A barometer measures air pressure. This simple design will let you record drops in air pressure (signifying bad weather approaching) and rising air pressure (good weather). You can calibrate it yourself, by recording values over several weeks. You'll need some sort of rigid-walled container, some plasticine, a piece of clear tubing, a bottle cap, and some mineral oil. You'll also need some wood to mount it all on, and something to fasten the container to the wood with. This design is from the Time Life Science series, in the volume entitled 'Weather'.
Here's what it looks like:
Keep the bottle cap full of mineral oil. The container shown here is the inside of a thermos bottle, but it could be any container that you can make air-tight, as long as it has rigid walls. This is so that the air pressure inside the container won't be directly affected by the outside air pressure; it will remain constant while the outside pressure changes.
Seal the opening where the tube enters the container with plasticine, to make an air-tight seal.
Attach a piece of paper behind the tube, on which to record the level of the mineral oil.
The container can be kept indoors or outside, since air pressure doesn't vary much inside or out. Just be sure to keep it in a shady location where the temperature doesn't change too dramatically. Hot sun shining on the container could cause the air inside to heat and expand, increasing its pressure, and distorting your readings.
As the air pressure increases, it pushes down on the mineral oil in the bottle cap; this pressure forces the mineral water up the tube, and you'll be able to record a higher reading. Similarly, when bad weather is approaching, the air pressure will drop; when it does, the level of mineral oil will fall.
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