When you look up at the sky, for a moment a big fluffy cloud might block the sun, and then you’re in the shade. You’re standing in the cloud’s shadow, and on a windy day when the clouds move fast, you can see the edge of the shadow come towards you on the ground as you enter the sunshine again. The cool thing is that the cloud shadow on the ground is about the same shape and size as the cloud itself (if the sun’s close to overhead). When your hand makes a bunny-rabbit shadow on the wall, that shadow becomes bigger if the lamp is close by, smaller as the lamp moves away. Same thing with our Sun. Sure, clouds are high over our heads: rainclouds start around 6,000 feet, wispy cirrus clouds at 19,000 feet and up, and giant storm clouds called cumulonimbus can reach 60,000 feet high. But the Sun is 93 million miles away, so the clouds are obviously a lot closer to us. Next time you chase the edge of a cloud shadow, see if you can do the math to find its size!
Wee ones: If your hand is 4 inches tall and you make a bunny-rabbit shadow that’s 1 inch taller, how tall is the bunny shadow?
Little kids: Which is higher in the sky, a horse-tail cirrus cloud at 21,000 feet or a raincloud at 12,000 feet? Bonus: If that chunk of raincloud is half as wide as it is high in the air, how wide is it (and its shadow)?
Big kids: If your street block has 15 houses on each side of the street and the bottom 1/3 of the block is in shadow, how many houses are still in the sun? Bonus: If it takes 10 minutes for a cloud to pass across the Sun and that cloud is blowing across at 12 miles an hour, how wide is that cloud? (Hint if needed: An hour has 60 minutes…so what fraction of an hour is 10 minutes?)
The sky’s the limit: If you’re riding a bike down a long, straight road, and the edge of the cloud shadow is 100 feet up ahead and moving 20 feet per second and you’re riding at 40 feet per second, how soon will you reach the edge and ride into the sunshine?
Wee ones: 5 inches tall.
Little kids: The cirrus at 21,000 feet. Bonus: 6,000 feet wide (a bit more than a mile).
Big kids: 20 houses are in the sun, because 10 are in shadow (5 on each side). Bonus: 2 miles wide.
The sky’s the limit: Just 5 seconds. You’re moving 20 more feet each second than the cloud, so it will take only 5 seconds to do 5 of those 20-foot chunks and catch up over the 100 feet.