The Thing You Cannot See

Why the Sky Is Doing That

Name Date
DIRECTIONS: Air has weight, and miles of it press on you. That push is pressure: you cannot feel it, but forecasters watch it.
1Read why air presses
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2Page 2: build a barometer
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3Page 3: two weeks of readings
one sheet of paperall the air above it, to the top of the sky620 kgabout the weight of a small carandbackpushingevery wayat once

So why is the paper not flattened? Air pushes just as hard from below, from the sides and from inside you. Pushed equally from every side, you feel nothing.

The number moves, a little.

Normal is about 1013 on a barometer. A big storm: near 950. A long settled spell: near 1050. All the world's weather fits in that 10% wobble.

Why falling pressure means rain

From sheet 10: low pressure means air rising.

  1. Pressure here is low, which means the air over this spot is rising.
  2. Rising air moves into thinner air above it, so it spreads out — it expands.
  3. Expanding cools it. This is the same reason a spray can goes cold in your hand.
  4. Cool air cannot hold as much invisible water as warm air can.
  5. So it lets some go, and the water it lets go of turns into droplets. That is cloud.

Falling pressure, weather coming. Rising pressure, clearing up.

Build One

Why the Sky Is Doing That

DIRECTIONS: Build a barometer: a jar, a balloon, a rubber band, a straw and a card.
  1. Cut the neck off a balloon. Stretch it tight over the jar and band it on. It must be airtight.
  2. Tape a straw flat on the skin: one end at the middle, the rest past the rim.
  3. Stand a card behind the free end. Mark where the tip points today, with the date.
  4. Mark it every morning and evening.
balloon skintrapped aircardPRESSURE RISINGskin pushed IN, pointer goes UP
High pressure pushes the skin in. The tip climbs.
balloon skintrapped aircardPRESSURE FALLINGskin bulges OUT, pointer goes DOWN
Low pressure: the trapped air bulges the skin up. The tip drops.
Its one fault: heat.

Warm air in the jar swells and drops the pointer, like falling pressure. Keep it in one indoor spot, away from windows, heaters and sun.

Two Weeks of Readings

Why the Sky Is Doing That

DIRECTIONS: Morning and evening: which way did the pointer move?
DateTimePointer: up, down or same?Sky todayWhat happened next day

A slow drift means little. A fast fall in a few hours means wind and rain, usually within a day.