I know this is not an absolute number, but on average how many Km is 1 Kg of atmosphere?

Question

Many times the Storm Predicition Center gives us numerical figures. They often use Joules per kilogram as a measure of energy. So I want to have an idea of how big a kilogram of air is (median, average, whatever)

Answer 1

Using 22.4 liters volume for one gram-molecular weight of a gas at standard temp. & press. (see ref.), for air with average MW = 29, 1 kg has a volume of 772 liters or 0.772 m^3.

Answer 2

OK, so we will have to make some assumptions to answer your question.

* Ground level... 1 atmosphere
* STP (Standard temperature and pressure) means the temperature is at 0 degrees celcius.
(http://wine1.sb.fsu.edu/chm1045/notes/Gases/IdealGas/Gases04.htm)

how many moles of air are there in 1kg (1kg =~ 1/0.029 moles =~ 34.5 moles)
(http://instruct.tri-c.edu/fgram/web/GASLAW.htm)

Ideal gasses take up 22.4L per mole.

22.4 L/mole * 34.5 moles = 772.8 L

Which is .7728 m^3, so a volume about 92cm x 92cm x92cm.

Using the ideal gas law (referenced above) and the atmospheric pressure at various altitudes, you can calculate the volume. At 5km up, you roughly double that volume.
(http://en.wikipedia.org/wiki/Atmospheric_pressure)

I know I don't exactly answer your question, but to phrase it another way 1kg of atmosphere is bounded by a box of approximately 0.00092 km in all dimensions.

Hope that helps.