What goes up…

I ran across this interesting graph showing the altitude, outside temperature, and inside temperature of a flight box as a balloon carried it up to about 32,000 feet and then popped. This is part of the Icarus Project, an adventurous experiment by a fellow named Robert Harrison. The graph is a treasure trove of artifacts to explain, and it is good practice for anyone interested in the sciences to try to interpret it. The comments under the graph show the thoughts of others intrigued by the graph, particularly by the rapid fall in temperature at 13:15.

Let’s start with the altitude graph, though — even that is interesting. You can clearly see where Robert turned on the instruments and then released the package a few minutes later, allowing the balloon to rise. Its ascent is very close to linear until the balloon pops and the flight box starts to fall. Notice its descent, though — at first glance it looks a little odd, convex instead of concave. Usually we expect things falling in a gravitational field to pick up speed as they fall, leading to a concave graph, but the flight box and deflated balloon quickly reach their maximum velocity and then slow down, leading to a convex graph, as the density of the air increases at lower altitudes. It looks like the flight box might have landed in a tree, from which Robert retrieved the package and turned it off.

Now let’s look at the internal and external temperatures. First take a look at the first graph on this page, which beautifully depicts the effect of altitude on temperature in earth’s atmosphere. In fact, from this graph you can clearly see why atmospheric scientists divide the atmosphere into layers. For the first 10 kilometers — about the height reached by Robert’s flight box — there is a remarkably linear drop in temperature on this graph as we get further and further away from the objects on the surface, which soak up and radiate/convect back the sun’s energy.

Back to the Icarus graph, temperature falls as the balloon rises, as expected. Not surprisingly, the exterior temperature drops faster. Then the exterior temperature mysteriously “bounces” off of 20 degrees — what could explain that? Could it be that the heater is controlled by a thermostat and came on at 12:03? If so, it looks like it is mounted close to the exterior sensor — the effect is much more dramatic on that sensor’s readings.

Then at 12:45, the temperature starts rising again in earnest, even though the air temperature is presumably still falling as the balloon continues to rise. This is likely the result of the fact that at higher altitudes the density of the air decreases — fewer collisions between air molecules and the flight box means less heat transfer out of the box, which is heated, so the exterior temperature gets closer to the interior temperature. Or again, if the heater is running close to that exterior sensor…

Then POP! — the flight box and deflated balloon begin to fall rapidly. This rapid descent means that many cold air molecules are colliding with the surface of the box, each taking heat from it. The internal temperature falls as well, but of course more slowly, as the increasingly cold exterior surface cools the air inside the box.

But as the flight box descends, the temperature of the air increases, so eventually the onslaught of air molecules becomes warmer than the exterior and the exterior starts heating up again, and for the first time the exterior sensor registers a higher temperature than the sensor in the heated interior. On the way up the exterior temperature bounced again — is that the heater turning off at 13:26?

Once the descent stopped, close to the ground, the temperatures came to match that of the surrounding air. It appears that at that point the heater was off, because the interior and exterior temperatures are about the same.

What an interesting graph! I wonder if this is how scientists feel when poring over data from probes crashing into comets…


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