Milibar to Altitude Conversion

Edo · Post by **Edo** » Mon Apr 09, 2007 9:10 am

I am looking for a conversion formula / memory trick to go from the MB levels in Upper air charts to the corresponding altitudes.

Short of straight regurgitation is there any easy way to remember or calculate these ?

50 Mb = 60,000
100 Mb = 45,000
150 Mb = 41,000
200 Mb = 39,000
250 Mb = 34,000
400 Mb = 25,000
500 Mb = 18,000
700 Mb = 10,000
850 Mb = 5,000

Thanks.

Crazymax · Post by **Crazymax** » Mon Apr 09, 2007 9:22 am

I'd have to dig in my books but the formula is exponential (not linear, which makes it pretty hard to memorize, probably harder than learning those numbers by heart). However, here is something that might be easier :

P/PSL H

1 0
1/2 18,000
1/3 27,480
1/10 52,926
1/100 101,381

P/PSL is the fraction of the pressure at altitude (in relation to the pressure at sea level)

Max

Edo · Post by **Edo** » Mon Apr 09, 2007 9:31 am

Thanks, for the quick answer - that helps for a few altitudes.

The non-linear formula sounds tough, I guess straight memory recall may be the best

chipmunk · Post by **chipmunk** » Mon Apr 09, 2007 10:28 am

I *think* that if you can find a buddy with one of those CX-2 calculators, there's a function in it that does it for you, and that's fully acceptable to take into an exam (what I assume you're needing this info for?) I'd check, but mine seems to have developed legs and walked away...

Edo · Post by **Edo** » Mon Apr 09, 2007 3:00 pm

I have the CX-1 I dont think I have the feature on it.

My old standby E6B doesn't do it either. Anyone have a CX-2 that can tell me how to do it. Maybe I am missing something on the CX-1

chipmunk · Post by **chipmunk** » Mon Apr 09, 2007 3:50 pm

Found the manual (CX-2) online here, and found out what I was talking about. If you go to the "Altitude" section and then "Standard Atmosphere" function, it will give you various info, including that of millibars. Their example is you enter 20,000' and it calculates Standard Atmosphere as 465.6 millibars.

Hope that helps.

Edo · Post by **Edo** » Mon Apr 09, 2007 5:20 pm

That's exactly what i was looking for thanks. The only problem is that the CX-1 doesn't have that option.

I'm not that interested to go buy the CX-2 so it will be good old memorisation I guess.

husky · Post by **husky** » Mon Apr 09, 2007 7:55 pm

If you want to be more accurate, you can always use the hypsometric equation:

Altitude = [(R * T)/g ] *ln(Ps/Pa)

Where:
-R is the gas contant appropriate to the units your are using, in this case milibars which is 83.14472,
-T is the average temperature of the layer of air between the pressure level and the surface
-g is gravitational acceleration (9.8 m/s^2)
-Ps is present sea level pressure
-Pa is the pressure you are at

Edo · Post by **Edo** » Mon Apr 09, 2007 8:23 pm

Husky - thats a little more than i need for the sama exam

- by the way whats In ?

Crazymax · Post by **Crazymax** » Mon Apr 09, 2007 8:25 pm

It's LN or Natural Logarithm.

Max

nyco · Post by **nyco** » Tue Apr 10, 2007 9:07 am

don t wanna sound like a nerd(or an ass, whichever is better), thou i wil, but it s
Neperien Logarithm