What if air had no weight?

You can't see air. You can hardly feel it.

But air has weight, and it pushes on you all the time! What if air weighed nothing?

Air seems like nothing at all. You can't see it, and you walk through it without effort. But air is made of trillions upon trillions of tiny moving molecules, and gravity pulls on every one of them. All that air piled up above you has weight.

So what if air had no weight? Would anything change? This thought experiment follows three everyday things that quietly depend on the weight of air: drinking through a straw, sticking up a suction cup, and the wind.

Air pressure is the force per area exerted by gas molecules colliding with a surface. Near the ground, the reason that pressure is so large is gravity: the atmosphere is held down and compressed by the weight of the air above.

This thought experiment removes that weight while keeping air as a gas, then asks which everyday phenomena rely on atmospheric pressure. The pressure numbers are measured facts; the consequences are reasoned ideas.

Air molecules bump into everything, all the time.

All those tiny bumps add up to a big push. We call it air pressure.

Air molecules zip around and bump into everything. NOAA explains that each bump is far too small to feel, but the huge number of bumps every moment adds up to a big push called air pressure.

At sea level, the air above every square meter of ground weighs about 101,000 newtons, roughly the weight of 10 metric tons. That's how hard the air presses down. NOAA also notes that air presses in every direction, from the sides and from below as well as from above, so it doesn't only push down on us.

OpenStax states that atmospheric pressure is due to the weight of the air above: on average, a column of air over 1.00 m² at sea level weighs 1.01 × 10⁵ N, so P_atm ≈ 101 kPa. NOAA gives standard sea-level pressure as 1013.25 hPa, or 29.92 inches of mercury.

Because the weight above decreases with height, so does pressure. Although the atmosphere extends hundreds of miles up, NOAA notes that half of its molecules lie within the first 18,000 feet (5.6 km).

Olly pretends that air weighs nothing.

Nothing presses down from above anymore. So straws, suction cups and even the wind would change!

In our pretend world, air still exists, but it has no weight, so nothing is piling it up and pressing it down near the ground. The big push of air pressure would mostly disappear.

Follow the paths in the lab: drinking through a straw, sticking up a suction cup, and weather and wind. Each card says whether it is a measured fact or an idea. Nobody has ever lived in a world like this, so the consequences are careful guesses.

Remove gravity's pull on air and the hydrostatic support that compresses the lower atmosphere vanishes; surface pressure would no longer be set by the column weight. With nothing holding it down, the atmosphere would not stay concentrated near the ground.

The scenarios trace three consequences of losing that pressure. Measured facts are labeled; the outcomes are ideas, since the premise breaks real physics.

When you drink with a straw, you don't really pull the juice up.

You take air out of the straw. Then the air outside pushes the juice up to you!

Here's the surprise: when you drink through a straw, you aren't really pulling the drink up. Sucking takes air out of the straw, lowering the pressure inside. The air pressing on the rest of the drink is now stronger, so it pushes the drink up the straw.

A suction cup works the same way. Squeezing it flat pushes air out from under it. The air outside then presses the cup against the wall harder than the little air left inside can push back.

A straw works because lowering the pressure inside it lets atmospheric pressure on the open surface push liquid up. With a perfect vacuum, sea-level pressure could lift water about 10.3 m (33.8 ft); that column exerts 101,325 N/m², or 14.7 psi.

A suction cup is held by the pressure difference: Tiwari and Persson's model balances the pull-off force plus the cup's own spring force against πr²(p_outside − p_inside). Air slowly leaking in through surface roughness is what eventually lets cups fall.

Weather maps show H for high pressure and L for low pressure.

Air moves from high to low. That moving air is the wind!

Air pressure is a big part of the weather. Weather maps show H for high-pressure areas and L for low-pressure areas. Air flows from high pressure toward low pressure, and that moving air is the wind. The bigger the difference in pressure, the faster the wind blows.

Scientists measure air pressure with barometers. In a mercury barometer, the air pushes a column of mercury up a tube; when the pressure changes, the mercury rises or falls, giving forecasters clues about the weather.

NOAA defines the pressure gradient as the difference between high and low pressure; wind speed is directly proportional to it. OpenStax notes that the large-scale flow of the atmosphere creates weather highs and lows around the 101 kPa average.

In a mercury barometer a near-vacuum above the mercury lets atmospheric pressure support a column of height h, where hρg = P_atm; its rise and fall gives forecasters clues. Meteorologists still report pressure in millibars, and NOAA notes that 'bar' comes from the Greek word for weight.