About air pressure

Have you ever drunk cocoa or a glass of lemonade through a straw? Of course you have! Have you ever wondered how the cocoa or lemonade gets into your mouth? Probably not! It seems so simple: you suck on the straw and the liquid flows into your mouth as if by itself. Then all you have to do is swallow. We are usually unaware that this process has a lot to do with the air around us. We cannot see it or feel it. But the air is always there. It is made up of tiny particles called molecules. There are countless numbers of them, spread out in a thin layer around the Earth.

All the molecules together form the atmosphere surrounding the Earth. It is very thin. If the Earth could be shrunk to the size of a 1.30 m-tall child, the atmosphere around it would be only a few millimetres thick. Every particle of air is drawn by an enormous force: gravity. All celestial bodies attract one another. The Earth exerts this force of attraction too. It acts everywhere and on everything near it. This force keeps particles of air close to the Earth’s surface so they don’t simply disappear into space.

But what does all this have to do with your cocoa, your lemonade and the straw? It’s simple: When you put a straw into a drink and suck on it, you’re actually only sucking in air. You’re pulling out the air that’s still inside the straw.

The rest is down to air pressure. It presses on the surface of the cocoa or lemonade and pushes some of it up the straw and into your mouth. Air pressure is not the same everywhere. When you climb a mountain, it decreases because there are fewer air particles above you. In other words, the air gets thinner. That means there is much less oxygen available, and every movement feels very strenuous.

Air pressure also plays a major role in the weather. When air warms up, its particles move faster. They need more space and rise. As a result, there are fewer air particles above a particular area than before, and the air pressure is now lower. Weather experts call this a low-pressure system, or simply a “low”. When the particles cool, their movement slows and they sink back down. Other particles flow in from outside, so there are now more air particles in the area overall. This causes the air pressure to rise, creating a high-pressure system, which experts call a “high”. Differences in pressure cause air to flow constantly from a high to a low. We call this movement of air wind, and it can sometimes be quite strong. But something else happens, too. As air particles sink in a high-pressure system, they warm up more. They can then absorb more moisture. The clouds disappear and the weather clears. By contrast, when warm air particles rise in a low-pressure system, they cool down. They can no longer hold as much water vapour, so clouds form and eventually it rains. That is why low-pressure systems are unpopular with many people: they often bring bad weather.