As the director of the Kirkpatrick
Planetarium for several decades, I received many questions from kids on school
field trips. They often centered around aliens: do I believed they exist (I do),
and do I believe they visit Earth (I don’t).
One of the more interesting
questions I ever received was “How much does the sky weigh?” At first blush, it
might seem that the answer is simple: nothing. That is not correct.
First, we need to define “the sky.”
To answer this question, I count as “sky” our atmosphere and all the water
vapor it contains. The Karman line, 60 miles above Earth’s surface, defines the
official boundary between our atmosphere and the edge of space, so that is what
we have to weigh.
Earth from space. Credit NASA
Sea level atmospheric pressure is 15 pounds per square inch, meaning every square inch of our planet’s surface
has 15 pounds of air above it. One square inch is roughly one-third to one-half
the size of an adult thumb. That corresponds 60,217,344,000 pounds of air for
every square mile. Earth has a total surface area of 201,061,929 square miles.
Do the math. Our atmosphere weighs 12,107,415,343,900,000,000 pounds.
According to meteorologists, water
vapor averages 0.04% of the volume of Earth’s atmosphere. That means the sky
holds 4,824,146,196 gallons of water. At eight pounds per gallon, that’s
another 38,593,169,564
pounds of water in the air.
Add it all up, and the total
weight of our sky is 12,107,415,382,493,169,564 pounds, more than 12
quintillion pounds! I didn’t say all of that at the time, because I can’t
multiply that quickly in my head. I simply told the child that it’s more than
all of us could hold.
Now, about those aliens …
On or about the first Tuesday of each month, I write an
astronomy-related column piece for the Oklahoman newspaper. On the following
day, I post that same column to my blog page.
This is reprinted by permission from the Oklahoman and www.newsok.com.
