Percy Saltzman Essay: Drinking Deep At An Isobar

Sunday, April 13, 1952

(CBC Trans-Canada Network)
P. P. Saltzman,
Dominion Public Weather Service, Toronto.

The other day my brother celebrated his tenth wedding anniversary and I was hard put to find a suitable gift. Good advice came from all sides and I soon learned that whereas diamonds are delightful for the sixtieth anniversary and gold is good for fiftieth, the tenth is triumphant with--tin.

Well, one thing leads to another, and tin is like brass and brass is what we weathermen have plenty of and moreover, brass sounds like glass- so I got my brother a--barometer. Not the kind you find in forecast offices which have a long tube chockful of mercury. No, my brother got the ordinary household kind, made of wood and leather and brass with a dial on it and two indicators, one a black hand and a red hand, and all you do is hang it on the wall and hope for the best. This kind has no mercury or any other liquid in it and is therefore called dry or--for the Greeks have a word for it--aneroid, which is Greek for dry only longer.

Now old time sailors are apt to be blunt about such matters and they disdained to call their barometer--made of glass and mercury--by such a landlubberly name, so they simply dubbed it the weather glass and they soon saw that when the glass was low a storm was brewing and they also learned to give the glass a good smart rap-a-tap-tap so as to bring the mercury down to its true level.

So you see it is quite natural to keep on talking of a weatherglass even though you mean the dry kind of barometer that hangs on your wall, although everyone associates glass with liquid, especially when talking of isobars. Isobars, I needly hardly say, have to do with meteorology and are the squiggly fingerprints seen on the weathermap. They are lines drawn to join places having the same air pressure at any given time.

A lot of people have weather glasses hanging on their wall, but may not know how to use them. Moreover, you may be getting one as a tenth anniversary wedding gift, so here's how to use your weatherglass without spilling a drop and I hope my brother is listening.

First of all, when you get the darn thing, don't shake it like my wife did and listen for the ticking, for it's not a clock, has no gears or motor, no spring to drive it, and you can't see the hands move, no matter how hard you try. You don't have to wind it, oil it, or flick a switch. But it does need adjustment, and I'll tell you about that in a minute. Just unpack it lovingly, handle , with care and put it on a wall or on your desk. It'll work by itself and the only thing that drives it is the weight of the air which changes all the time.

The best place to put it is on the "hall wall, where you won't forget to look at it on your way to work or play_ Then too, if you put it in a room with a high temperature, you may throw it off its feed a bit.

The way your weather glass shows the change in air pressure is this-- it has a little metal pillbox inside it that's hermetically sealed after most of the air in it has been vacuumed out. This is to make it sensitive to the weight of the atmosphere which as it rises and falls, causes the top of the pillbox to bend in and out like a sounding drum. Suitable springs and levers carry this movement to an indicator on a dial.

The indicator looks like the minute hand of your clock but the dial doesn't. Its numbers run from 27 to 31. These stand for inches, patterned after the mercury barometer, where the height of the mercury column is often shown in inches. So when the indicator shows 30, let us say, it means the air pressure is equal to the weight of 30 inches of mercury standing straight and tall and thin.

There are two hands on the dial--one black and one red. However it's the black hand that's read and the red hand that's not. This is because it's the black hand that's connected with our vacuumed pillbox. The red hand is connected only to a brass knob that sticks out at you from the face of the dial and the only thing to move it is yourself. You set the red hand over the black for reference, so that you can tell where the black hand was and compare it with where it is now. This tells you if the pressure has risen or fallen in the time since you set the red hand.

You should set the red hand over the black twice a day, preferably in the morning when you leave for work and the evening when you get back home. Then you can see which way the hand has moved and how far the air pressure has risen or fallen during the day.

But when you get to your tenth wedding anniversary and someone gives you a weatherglass, don't expect it to read correctly by itself. You have to adjust it. Here's why--when the instrument is made, it's adjusted to read right for air pressure at sea level. And supposing you get it delivered without its having gone out of kilter, and further supposing you happen to live down beside the sandy sea shore, then and only then will you not have to set about setting it. Anyone not so lucky will have to adjust it, but the job's easily done.

So turn the instrument around and you'll find a small hole drilled in its back. In the hole is a little set screw which you can move easily with a small screwdriver. Test it. Turn it slightly back and forth and if you lean over and gaze at the dial in front, you'll see the black hand weaving gently back and forth.

Now to get the right setting, you can do two things--first if you live near a weather office, phone them and ask for the barometer reading corrected to sea level. Ask for the reading in inches. Then set the black hand to read the same as what the man said.

Of course if you live near a friend or anyone who has a reliable barometer already set to sea level, you can ask for a reading and set yours to that.

But if all this doesn't apply then you use a correction table, which tells you what correction to make for any elevation above sea level. Such a table for instance will tell you that for an elevation of 250 feet you move the black hand so as to add 0.29 inches to whatever it happens to read at the time. For 500 feet you add a little less than twice that. So far then so good. It's easy to use the table and you can get one from the people who make weather glasses, or from any weather office you care to ask. The catch is- what is your elevation. You can find that by consulting your city engineer, your local railroad station, a good road map or your local weather office.

This may all sound complicated and a bit of a bother and so it is- but the payoff is-.-you have to do this only once. That is, unless you move to a house with a different elevation. But then you can always ask your real estate agent to choose a place with the same height above sea level--if your a weather fiend, and I'm sure we all are.

Now I think we've all had enough for one night, so set down your weather glasses, and I'll meet you back at this same Isobar next week and we'll have another chat over a tall, cool glass of mercury.