Everything on Earth, including the atmosphere, is pulled toward the Earth's center by gravity. This pulling causes things to have weight. Even the atmosphere has weight. Individual gas molecules may not weigh much, but collectively, they're quite heavy.
For air molecules at sea level, the entire atmosphere above is pushing down on them. That's a lot of weight or, in meteorological terms, high pressure. Conversely, fewer air molecules push down on those atop Mt. Everest, so the pressure is less. In short, the higher you ascend in the atmosphere, the fewer molecules are pushing down, meaning that there's less weight and lower pressure.
Think about this in terms of tomatoes One tomato isn't very heavy. Put it at the bottom of an empty 50-gallon barrel and it'll be pretty safe. But fill the barrel with hundreds of similarly sized tomatoes and the one at the bottom will be squashed. That's because of the weight of the other tomatoes, which are applying pressure.
Consider another example. The graphic shows 2 columns of air, A and B. Which column has more molecules in it? Which has greater weight? Which has greater pressure at the bottom? If you answered A to all of the questions, you're right! There are more molecules in column A, meaning that it weighs more. Greater weight means greater pressure. Thus, if you stand under column A at the beach, you'll experience greater atmospheric pressure than if you stand under column B on the top of Mt. Everest.
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