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The weather-person on the television and radio news programs frequently make a comment such as "The barometer is falling," or "The barometer is rising." What on earth does that mean?

A barometer (say "bar-ROM-mit-tur.") measures the weight (pressure) of the atmosphere (the air around us). Air in the atmosphere varies according to the weather, and maybe according to some other stuff. Your writer does not know if different weather makes the pressure change, or if changes in pressure cause the weather to change. Students will have to find out somewhere else!

We'll start from the beginning, and build a water barometer. After we build it and it works, we'll find out how important and valuable it really is to know what the air pressure currently weighs.

Get a bottle with a long slender neck, such as a wine bottle, a salad dressing bottle, or a wine vinegar bottle, and invert it in a quart or liter jar that is half-full of water.

Warm the bottle with both hands until several large bubbles of air escape from it. Let the bottle sit undisturbed for several minutes.

Why does the water rise in the bottle as it cools? Probably because cool air molecules take up less space than warm air molecules, thereby reducing the air pressure inside the bottle, and freeing some space for the water to fill.

Place the jar and bottle in a spot where the temperature remains constant. (A thermometer may be placed nearby to make sure the temperature is constant.)

Mark a large index card lengthwise with lines 1/4 inch apart. Tape the card, with the lined side against the glass, on the side of the jar.

Periodically read and record the level of the water inside the bottle.

You will realize that the water level is an indicator of the pressure of the outside air (providing the pressure inside the bottle does not vary as a result of temperature changes.)

The device you have made is called a water barometer.

Many people have barometers inside their homes. They are usually kind of a decorative item, but they do function. Not realizing that they are for the purpose of measuring air pressure, I used to wonder how an instrument inside the house could tell us anything about the world outside.

After all, a thermometer inside the house may register 72o when outside, there is snow on the ground and it feels like a million below zero. I reasoned, then, that another instrument inside the house could tell us nothing about outside the house. Apparently air pressure weighs the same inside the house as it does outside the house.

Sometimes when people look at their barometers, they tap on them a little bit. Why do you suppose that is?

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