Barometric pressure and elevation

[wally]

when tracking weather on the baro does anyone know the ratio change in pressure per 100'ft elevation gain. when climbing up in questionable weather I notice the change but would like to know how much is due to elevation gain

[Gary_Yngve]

whew, my aeronautical texts are at the lab, but luckily, I had the numbers entered into some old code:

 

typedef struct {

float f;

float pres;

float dens;

} weather;

 

 

weather atmo[]={

{0,101.32,1.2250},

{500,95.46,1.1673},

{1000,89.88,1.1117},

{1500,84.56,1.0581},

{2000,79.50,1.0066},

{2500,74.69,0.9569},

{3000,70.12,0.9092},

{3500,65.78,0.8634},

{4000,61.66,0.8194},

{4500,57.75,0.7770},

{5000,54.05,0.7364},

{5500,50.54,0.6975},

{6000,47.22,0.6601},

{6500,44.08,0.6243},

{7000,41.10,0.5900},

{7500,38.30,0.5572},

{8000,35.95,0.5258},

{8500,33.15,0.4958},

{9000,30.80,0.4671},

{9500,28.58,0.4397},

{10000,26.50,0.4140},

{15000,12.11,0.1948},

{20000,5.53,0.0889},

{30000,1.20,0.0184},

{40000,0.29,0.0040}

 

first column is altitude in meters

second column is pressure in Pascals (N/m)

third column is density in 1/meters^2

 

You can interpolate in between.

 

If you want to convert Pascals to inches of Mercury,

1 Pascal is .0002953 inches of Mercury.

 

As a sidenote, you can use P=\rho R T to calculate temperatures*... temperatures decrease in the troposphere, but increase higher up (in the ionosphere, if my memory is correct?)

 

* assuming atmospheric gases remain in proportion... you can find specific temperature data, I just don't have it on me

[mattp]

You can interpolate from Gary's table, but the more direct answer to your question is that there is not a linear relationship between pressure and altitude, so I don't think there is any simple equation for the change in pressure for a 100 foot gain. Also, there is some reduction in atmosphere pressure as you get further from the equator (the air is thinner at the poles so 20,000 feet in Alaska is equivalent to a slightly higher elevation in Nepal), so the equation would be different in Alaska as compared to California.

 

To monitor changes in pressure, watch your altimeter when you are at camp, or compare the reading on your ascent and descent past the same location. Also, recalibrate your altimeter when you pass a known elevation point such as a col or a summit, and you can detect relative changes in barometric pressure during the day (was it reading "high" - meaning pressure has dropped slightly?). Realize, however, there is often a daily cycle with slightly reduced barometric pressuring during the middle of the day, and this occurs apart from any changes associated with changes in the weather.

 

Lastly, it has been my general experience that changes in barometric pressure often do not precede changes in the weather but merely accompany those changes. I haven't found the altimeter nearly as useful a tool for predicting the weather as looking around and noting changes in cloud formation, wind direction, etc.

 

[Bug]

I find that my Suunto is very good at predicting a change in the weather. It only works if I am stationary though. Like at camp or hangin for lunch etc. There are minor fluctuations but any trend lasting 30 minutes or more is worth noting.

[Toast]

There's a rough rule of thumb that every 25 foot increase in elevation corresponds to a drop of about one millibar of pressure. I'm not quite sure what the conversion is to inches mercury, though.

 

[klenke]

A handy dandy atmospheric pressure conversion page

 

Conversion Chart

[wally]

thanks for the information I'll test how accurate the charts are in the field, I feel smarter already.

[Alex]

  Quote

typedef struct {

float f;

float pres;

float dens;

} weather;

 

classic!

[jmace]

As MattP said Pressure and altitude do not have a linear relationship, they more or less have a exponential relationship. Therefore using the ideal gas law as Gary suggests wont really work at altitude. What density would you use? What about liquid and water vapour in the air...?What you want is the hypsometric eqaution. This gives you everything you need including liquid and vapour water loading effects on density. Ill leave it you to search out the eqn if you like, its quite simple.