches of mercury, still used in the US. One of those equals 33.864 mb.

Second, meteorologists use at least three kinds of pressure for surface-based observations. First comes station pressure. Never reported to the public, it is the pressure seen by the station barometer. If that is a mercury barometer, it must be corrected by the barometer temperature, because its mercury column does not reflect only pressure: it also dilates with temperature and thus also acts as a thermometer.

Station pressure is valid for the elevation of the barometer, which may be housed in a 100-metre tall air-traffic control tower. Pilots want to know their height above the ground. For that reason, airport weather stations and air-traffic-control (ATC) give pilots an `altimeter setting, which lets cockpit altimeters to show approximate height above sea level..

At this point, purists may note two omissions, so let me assuage them. First, the altitude shown by an altimeter is seldom the exact height above mean sea level. In very cold weather you'll be lower than you think. For this reason some Transport Canada publications offer temperature-correction factors. Second, the terrain of most airports is not flat. The touchdown zone of a runway may be higher or lower than the airport reference point on which the altimeter setting is predicated.

So much for altimeter settings. For surface and upper-air mapping, meteorologists use pressure reduced to mean sea level (SLP in the METAR code for airport weather reports, used in Europe since the early 60s and recently adopted in the US and Canada.) Like the altimeter setting, SLP is an approximation of the pressure you would see at mean sea level if you dug a hole down to sea level and read a barometer at the bottom. However, SLP further adjusts station pressure by the average outdoor temperature in the past 12 hours and, in North America by some other, empirical correction factors

Most amateur meteorologists will be happy if their aneroid barometer reads true within 0.5 mb or 0.01 inch. Here's how to correct a home aneroid barometer, if it has an adjusting screw in the back.

1. Wait for a calm day with no recent change in indicated pressure.

2. Record your barometer reading on the hour. (Yes, gently tap it first. That is recommended even for mercury barometers, not covered here.)

3. Get the hourly altimeter setting for the nearest airport, or nearby ones, for your observation time. You may need to call a NavCanada Flight Service Station, e.g. at 1800-INFO-FSS. Tell the briefer Athis is not for flying purposes; he'll probably accommodate you. In North America altimeter settings are given in inches of mercury. To convert to millibars, multiply by 33.864.

4. Note the error of your barometer.

5. Adjust your barometer by the requisite amount. It now approximates sea-level pressure.

I thank my friend Peter Bowman of AES for his comments on the draft of this article.

This article is Copyrighted 1999: Sander Schimmelpenninck and used with his expressed written permission.  No unauthorized use allowed without prior written consent by him.