Black Cup, White Cup: Albedo and a Warming Planet

Two identical cups, the same water, the same sun, one wrapped in black paper and one in white. An hour later the black one is several degrees warmer, and your child has measured albedo. That word covers one of the most consequential numbers in climate science: the fraction of sunlight a surface sends straight back to space. This albedo experiment for kids takes an hour of waiting and a thermometer, and the step almost everyone leaves out is putting lids on both cups, because without them you are measuring evaporation instead.

Key Takeaways

• You’ll put two identical covered cups of water in full sun, one black and one white, and log both temperatures every five minutes for an hour.
• It teaches albedo as a measurable quantity and introduces the ice-albedo feedback.
• Best for ages 8 to 14, on a clear still day.
• Cover both cups. Evaporation cools the warmer cup faster and will wreck your result.
• Expect a difference of 3 to 8 °C, not 20. The honest number is more interesting than an exaggerated one.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Two identical cups, tins or small jars	2	Kitchen. They must be the same size, shape and material	Two identical drinks cans with the tops cut off by an adult
Black paper and white paper	1 sheet each	Stationery	Matte black and white paint. Matte, not gloss
Tape	—	—	Rubber bands to hold the paper on
Water	200 mL per cup, measured	Tap	Use a measuring jug. Equal volumes are not optional
Two thermometers reading 0–60 °C	2	Supermarket or aquarium shop	One thermometer moved between cups, dried each time. Less accurate but workable
Cling film or two lids	2	Kitchen	Do not skip this. It is the difference between a result and noise
Notebook and pen	1	—	A phone timer plus a photo of each reading
Hat, sunscreen, water	—	—	An hour in full sun

Time: prep 15 min, experiment 60 min, cleanup 5 min Cost: everything is probably in your house Mess level: low

Do the Experiment

Step 1 — Make the two cups identical apart from colour

Wrap one cup completely in black paper and the other completely in white, including the base, and tape them so no bare surface shows. If you paint instead, use matte paint and let it dry fully. Gloss paint reflects light off its surface in a mirror-like way and muddies the comparison.

Step 2 — Measure equal volumes at the same starting temperature

Pour exactly 200 mL into each cup from the same jug, so both start at the same temperature. Write the starting temperature down for both. If they differ by more than half a degree, stir and wait.

Step 3 — Cover them, and say why out loud

Seal both cups with cling film or lids. Here is the reason, and it is the whole credibility of the experiment: evaporation carries away a large amount of heat, and it happens faster from the warmer cup. Leave the cups open and the black one loses much of its advantage through its own surface, which is why uncovered versions of this experiment produce small or confusing differences.

Step 4 — Take the prediction and set them in the sun

Ask: “How many degrees apart do you think they will be after an hour?” Most children say 20 or more. Write the number down. Then put both cups side by side in full sun on the same surface, with neither shading the other, and start a timer.

Step 5 — Log both temperatures every five minutes

Read both thermometers at the same moment, with the tips at the same depth. Record the time, both temperatures, and any cloud.

What you should see: both cups warm, and the gap between them widens for the first 30 to 40 minutes and then stabilises. On a clear day with the sun high, expect the black cup to end 3 to 8 °C above the white one. Both will be well above the air temperature. If a cloud passes, both curves flatten at once, which is a useful reminder that sunlight is the whole input.

Step 6 — Add a shaded control, then graph it

Set up a third cup, any colour, in full shade with the same 200 mL of water, and read it too.

What you should see: the shaded cup barely moves. That is the control that proves the sun, not the air, is doing the work. Plot all three temperature curves on one graph. The two sunlit lines separating while the shaded line stays flat is the clearest single picture of what albedo does.

What’s Actually Going On in This Albedo Experiment for Kids

Albedo is the fraction of incoming solar energy a surface reflects, on a scale from 0 to 1. The National Snow and Ice Data Center defines it plainly: a value of 0 is a perfect absorber and 1 a perfect reflector. Whatever is not reflected is absorbed, and absorbed sunlight becomes heat. Your black wrapping has a low albedo, so most of the light that lands on it is absorbed and warms the cup and the water inside. The white wrapping has a high albedo, so a large share of the light leaves again before it can do anything.

The real-world numbers are worth reading out. NSIDC gives a typical ocean albedo of about 0.06, meaning the sea reflects only 6% of the sunlight hitting it and absorbs 94%. Bare sea ice is around 0.5 to 0.7, and sea ice covered with fresh snow reflects as much as 90% of incoming sunlight. Those are not small differences. They are the difference between a surface that absorbs almost everything and one that absorbs almost nothing.

Scaled up to the whole planet, NASA’s Earth Observatory reports that roughly 340 watts per square metre of solar energy reach Earth on average, about a third of it is reflected, and the remaining 240 watts per square metre is absorbed by land, ocean and atmosphere. Satellite measurements put Earth’s average albedo at about 0.30. For comparison, the same article notes that an Earth completely covered in ice would have an albedo near 0.84, and one covered in dark forest canopy near 0.14. The planet’s temperature depends on where in that range we sit, and measuring it is a long-running satellite programme.

This is also why the Arctic is the part of the planet where albedo matters most. Sea ice reflects most of the sunlight that hits it; the ocean underneath absorbs almost all of it. Melt some ice and you replace a bright surface with a dark one, which absorbs more energy, which melts more ice. NSIDC describes the intermediate stage too: once meltwater pools on top of the ice, those ponds have an albedo of roughly 0.4 to 0.5, so the surface albedo drops to about 0.75 and keeps falling as the ponds grow. A process that amplifies itself like this is called a positive feedback, and the ice-albedo feedback is one of the main reasons the Arctic warms faster than the global average.

Two honest caveats. First, your experiment does not measure albedo; it measures a temperature difference and lets you infer that one cup absorbed more energy. Measuring albedo properly needs a light meter pointed at the surface and then at the sky. Second, dark surfaces are better at radiating heat away as well as absorbing it, and at the temperatures involved almost all paints, black or white, are good thermal radiators. That is exactly why a white roof works so well: it reflects most of the visible sunlight and still radiates its own heat away efficiently. The equilibrium temperature difference between your cups is therefore smaller than the difference in absorbed sunlight alone would suggest, and noticing that is a more sophisticated observation than the result itself.

How to Explain It to Your Kid

Ages 5–8: Dark things drink up the sun

Put a black T-shirt and a white one on a sunny step for ten minutes and let them press a palm on each. The difference is instant and needs no vocabulary. Then say that white sends the sunshine back and black keeps it, and let them read the two thermometers and call out the numbers. That is enough. Snow being white and the sea being dark is the version of the planet story they can hold.

Ages 9–12: Make the graph and name the feedback

Give them the logging job and the graph. Two climbing lines separating and one flat line is a picture they can explain to someone else. Then introduce the numbers: 6% reflected by the ocean, up to 90% by snow-covered ice. Ask what happens when the ice melts, and let them construct the loop themselves. Children find positive feedback loops genuinely interesting once they have drawn one.

Ages 13+: Separate absorption from emission

Push them past the obvious result. If black absorbs more, why is the gap only 5 degrees and not 30? Introduce the idea that a hot object also loses heat, by radiation and by conduction, and that the cups settle where gains balance losses. Then design a test: wrap a third cup in shiny aluminium foil, which reflects visible light well but is a poor thermal radiator, and predict where it lands relative to the other two before running it.

The question to ask: “A city wants to stay cooler in summer. Would painting every roof white help, and what would the same change do in a city with cold winters?”

Why Teaching This Early Pays Off

Albedo is one of the few climate concepts that a child can measure rather than be told, and the measurement is honest: a few degrees, not a dramatic one. That matters, because a great deal of climate education is delivered at a scale nobody can verify. An hour with two cups and a thermometer produces a number your child owns.

The second payoff is the feedback loop. Positive feedback is the reason small changes sometimes produce large outcomes, and it appears in population biology, economics, electronics and epidemiology as well as climate. A child who has reasoned through ice melting to darker water to more absorption to more melting has a template for that kind of thinking. It is also the conceptual difference between “the Arctic is warming” and “the Arctic is warming faster than everywhere else, and here is the mechanism”.

The practical applications are immediate, too: light-coloured roofs and pavements measurably reduce urban heat, light clothing is cooler in summer, and a dark car in a car park is a genuinely hotter car. Chi and Wylie’s ICAP framework finds that learners who explain and construct outperform those who watch (Chi & Wylie, 2014), and here the construction is the graph. Without it, this is two cups on a step. No study shows that an hour of this improves school performance, and the useful claim is narrower than that.

Before You Start: What Parents Should Know

Best age	8–14. From 8 for the readings, from 11 for the feedback loop and the foil control
Supervision	Light, but an adult cuts any tins and manages sun exposure over a full hour
Real hazards	Sunburn and heat: hat, sunscreen, water, shade between readings. Water in a dark cup in strong sun can reach 45 to 50 °C, which is hot enough to be unpleasant on skin. Cut tin edges are sharp, so an adult does that and tapes the rims. Use a spirit or digital thermometer, never a mercury one
Don’t	Do not use spray paint indoors or near a child’s face; black and white paper avoids the issue entirely. Do not leave the cups uncovered, and do not compare two different cup sizes or materials, which destroys the comparison

When It Doesn’t Work

• There is almost no difference between the cups. Either they were uncovered, so evaporation equalised them, or it is windy, or the sun is low and weak. Cover both, pick a still day, and run it within two hours of solar noon.
• The white cup ends up warmer. Check for shading: a fence post, a railing or the other cup. Also check that both thermometers read the same when sitting in the same glass of water, because cheap thermometers can disagree by a degree or two.
• Both cups cool down instead of warming. The water started warmer than the air and the sun is too weak to compensate. Start with cool tap water and run the experiment in strong sun.
• The readings jump about. You are moving the thermometers in and out, or the tips are at different depths. Leave both in place and read without lifting them.
• The gap stops growing after 40 minutes. That is the correct result, not a failure. It means both cups have reached the point where heat losses match heat gains, which is the same equilibrium idea behind every thermal system.

Take It Further

1. Add a third and fourth surface. Run shiny aluminium foil and mid-grey alongside black and white. Rank them, then compare your ranking with published albedo values for sand, grass, concrete and water. This is where the experiment becomes a real measurement series.
2. Do it on real surfaces. Lay a thermometer flat on grass, on light pavement and on asphalt in the same sun, and record all three. The asphalt reading is usually startling, and it is the mechanism behind urban heat islands. Pair it with the asphalt-and-grass heat-island experiment.
3. Model the ice-albedo feedback. Float a white polystyrene disc on dark water in a tray, and a second tray with no disc, and compare water temperatures in the sun. Then shrink the disc and repeat. A graph of water temperature against covered area is a model of an amplifying feedback, and it links directly to the two-jar greenhouse-effect model and the solar oven.

What to Watch For Over the Next 3 Months

• Week 4: they comment on the colour of a surface in relation to how hot it is, usually a car roof or a playground slide.
• Month 2: they use the word albedo, or “reflects more”, about something new without prompting.
• Month 3: they reason about a feedback loop unprompted. “If the ice melts it gets darker so more melts” is the sentence to listen for, and it is worth taking seriously when it arrives.

Frequently Asked Questions

Why do the cups have to be covered?

Because evaporation removes a lot of heat and removes more from the warmer cup, which flattens the very difference you are trying to measure. Covering both cups with cling film blocks that path and makes the comparison about absorbed sunlight rather than about which cup dried out faster.

How big a difference should we actually get?

Three to eight degrees Celsius after an hour in strong, still sun. Bigger claimed differences usually come from dark cups left uncovered in a greenhouse, or from longer runs. Getting a modest, repeatable number is a better outcome than getting a dramatic one.

Does this prove that painting roofs white fixes climate change?

No, and it is worth being clear. Light roofs measurably reduce how hot a building and a city get, which is a real benefit for comfort and cooling energy. Global temperature is set by the whole planet’s energy budget, and surface albedo is only one term in it alongside greenhouse gases, clouds and aerosols.

Why is the Arctic warming faster than the rest of the planet?

Partly because of this effect. Sea ice reflects most sunlight and the ocean beneath absorbs almost all of it, so losing ice replaces a bright surface with a dark one, which absorbs more energy and melts more ice. That self-amplifying loop is called the ice-albedo feedback.

Can we measure albedo properly at home?

Roughly, with a light meter or a phone light-sensor app. Point it at the surface to measure reflected light, then at the sky to measure incoming light, and take the ratio. It is crude, because the sensor sees only a fraction of the sky, but it gets you the right order of magnitude and makes the definition concrete.

What school topic does this prepare?

Energy transfer, reflection and absorption, and climate systems. The National Research Council’s framework names “analysing and interpreting data” and “constructing explanations” as core practices, and a three-line graph plus a feedback loop is both at once (NRC, 2012).

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. National Snow and Ice Data Center. “Science of Sea Ice.” https://nsidc.org/learn/parts-cryosphere/sea-ice/science-sea-ice
2. NASA Earth Observatory. “Measuring Earth’s Albedo.” https://earthobservatory.nasa.gov/images/84499/measuring-earths-albedo
3. NASA Earth Observatory. “Climate and Earth’s Energy Budget.” https://earthobservatory.nasa.gov/features/EnergyBalance
4. NASA Goddard Space Flight Center. “Earth Fact Sheet.” NSSDCA. https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html
5. Chi, M. T. H., & Wylie, R. (2014). “The ICAP Framework: Linking Cognitive Engagement to Active Learning Outcomes.” Educational Psychologist, 49(4), 219–243. https://doi.org/10.1080/00461520.2014.965823
6. Alfieri, L., Brooks, P. J., Aldrich, N. J., & Tenenbaum, H. R. (2011). “Does discovery-based instruction enhance learning?” Journal of Educational Psychology, 103(1), 1–18. https://doi.org/10.1037/a0021017
7. National Research Council. (2012). A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas. National Academies Press. https://doi.org/10.17226/13165