Standard sea-level conditions

Imagine that you and your friend are playing with a ball on the beach. The beach is flat, and you're standing right next to the ocean, where the waves are crashing onto the sand.

Now imagine that you have a little bag of air with you, and you filled it up with the air around you. When you first filled up the bag, the air inside it had a certain pressure - that's like the way the ball feels when you squeeze it, or when you blow it up.

But as you and your friend keep playing, you might notice that the air pressure inside your bag changes a little bit. Maybe the wind shifts direction and blows some more air into your bag, or maybe you move further away from the ocean and the air pressure decreases a little bit.

Standard sea-level conditions mean that we're imagining a very specific situation: you're standing at the beach, right by the ocean, and the air around you has a certain pressure. Scientists use sea level as a way to standardize these conditions, because the air pressure changes depending on how high above sea level you are.

At sea level, the air pressure is about 14.7 pounds per square inch (PSI), or around 101,325 Pascals - that's the units scientists use to measure air pressure. This pressure is what we could call "normal" or "standard" conditions, because it's what most scientists use to compare other measurements.

So when scientists talk about standard sea-level conditions, they mean a situation where the air pressure is around 14.7 PSI or 101,325 Pascals, and you're standing at sea level, right next to the ocean. This is helpful because it gives us a baseline to use when we're making measurements or doing experiments - we can compare our results to what we would expect under normal, everyday conditions.