Monday

Rocket holds up two balloons, blown up to the same size and taped to the ends of a meter stick.

A string at the middle holds the stick level. "Perfectly balanced," he says. "Now watch."

Nova hovers close. "Before you do anything, predict. If one end gets heavier, which way will it move?"

"Down," Raven says. "But air does not weigh anything. Does it?"

Rocket carefully unties one balloon and lets the air hiss out. The stick tips. The full balloon sinks.

Raven stares. "What do you notice? The one with air in it is heavier."

"Now imagine a balloon as tall as the whole atmosphere," Nova says. "That weight is pressing on you right now."

Air is all around us, and it is made of atoms and molecules. They are tiny, but there are a great many of them.

The National Weather Service explains that these molecules are constantly moving in random directions.

When they strike a surface, they exert a force on it. We observe that force as pressure.

Despite their small size, the huge number of molecules exerts a weight on us. We call that weight air pressure.

Our bodies are adapted to live under this weight, so we do not notice it.

Scientists at the University Corporation for Atmospheric Research describe air pressure as the weight of all the air above us.

Above every square inch of the Earth's surface is 14.7 pounds of air. So air presses with 14.7 pounds per square inch at the surface.

When we measure air pressure with a barometer, we are measuring the weight of the column of air directly over us.

A barometer is simply an instrument that measures atmospheric pressure. The word comes from the Greek "baros", meaning weight.

A weighty idea, well handled. Tomorrow you climb a mountain in numbers and learn the units of pressure.