Q.If the surface air pressure is 1,000 mb, the air pressure at 1 km above the surface will be:

Think about the last time you flew in a plane, or watched a video of someone climbing a high mountain. As the plane climbs or the climber ascends, something subtle happens to your body — your ears might pop. That popping is your body reacting to a change in the pressure of the air around you. And that change is the heart of the pressure-altitude relationship.

At ground level, you are at the bottom of a giant ocean of air. All that air above you has weight, and it presses down on everything, including you. We call that pressing force atmospheric pressure. Now, here is the key intuition: the higher you go, the less air there is above you. So the weight pressing down on you decreases. Less air above means less pressure. That is the entire relationship in one sentence — as altitude increases, air pressure decreases.

But why does it matter beyond your ears popping? Because this simple fact shapes how we live, travel, and even breathe. The air you breathe is a mixture of gases, and pressure is what pushes those gases into your lungs. At high altitudes, with less pressure, each breath delivers less oxygen to your body. That is why mountaineers carry oxygen cylinders, and why airplane cabins are pressurised — to keep the air inside feeling like it does at a lower altitude, even when the plane is flying miles above the ground.

There are a few things worth holding onto about this relationship:

• It is not a straight line. The pressure drops quickly at first, then more slowly as you go higher. The first few thousand metres make a big difference; the next few thousand make a smaller one.
• It is predictable. Because the change follows a consistent pattern, pilots, weather forecasters, and climbers can use altitude to estimate pressure, and vice versa.
• It affects weather. Low pressure at the surface usually means cloudy, rainy conditions; high pressure means clear skies. So the way pressure changes with altitude helps meteorologists understand and predict the weather.

One more everyday example to cement it. When you open a packet of chips on a mountain, it often puffs up or even bursts. The air inside the packet was sealed at a lower altitude, where pressure was higher. When you carry it up, the outside pressure drops, but the air inside still has its original pressure — so it pushes outward and the packet expands. That is the pressure-altitude relationship working right in your hands.