Every second, the Sun shines on the Earth. Of every 100 units of sunlight that arrive, about 30 bounce straight back off clouds, ice and bright ground. About 70 are soaked up by the ground, the sea and the air, and become heat.
Sunlight arrives
↓→
The ground and sea warm up
↓→
Warm things glow heat out to space
Anything warm gives off heat. You cannot see it, but your hand feels it near a radiator. The Earth does the same. It glows heat out to space, all day and all night. The warmer it is, the harder it glows.
Sunlight soaked up70 units
Heat sent out to space0 units
The second bar is the question.
The Earth soaks up 70 units of sunlight. To stay at the same temperature, how much heat must it send back out to space?
About 100 units — all the sunlight that arrived.
Not yet. Thirty of the hundred never became heat. They bounced off clouds and ice before anything warmed. Only the 70 that were soaked up have to leave again.
About 70 units — the same as it takes in.
Right. If out matches in, nothing is left over to warm the planet and nothing is missing to cool it. That is what staying at one temperature means.
Less than 70 — some heat stays in the ground for good.
Not yet. Heat that stays warms the ground, and a warmer ground glows harder. Nothing can hide for good; it only raises the temperature until it does leave.
Sunlight soaked up70 units
Heat sent out to space65 units
The gap is heat that stays and warms the planet.
Now suppose the planet sends out only 65 while 70 come in. The 5 left over stay, and the planet warms. A warmer planet glows harder, so the out number rises. It keeps rising until it meets 70. That meeting point is the temperature the planet settles at.
Move the control to set how warm the planet is, and watch what it sends out. The out numbers follow the real rule for glowing things: hotter glows harder, and much harder.
Sunlight soaked up70 units
Heat sent out to space49 units
-40°CIt sends out 49 units and takes in 70. The 21 left over warm it, so it will not stay this cold.
Sunlight soaked up70 units
Heat sent out to space70 units
-18°CIt sends out 70 and takes in 70. Nothing is left over and nothing is missing. It stays here.
Sunlight soaked up70 units
Heat sent out to space92 units
0°CIt sends out 92 units and takes in 70. It is losing 22 more than it gets, so it cools back down.
Sunlight soaked up70 units
Heat sent out to space114 units
15°CIt sends out 114 units and takes in 70. It is losing 44 more than it gets, so it cools back down.
Sunlight soaked up70 units
Heat sent out to space140 units
30°CIt sends out 140 units and takes in 70. It is losing 70 more than it gets, so it cools back down.
Where does this planet settle?
At the hottest setting — the Sun keeps adding heat.
Not yet. The Sun adds the same 70 at every setting. At 30°C the planet sends out far more than 70, so it loses heat and cools back towards −18°C.
At −18°C, where out equals in.
Right. Colder than that and it keeps some heat and warms. Warmer than that and it loses more than it gets and cools. Only at −18°C does neither happen.
At the coldest setting — space is cold and pulls heat away.
Not yet. Space does not pull. A cold planet simply glows less. At −40°C it sends out under 50 and gets 70, so it keeps the rest and warms.
Move the control.
A bare Earth with no air−18°C
The real Earth15°C
Do that sum for a bare Earth with no air, and the ground would settle near −18°C. The oceans would be ice. The real ground averages about 15°C. So the air is doing about 33 degrees of work. The next lesson is about how.
Sunlight soaked up70 units
Heat sent out to space62 units
A planet takes in 70 units and sends out 62. How many units stay behind each second to warm it?
units
Right. The gap looks small next to the 70, but it never stops. Eight units every second, all year, is what moves a planet's temperature.
Each of these changes one of the two numbers. Which way does the temperature go?
One card at a time. Tap the pile it belongs to.
Card 1 of 5
Fresh snow covers the land and bounces more sunlight back.
Dark sea replaces bright ice and soaks up more sunlight.
Something in the air slows the heat on its way out.
Ash from a giant volcano shades the ground for a year.
The Sun brightens a little.
Fresh snow covers the land and bounces more sunlight back. → Cooler
Not yet. This makes it cooler. Less sunlight is soaked up, so less heat comes in, and the planet settles lower.
Dark sea replaces bright ice and soaks up more sunlight. → Warmer
Not yet. This makes it warmer. More sunlight is soaked up, so more heat comes in than goes out, until it settles higher.
Something in the air slows the heat on its way out. → Warmer
Not yet. This makes it warmer. With heat leaving more slowly, the planet has to get hotter to push the same 70 units out.
Ash from a giant volcano shades the ground for a year. → Cooler
Not yet. This makes it cooler. The ash bounces sunlight away before the ground can soak it up, so less heat comes in.
The Sun brightens a little. → Warmer
Not yet. This makes it warmer. A brighter Sun means more sunlight arrives to be soaked up, so more heat comes in.
More soaked up, or heat slowed on the way out→Warmer
Right. Everything on the list changes one of two numbers: how much sunlight is soaked up, or how easily heat gets out. Snow and ash cut the first. Dark sea and a brighter Sun raise it. Slowing the heat out means the planet must get warmer to push the same 70 through.
Sunlight per square metre, Earth100 units
Sunlight per square metre, Mars43 units
Mars is farther from the Sun. Each square metre gets about 43 units of sunlight to the Earth's 100, and its air is very thin. Where does Mars settle?
Warmer than Earth. A planet farther out has had more time to warm.
Not yet. Time does not add heat. A planet warms only while in beats out, and on Mars that stops at a much colder point because in is so much smaller.
Far below freezing. It needs to glow much less to match a smaller in, so it settles colder.
Right. Mars averages about −60°C. Less comes in, so it balances at a lower glow, and a lower glow is a colder planet. With almost no air, there is no blanket to lift it.
The same as Earth. Every planet settles at the temperature where out equals in.
Not yet. Every planet does settle where out equals in. But in is different on Mars, so the out that matches it is smaller, and a smaller out means colder.
Lesson complete
A planet warms or cools until the heat it sends out matches the sunlight it takes in.