---
title: "Air Pressure and Weather"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 36
exercises: 11
source: https://one-course.com/books/physics/1/en/chapter/36-air-pressure-and-weather
---

# Chapter 36 — Air Pressure and Weather

You live at the bottom of an ocean — an ocean of [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air), hundreds of kilometres deep, with the whole [weight](https://one-course.com/books/physics/1/en/chapter/24-weight-and-mass#def-g4-weight-and-mass-weight) of it resting on your shoulders. Why does it not crush you? Why do you not even feel it? And what does any of this have to do with tomorrow’s weather? [Air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air), our old invisible friend, has one more big secret to give up.

## 36.1 Air has weight

**Example 36.1 (Weighing the invisible).**

A firm football is placed on a fine [balance scale](https://one-course.com/books/physics/1/en/chapter/17-levers-and-balance-scales#def-g3-levers-and-scales-scale) and balanced exactly. Then its valve is opened and the extra [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) hisses out — same ball, same valve, same everything, minus some [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air). Back on the scale: the deflated ball is *lighter*. The difference is the [weight](https://one-course.com/books/physics/1/en/chapter/24-weight-and-mass#def-g4-weight-and-mass-weight) of the escaped [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air). [Air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) is stuff, stuff has [mass](https://one-course.com/books/physics/1/en/chapter/20-measuring-mass#def-g3-measuring-mass-mass), and [mass](https://one-course.com/books/physics/1/en/chapter/20-measuring-mass#def-g3-measuring-mass-mass) is pulled down by the Earth: the ocean above us is heavy.

**Definition 36.2 (The atmosphere and its pressure).**

The *atmosphere* is the deep layer of [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) wrapped around the whole Earth. Its [weight](https://one-course.com/books/physics/1/en/chapter/24-weight-and-mass#def-g4-weight-and-mass-weight) squeezes the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) at the bottom — where we live — and squeezed [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) pushes hard on every surface it touches. That push is called *air pressure*.

**Proposition 36.3 (The push from all sides).**

[Air pressure](#def-g5-air-pressure-weather-pressure) pushes on every surface from *every* direction — downward on tables, sideways on walls, even *upward* on ceilings and palms. And it is mighty: on an area the size of your palm, the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) pushes about as hard as the [weight](https://one-course.com/books/physics/1/en/chapter/24-weight-and-mass#def-g4-weight-and-mass-weight) of a large, full suitcase. We never notice, because the pushes come balanced: [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) inside us and behind every surface pushes back just as hard.

**Method 36.4 (The card that holds back water).**

[Air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air)’s upward push, caught red-handed (do it over the [sink](https://one-course.com/books/physics/1/en/chapter/5-floating-and-sinking#def-g1-floating-and-sinking-words)):

1. fill a glass with water to the very brim;
2. lay a stiff, smooth card flat on top, touching the water all around;
3. hold the card in place, turn the glass exactly upside down, and gently let the card go.

The card stays — holding back the whole glassful. The water’s [weight](https://one-course.com/books/physics/1/en/chapter/24-weight-and-mass#def-g4-weight-and-mass-weight) presses down on the card, but the [atmosphere](#def-g5-air-pressure-weather-pressure) presses *up* on it harder still. You have just seen the suitcase-push from below.

![The upside-down glass: the air’s upward push on the card out-muscles the water’s weight. The invisible ocean is strong.](https://one-course.com/images/onecourse/chapters/physics-1/g5-air-pressure-weather/fig-d3c46dc5c9bb.svg)

*The upside-down glass: the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air)’s upward push on the card out-muscles the water’s [weight](https://one-course.com/books/physics/1/en/chapter/24-weight-and-mass#def-g4-weight-and-mass-weight). The invisible ocean is strong.*

## 36.2 Machines run by the atmosphere

**Example 36.5 (The straw).**

You think you pull juice up a straw. In truth, you cannot pull a [liquid](https://one-course.com/books/physics/1/en/chapter/14-solids-liquids-gases#def-g3-solids-liquids-gases-liquid) at all — try pulling water with your fingers! What you really do is suck the *[air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air)* out of the straw. With the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) gone from inside, the [atmosphere](#def-g5-air-pressure-weather-pressure) pressing down on the juice in the glass pushes juice up the empty straw to your mouth. Every sip is the [atmosphere](#def-g5-air-pressure-weather-pressure) doing the lifting; you only clear the road.

**Example 36.6 (Suction cups and crushed bottles).**

The bathroom hook’s rubber cup is pressed flat to the tiles, squeezing the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) out from underneath: now the [atmosphere](#def-g5-air-pressure-weather-pressure) pushes the cup against the wall with nothing behind it pushing back — pinned by the sky itself. And screw the cap onto an empty plastic bottle high on a mountain, then drive down into the valley: the bottle crumples as if squeezed by a ghost hand. More [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) presses outside than the thin mountain [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) sealed inside — the [atmosphere](#def-g5-air-pressure-weather-pressure) wins, and the bottle folds.

**Example 36.7 (Thinner with height).**

Climb, and you leave part of the ocean below you: less [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) above means a weaker push. Your ears pop in a fast cable car as the pressure outside changes; mountaineers gasp in [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) grown thin. And here the mountain-tea mystery of last year surrenders: with the [atmosphere](#def-g5-air-pressure-weather-pressure) pressing less on the water, boiling comes easier — water on great peaks boils well below $100\,{}^{\circ}\mathrm{C}$, too cool for a proper brew.

![A thundercloud towering over the fields: weather is the atmosphere on the move.](https://one-course.com/images/onecourse/chapters/physics-1/g5-air-pressure-weather/img-80e45db67bb7.jpg)

*A thundercloud towering over the fields: weather is the [atmosphere](#def-g5-air-pressure-weather-pressure) on the move.*

## 36.3 The barometer and the weather

**Definition 36.8 (Barometer).**

A *barometer* is the instrument that [measures](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-measure) [air pressure](#def-g5-air-pressure-weather-pressure) — one more member of our family of honest judges, beside the [thermometer](https://one-course.com/books/physics/1/en/chapter/15-temperature-and-thermometers#def-g3-temperature-thermometers-temperature) and the scale. Its needle rises when the [atmosphere](#def-g5-air-pressure-weather-pressure) above presses harder, and falls when the press lightens.

**Proposition 36.9 (Pressure talks about tomorrow).**

The [atmosphere](#def-g5-air-pressure-weather-pressure)’s pressure is restless: heavy, sinking [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) brings calm, clear skies; [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source), rising [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) invites clouds and rain. So the [barometer](#def-g5-air-pressure-weather-barometer) is a fortune teller of a modest, honest kind: *steady or rising* pressure promises fair weather; *falling* pressure warns of clouds coming; a *fast fall* means a storm may be on its way.

![Reading a barometer: not where the needle stands so much as which way it moves — the atmosphere announces its plans.](https://one-course.com/images/onecourse/chapters/physics-1/g5-air-pressure-weather/fig-a12b24c26ce7.svg)

*Reading a [barometer](#def-g5-air-pressure-weather-barometer): not where the needle stands so much as which way it moves — the [atmosphere](#def-g5-air-pressure-weather-pressure) announces its plans.*

**Remark 36.10 (The weather map’s H and L).**

Tonight’s weather report will show great swirls labeled H and L: regions of high and low pressure drifting across the map like slow whales. [Air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) forever pours from the heavy H regions toward the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) L regions — and that pouring *is the [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind)*. One chapter, three old friends explained: the [barometer](#def-g5-air-pressure-weather-barometer)’s forecast, the [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind)’s origin, and the reason your ears know when the weather is about to break.

## 36.4 Exercises

**Exercise 36.1 ★.**

How does the football experiment show that [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) has [weight](https://one-course.com/books/physics/1/en/chapter/24-weight-and-mass#def-g4-weight-and-mass-weight)? What exactly is compared?

**Solution of Exercise 36.1.**

The same ball is weighed firm and then deflated: only [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) left it, and the scale reads less — the difference is the escaped [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air)’s [weight](https://one-course.com/books/physics/1/en/chapter/24-weight-and-mass#def-g4-weight-and-mass-weight). [Air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) is stuff with [mass](https://one-course.com/books/physics/1/en/chapter/20-measuring-mass#def-g3-measuring-mass-mass).

**Exercise 36.2 ★.**

What is [air pressure](#def-g5-air-pressure-weather-pressure), and where does it come from? In which directions does it push?

**Solution of Exercise 36.2.**

The push of the squeezed [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) at the bottom of the [atmosphere](#def-g5-air-pressure-weather-pressure) on every surface it touches. It comes from the [weight](https://one-course.com/books/physics/1/en/chapter/24-weight-and-mass#def-g4-weight-and-mass-weight) of the deep ocean of [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) above us, and it pushes from every direction — down, sideways, and up.

**Exercise 36.3 ★.**

The [atmosphere](#def-g5-air-pressure-weather-pressure) presses on your palm about as hard as a full suitcase weighs — yet you feel nothing. Why not?

**Solution of Exercise 36.3.**

Because the pushes come balanced: the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) on the other side of your palm — and inside your body — pushes back exactly as hard. Only an *unbalanced* push, like on the card or the crumpling bottle, shows its might.

**Exercise 36.4 ★.**

In the card trick, two pushes fight over the card. Name them, and say which one wins.

**Solution of Exercise 36.4.**

The water’s [weight](https://one-course.com/books/physics/1/en/chapter/24-weight-and-mass#def-g4-weight-and-mass-weight) pushes the card down; the [atmosphere](#def-g5-air-pressure-weather-pressure) pushes the card up. The [atmosphere](#def-g5-air-pressure-weather-pressure) wins — the card stays and the water is held in.

**Exercise 36.5 ★.**

“I pull the juice up the straw.” Correct this sentence: what do you really remove, and who does the lifting?

**Solution of Exercise 36.5.**

You remove the *[air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air)* from inside the straw. The [atmosphere](#def-g5-air-pressure-weather-pressure), pressing down on the juice in the glass, then pushes juice up the cleared straw: the sky does the lifting.

**Exercise 36.6 ★.**

Why does a suction cup grip the tiles? What was squeezed out, and what holds the cup on?

**Solution of Exercise 36.6.**

Pressing the cup flat squeezed out the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) behind it. With nothing pushing back from underneath, the [atmosphere](#def-g5-air-pressure-weather-pressure)’s push pins the cup to the tiles.

**Exercise 36.7 ★.**

What happens to [air pressure](#def-g5-air-pressure-weather-pressure) as you climb a mountain, and why? Give two signs of it a traveler can notice.

**Solution of Exercise 36.7.**

It weakens — less ocean of [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) remains above to press down. Signs: ears popping on a fast climb, breathlessness in the thin [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) (and tea that boils too cool).

**Exercise 36.8 ★.**

What does a [barometer](#def-g5-air-pressure-weather-barometer) [measure](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-measure)? Your [barometer](#def-g5-air-pressure-weather-barometer) has been falling fast since breakfast — what should the picnic plans expect?

**Solution of Exercise 36.8.**

The push of the [atmosphere](#def-g5-air-pressure-weather-pressure) — [air pressure](#def-g5-air-pressure-weather-pressure). A fast fall warns of a storm on the way: move the picnic indoors.

**Exercise 36.9 ★★.**

The sealed bottle crumpled on the drive down from the mountain. Explain the crumpling with pushes inside and outside — and predict: what would happen instead to a bottle sealed in the valley and carried *up*?

**Solution of Exercise 36.9.**

Sealed on the peak, the bottle kept thin mountain [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) inside; in the valley the outside push grew stronger than the trapped push inside, and the bottle folded. Carried the other way, a valley-sealed bottle keeps *strong* [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) inside while the outside push weakens with height: the bottle swells taut — and a sealed bag of crisps does the same on every mountain drive.

**Exercise 36.10 ★★.**

Where does [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind) come from, according to the weather map’s H and L swirls? Connect it to the oldest definition in this book: [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind) is [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) on the move — moved by what?

**Solution of Exercise 36.10.**

[Air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) pours steadily out of the heavy high-pressure regions toward the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) low-pressure ones, and that pouring is the [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind). So the old definition completes itself: [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind) is [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) on the move — moved by differences in [air pressure](#def-g5-air-pressure-weather-pressure).

**Exercise 36.11 ★★.**

Mountain climbers complain that on great peaks, tea never brews strong and pasta stays hard however long it boils. Gather the whole explanation across two years: what does the thin [atmosphere](#def-g5-air-pressure-weather-pressure) do to the [boiling point](https://one-course.com/books/physics/1/en/chapter/21-melting-and-boiling-changes-of-state#def-g4-changes-of-state-boiling-point), and why does longer boiling not help? (Two propositions, one from last year, close this case.)

**Solution of Exercise 36.11.**

With height, the [atmosphere](#def-g5-air-pressure-weather-pressure)’s push on the water weakens, so boiling comes easier: the [boiling point](https://one-course.com/books/physics/1/en/chapter/21-melting-and-boiling-changes-of-state#def-g4-changes-of-state-boiling-point) slides well below $100\,{}^{\circ}\mathrm{C}$. And by the plateau law, boiling water can never climb past its [boiling point](https://one-course.com/books/physics/1/en/chapter/21-melting-and-boiling-changes-of-state#def-g4-changes-of-state-boiling-point) — extra flame only makes steam. So peak-top water boils cool *and is stuck there*: tea weak, pasta hard, physics satisfied.