When you sit down to a glass of iced tea with a straw in it, you might say that you are sucking the tea up through the straw. In fact, a more accurate description would be to say that the atmospheric pressure is pushing the liquid up the straw.
Decreasing Air Pressure
With the straw just sitting in the glass, the pressure on the surface of the tea is the same all over, including on the little bit of surface inside the straw.
When you suck the air out of the straw, you decrease the pressure inside the straw, allowing the higher pressure on the rest of the surface to push the tea up the straw and into your mouth.
Why Do Straws Work?
Because it is really the atmosphere that is doing the pushing, the atmospheric pressure limits how high water will go up a straw. At sea level, the air pressure is enough to support a column of water about thirty feet high.
Even if you could suck all the air out of a forty-foot straw, the water wouldn't rise more than thirty feet. In space, where there isn't any atmospheric pressure, a straw wouldn't work at all.
How Does A Barometer Work?
This same principal explains how a barometer measures the atmospheric pressure. Since mercury is much heavier than water, the atmospheric pressure can only support a column of mercury about thirty inches high. But as the atmospheric pressure changes, so does the height of the column it can support.
When the meteorologist announces that the barometric pressure is twenty-nine inches of mercury, it means that the air pressure is capable of supporting a column of mercury twenty-nine inches high.
You could measure the atmospheric pressure with a column of water, but a thirty-foot water barometer would be much more cumbersome than a three-foot mercury barometer.