The air pressure at the Earth's surface is greatest because of the weight of the air above it. The air is made up of tiny particles called molecules, and these molecules are constantly colliding with each other and with the ground. The weight of all these collisions creates pressure.

The air pressure is also affected by temperature. When the air temperature is higher, the air molecules move faster and collide with each other more often. This increases the pressure. When the air temperature is lower, the air molecules move slower and collide with each other less often. This decreases the pressure.

The air pressure also varies with altitude. The higher you go above sea level, the less air there is above you. This means that the weight of the air above you is less, and the air pressure is lower.

At sea level, the average air pressure is 14.7 pounds per square inch (psi). This means that for every square inch of surface area, there is a force of 14.7 pounds pressing down on it. The air pressure decreases by about 1 psi for every 1,000 feet you go above sea level.