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my mom got this Letter From Steve Horstmeyer

Sorry for not answering immediately, but a construction crane fell on power lines in Downtown Cincinnati we lost power and it took a couple hours to get all the weather office back on line.


Now, after my daily nap here is the answer. Bob told me his answer is -32°F and he is way...way off.

Forgive me but I always fully explain my answers so this will be a bit longer than just an answer.


When we say "temperature" what do we mean? We all have an intuitive feel for the difference between "hot" weather and "cold" weather and we know that what we will wear at 90°F and a 0°F will be different.


In the world of the scientist air is like a jar of different colored marbles - 78% of the marbles are green, 21% are blue and the other 1% are made up of a variety of other colors.

In the atmosphere the green marbles are nitrogen molecules, blue are oxygen and for most explanations, like this one, we can ignore all the other colors.

There is a BIG difference between the jar of marbles and the atmosphere, the marbles just sit there but molecules in the atmosphere move at incredible speeds.

In one cubic meter of air, about a cubic yard, there are more molecules of air than there are stars in the universe, the number of molecules in a single cubic meter at sea level is:

26,877,618,225,750,776,665,516,051 molecules

The number is 26.87 septillion, and if you had that many dollars and spent $1 trillion per day it would take 73.6 billion years to spend it!!!

When big numbers come into play scientists love to talk about "averages".

So what about temperature? - temperature is just a measure of how fast those molecules move and because there are so ... so many we get around the big numbers by talking about an "average molecule".

To a scientist the temperature of the air is a measure of how fast the average molecule moves. The branch of science that deals with this is "kinetic theory".

At 0°F (-18°C or -255°Kelvin) the average dry air molecule (nitrogen or oxygen) is moving at a speed of 965 mph, but molecules are so small it does not hurt when they hit your skin. That is our "average molecule", some are going faster, some slower.

The warmer a mass of air is the faster the average molecule moves.

So you daughter's question, translated into the language of a scientist would be:

At what temperature is the velocity of the "average air molecule" half of the velocity it is at 0°F?

When I use a calculator at the great website "Hyperphysics"

the velocity of the average air molecule at 0°F is 965 mph and the air temperature has to cool to -344°F (-209°C and 64°Kelvin) for the average molecule to move half as fast, 428 mph. In case you daughter knows the answer and is just testing us there are two other measures of "average velocity" and the speeds vary a bit, but the answer remains the same -344°F.

If your daughter is interested in a simple explanation of kinetic theory guide her to my website and a page I use when I teach meteorology and oceanography at The College of Mt. Saint Joseph:

Your question makes such a great "science meal" that I will use it tomorrow as the question of the day at 6:10am

Thanks for watching and sorry this is so long but I just love to discuss science and weather.

-Steve Horstmeyer