The Sun Heats It Unevenly

Why the Sky Is Doing That

Name Date
DIRECTIONS: If the sun warmed the earth evenly, there would be no wind, cloud or rain. Find out why, then test it.
1Read pages 1 and 2
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2Page 2: test a coast breeze
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3Page 2: the harder question
equatorspread over 31squeezed into 14the same sunlightarriving at both

Same beam, same sunlight. Near the pole the ground tilts away, so the beam spreads over about 2.2 times as much ground and each patch gets less. It is the angle, not the distance from the sun.

The same thing, at the size of a beach

HOTall of it staysin the top inchlandseashared all the way down,and stirred about

Same sun on land and sea: the land keeps its heat in the top few inches, the sea spreads it deep. The land is hot by lunchtime; the sea has barely noticed. After dark the land cools faster too.

A Prediction You Can Go and Check

Why the Sky Is Doing That

Wind is air evening out a temperature difference.

One patch getshotterThe air over itrisesLess air isleft pressingdownAir slides infrom beside itthe whole of weather, in four steps
  1. One patch gets hotter. Because of the angle, or because it is land and next to it is water.
  2. The air over it rises. Warm air is thinner for its size, so it floats up through the cooler air around it.
  3. Less air is left pressing down. The weight of air above that patch drops. That is what LOW PRESSURE means.
  4. Air slides in from beside it. From wherever there is more, towards wherever there is less. That sideways flow IS the wind.

If the chain is right, a coast breeze blows in off the water in the afternoon and the other way before dawn.

landseawarm air risesonshoreAFTERNOON — land is warmer
A flag on the beach points inland.
landseawarm air risesoffshoreBEFORE DAWN — sea is warmer
The land is cooler now. The flag has swung around.
Test it.

On a still, sunny day at a coast, check a flag or a wet finger in the afternoon, then late evening. No swing? The chain is wrong, or bigger weather got in the way.

afternoon: the wind comes from late evening: the wind comes from

Harder: the equator and poles do the same, on a giant scale. Which way does the air move at the bottom, and along the top?

at the bottom: from … to … along the top: from … to …