How to make a Barometer

What you need:
One clear glass or plastic jar
One piece of transparent tube, a drinking straw is ideal
Sticky tape
A little bit of modelling clay
Some cold water
A few drops of food colouring.

Put some cold water into the jar, so that it's less than half full.

Add some food colouring, this will make it easier to make your readings.

Put the straw into the jar and tape it to the inside, so that the bottom end of the straw is in the water, but not touching the bottom of the jar.

Suck the water halfway up into the straw and pinch it to trap the water.

When you take your mouth off the straw, keep pinching it until you get the piece of modelling clay over the top to block it.

That's your barometer made.
How does it work  

The only other thing you need is some method of recording changes in the level in the straw. You could attach a piece of paper on to the outside of the jar and mark it each day, or you could use an elastic band around the jar.
 Leave in a place where it's not too draughty, and not too sunny.

When the atmospheric pressure rises, the water in the jar will be forced downwards which will, in turn force the water in the straw to rise. So,rising air pressure, rising water in the straw and vice versa. This is your first step in observing the weather patterns. As a rule of thumb, when the air pressure is rising, the weather is generally going to be "quiet and settled", generally dry. But when the pressure is falling, the weather will be turning more unsettled, perhaps with the approach of a frontal system, for instance.