Atmospheric pressure is caused by the weight of the air above you being pulled down by gravity and by constantly moving air molecules colliding with surfaces.

What is atmospheric pressure?

Atmospheric pressure is the force per unit area exerted by the air in Earth’s atmosphere on any surface. At sea level this averages about 101,000 pascals (101 kPa), which is the same as about 14.7 pounds of force on every square inch.

The two main causes

1. Gravity and the weight of air

  • Earth’s gravity pulls air molecules toward the surface, giving the air weight.
  • The air above you forms a huge “column” that presses down on you and everything else.
  • The lower you are (closer to sea level), the more air is stacked above you, so the pressure is higher.
  • As altitude increases, there is less overlying air, so atmospheric pressure decreases with height.

2. Molecules in constant motion

  • Air is a gas made of fast-moving molecules that move randomly and collide with surfaces.
  • Each collision transfers a tiny bit of momentum; the combined effect of countless impacts per second creates a measurable pressure.
  • The higher the number of molecules and their average energy (related to temperature), the higher the pressure.

These two views—“weight of a column of air” and “molecular motion and impacts”—are just different but compatible ways of describing the same physical reality.

Why pressure changes from place to place

  • Temperature: Warm air expands and becomes less dense, often lowering surface pressure; cold air contracts and becomes denser, raising pressure.
  • Amount of air overhead: Regions where the air column has more mass have higher pressure; where there is less mass, pressure is lower.
  • Weather systems: High-pressure areas are where more air has piled up; low-pressure areas are where air has been drawn away upward or sideways, which is why they’re so important in forecasts.

A quick mental picture

Imagine you are standing at the bottom of a very light, invisible ocean made of air. Gravity pulls this air “ocean” down, so the layers near the ground are squashed by the weight of all the air above, and the jostling motion of the squeezed molecules banging into you and everything else is what we measure as atmospheric pressure.

TL;DR: Atmospheric pressure comes from gravity pulling the mass of the atmosphere down (so the air has weight) and from the continuous impacts of fast-moving air molecules on surfaces; together, these create the pressure that changes with altitude, temperature, and weather patterns.