Tuesday, August 13, 2019

How Much Does the Sky Weigh?


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.


No comments:

Post a Comment