Ive read the other posts about the altitude box on the jbm progrm but I stil dont get it..lets say im going to shoot at the airprt . i listen to the pilots weather report and it gives me the altimeter setting which i believe is the real baro pressure at the airport.it also give the temp and humidity.I also know the field elevation of the airport. If i enter the pressure ,temp,humidity, and field elevation in the boxes do I uncheck the corrected pressure box t the bottom?
Range Report JBM altitude hangup
- Thread starter cl60cruzer
- Start date
Re: JBM altitude hangup
Yes, if you enter the "station" pressure, then you uncheck the box. Otherwise it will use the temp and altitude to predict a pressure using a meteorological model and override the actual pressure you entered.
PS To be crystal clear, the station pressure is the actual pressure at your location. The airport reports the corrected pressure, which is the corresponding pressure at sea level. I thought you were a pilot and knew the difference, but perhaps not.
Yes, if you enter the "station" pressure, then you uncheck the box. Otherwise it will use the temp and altitude to predict a pressure using a meteorological model and override the actual pressure you entered.
PS To be crystal clear, the station pressure is the actual pressure at your location. The airport reports the corrected pressure, which is the corresponding pressure at sea level. I thought you were a pilot and knew the difference, but perhaps not.
Re: JBM altitude hangup
Sorry - I'm afraid that turns out not to be the case.
The airport doesn't report the station pressure. The altimeter setting it reports is the setting required to get a pressure altimeter to read the airport elevation - which means it's basically the sea-level-referenced barometric pressure. (Not quite - but the difference is not relevant here.)
See:
Barometric Pressure and Ballistic Software
Also note:
This applies to the standard trajectory screen, http://www.jbmballistics.com/cgi-bin/jbmtraj-5.0.cgi
If you check "Standard Atmosphere at Altitude", you get the ICAO temperature and pressure at whatever altitude you specify, with the humidity set to zero. That means that whatever temperature, pressure, and relative humidity you might have entered in the input screen are completely ignored. See the chart of the ICAO Standard Atmosphere below. The temperature and pressure listed for the appropriate altitude in that chart are what the program will use.
If "Standard Atmosphere at Altitude" is not checked, temperature and RH entered in the appropriate boxes always affect the air density calculation.
If "Pressure is corrected" is checked, the density is calculated using the pressure corrected for whatever altitude is entered in the altitude box. It means that the pressure you entered is from a source which corrected the station pressure to the sea-level-referenced barometric pressure, and the program corrects <span style="font-style: italic">that</span> pressure for altitude.
That's confusing to me, but I didn't write the program. It still uses the temperature and relative humidity entered in the appropriate places.
If "Pressure is corrected" is not checked, the density is calculated ignoring the contents of the altitude box. In other words, the programs assumes that you have entered the station pressure. It still uses the entered temperature and relative humidity in the air density calculation.
The most accurate way to get any ballistic program to work is to have an instrument like a Kestrel or many watches which measures local atmospheric pressure, which we call the station pressure. Enter the station pressure, the temperature, and the relative humidity, and make sure that you're not using a program option to calculate the "standard atmosphere".
Sorry - I'm afraid that turns out not to be the case.
The airport doesn't report the station pressure. The altimeter setting it reports is the setting required to get a pressure altimeter to read the airport elevation - which means it's basically the sea-level-referenced barometric pressure. (Not quite - but the difference is not relevant here.)
See:
Barometric Pressure and Ballistic Software
Also note:
This applies to the standard trajectory screen, http://www.jbmballistics.com/cgi-bin/jbmtraj-5.0.cgi
If you check "Standard Atmosphere at Altitude", you get the ICAO temperature and pressure at whatever altitude you specify, with the humidity set to zero. That means that whatever temperature, pressure, and relative humidity you might have entered in the input screen are completely ignored. See the chart of the ICAO Standard Atmosphere below. The temperature and pressure listed for the appropriate altitude in that chart are what the program will use.
If "Standard Atmosphere at Altitude" is not checked, temperature and RH entered in the appropriate boxes always affect the air density calculation.
If "Pressure is corrected" is checked, the density is calculated using the pressure corrected for whatever altitude is entered in the altitude box. It means that the pressure you entered is from a source which corrected the station pressure to the sea-level-referenced barometric pressure, and the program corrects <span style="font-style: italic">that</span> pressure for altitude.
That's confusing to me, but I didn't write the program. It still uses the temperature and relative humidity entered in the appropriate places.
If "Pressure is corrected" is not checked, the density is calculated ignoring the contents of the altitude box. In other words, the programs assumes that you have entered the station pressure. It still uses the entered temperature and relative humidity in the air density calculation.
The most accurate way to get any ballistic program to work is to have an instrument like a Kestrel or many watches which measures local atmospheric pressure, which we call the station pressure. Enter the station pressure, the temperature, and the relative humidity, and make sure that you're not using a program option to calculate the "standard atmosphere".
Re: JBM altitude hangup
So Lindy if I want to use the airport info wherever I go what is my selection on the boxes?
So Lindy if I want to use the airport info wherever I go what is my selection on the boxes?
Re: JBM altitude hangup
Put the altitude at which you are shooting (not the altitude of the airport) in the altitude box, put the pressure the airport provides in the pressure box, and check "Pressure is corrected". Also enter your local temperature, and the humidity if you know it - if not, I just set it at 50 percent, where it will make little difference either way.
And note that JBM has a little blue question mark at the end of the label of every entry field. Clicking on those questions marks will often get you an answer to your question.
Put the altitude at which you are shooting (not the altitude of the airport) in the altitude box, put the pressure the airport provides in the pressure box, and check "Pressure is corrected". Also enter your local temperature, and the humidity if you know it - if not, I just set it at 50 percent, where it will make little difference either way.
And note that JBM has a little blue question mark at the end of the label of every entry field. Clicking on those questions marks will often get you an answer to your question.
Re: JBM altitude hangup
thanks, I can always get that information from airport ATIS and then get my elevation from maps or GPS. it makes thing simpler for me
thanks, I can always get that information from airport ATIS and then get my elevation from maps or GPS. it makes thing simpler for me
Re: JBM altitude hangup
So, for example, if you see the airport reading is 30.06 @ an altitude and temperature of 4000'/50F, use just the pressure, but enter the following in JBM like this, e.g. for an altitude of 5320 and temperature of 40F:
So, for example, if you see the airport reading is 30.06 @ an altitude and temperature of 4000'/50F, use just the pressure, but enter the following in JBM like this, e.g. for an altitude of 5320 and temperature of 40F:
Re: JBM altitude hangup
Avoid the hassle and buy yourself a gizmo that will give you the station baro pressure (the value where you are) and the temperature. The relative humidity makes little difference and is not critical to know.
I dunno what gizmos are out there besides a Kestrel, but you can avoid phone calls to the FAA for the automated airport weather and figuring your altitude. You may not even be within cell coverage way out yonder!
Here's a gizmo for under $120 on Amazon:
http://www.amazon.com/Brunton-ADC-Atmosp...7602&sr=8-2
There are three more near the bottom of the page for under $100.
Avoid the hassle and buy yourself a gizmo that will give you the station baro pressure (the value where you are) and the temperature. The relative humidity makes little difference and is not critical to know.
I dunno what gizmos are out there besides a Kestrel, but you can avoid phone calls to the FAA for the automated airport weather and figuring your altitude. You may not even be within cell coverage way out yonder!
Here's a gizmo for under $120 on Amazon:
http://www.amazon.com/Brunton-ADC-Atmosp...7602&sr=8-2
There are three more near the bottom of the page for under $100.
Re: JBM altitude hangup
If you don't have a device and are just checking before heading to the range:
http://www.weather.gov/ - enter city and state, hit go, then look at Current Conditions...no need to call anyone.
If you don't have a device and are just checking before heading to the range:
http://www.weather.gov/ - enter city and state, hit go, then look at Current Conditions...no need to call anyone.
Re: JBM altitude hangup
Another good site you can use http://www.wunderground.com/ for current and historical data.
Another good site you can use http://www.wunderground.com/ for current and historical data.
Re: JBM altitude hangup
I can also vouch for weather underground. Between that and Google maps/Google Earth for topography and actual altitudes, you should be well covered.
Of course, my Kestrel happens to travel with me during certain courses of fire along with a Density Altitude ballistics card so I can vary my scope adjustments as the day's environmentals change.....
I can also vouch for weather underground. Between that and Google maps/Google Earth for topography and actual altitudes, you should be well covered.
Of course, my Kestrel happens to travel with me during certain courses of fire along with a Density Altitude ballistics card so I can vary my scope adjustments as the day's environmentals change.....
Re: JBM altitude hangup
My Casio Pathfinder watch has a pressure sensor, and displays station pressure.
My Casio Pathfinder watch has a pressure sensor, and displays station pressure.
Re: JBM altitude hangup
I'm going to calibrate my GPS unit before heading to the range tomorrow and see how useful it's pressure sensor is.
Niles, thanks for the link to wunderground.com. I've used 30.06" corrected pressure as a baseline in my ballistics calculations, based on the long-term average reading at my local airport (30 years). Just out of curiosity, I downloaded a year's worth of data and saw that last year the average was only slightly higher. The low was 29.24 and the high 30.97 (both in January this year). Neither of these extremes has as much effect as changing altitude 1000'. Obviously, if you combined high pressure with an altitude drop or low pressure with an altitude gain, that would be a case for not relying on averages.
I'm going to calibrate my GPS unit before heading to the range tomorrow and see how useful it's pressure sensor is.
Niles, thanks for the link to wunderground.com. I've used 30.06" corrected pressure as a baseline in my ballistics calculations, based on the long-term average reading at my local airport (30 years). Just out of curiosity, I downloaded a year's worth of data and saw that last year the average was only slightly higher. The low was 29.24 and the high 30.97 (both in January this year). Neither of these extremes has as much effect as changing altitude 1000'. Obviously, if you combined high pressure with an altitude drop or low pressure with an altitude gain, that would be a case for not relying on averages.
Re: JBM altitude hangup
but you will learn propper camoflauge under the pressure of incarceration
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BOLTRIPPER</div><div class="ubbcode-body">shooting at the airport would require TSA approval.....i'm just sayin </div></div>
but you will learn propper camoflauge under the pressure of incarceration
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