Manual for the Computer Program   ATMO99

The computer program "ATMO99" has been developed for PC computer running Windows® 3.1/95/99. The article ("The Atmosphere") on this site is general information which may be of interest and supports the use of this program. In addition the program has extensive help files and technical references in the help files. Notes on program instalation operation are provided below.

If you want to download the program now click on ATMO99

The copyright notice is to prevent this code from being sold, except for the cost of distribution, like the disk, copying, and postage. It can be given away, that is, this is true "freeware".

Installation and Running

The executable file (ATMO99.EXE) and the required support files (the icon, dynamic link library, and help files with extensions ICO, DLL and HLP) should all be in the same directory — unless, of course, you are Windows® wizard and can modify the PIFand INI files to take care of files in dispersed directories. The program can be run directly from a floppy to see how it works. This mode will be slow, so copyingATMO99 to your hard drive will be more convenient if you plan to use it.

Run the program from the program manager. Either enter a Run command with the file path and name or from the File Manager Click on the ATMO99.EXE file name. The program screen should appear with a partial listing of the computed properties for the standard atmosphere at sea level.

Overview

The program was designed to generate quantities used in aerodynamic, heat transfer, and propulsion computations that depend on atmospheric conditions as well as producing and comparing specific conditions with those in the U.S. Standard Atmosphere, 1976. The objective is to produce correct values of, for example, Reynolds number, for the atmospheric conditions encountered in flight as opposed to using the usual standard atmosphere at sea level figures.

The standards and physical constants used in the program are up to date and consistent with those in use by the NASA and other government agencies. These are now generally consistent with both International standards, those used by the NBS, and in meteorological work. There are, quite naturally, small differences between the values here and earlier ‘standards’ – the arbitrarily assumed numbers such as standard temperature– here and those in older textbooks and references. For example, the standard pressure at sea level, used as a reference pressure, is slightly different from that used in some older thermodynamic books and tables. The program help fiels has a list of the appropriate references.

The program is also a possible element in a larger program ‘set’ which would use these values in engineering calculations. My experience with programming for the Microsoft Windows® operating system is limited and this program was partly an exercise for me to see how difficult it was.

The general idea is that the values for ‘standard’ atmosphere properties often used in magazine articles, symposia papers and so–on, are not directly applicable to real flying situations. If, for example, one had the temperature, barometric pressure and relative humidity at the flying site then this code (or its scientific calculator and table equivalent) would provide better estimates for things like density, Reynolds number, and corrections for sink speed and engine power output.

One other notion was to have all data given in the SI system (‘metric’) to help or inspire those who would convert to it.

There are two different ways of capturing any calculation or set of calculations generated by the ATMO99 program.

1. You can send the results of the present calculation directly to the printer and get a one page output of what is shown on the screen.
2. You can add the results to the present calculation to a store kept in memory, then write the whole collection to an ASCII text file.

This data saving could be improved, I’m sure, but it is serviceable. The menu items are discussed next followed by a discussion of a few examples of use.

Menu Items

There are five top level menu items. Click on the Help command for on-line help on how to use any of the menu items as well as references, some description of program operation, and so-on. This should be your primary guide.

The first thing you might want to do is select the input unit system, which can either be English or SI ("metric"). Selecting this is done from the Units menu and clicking on the ‘radio button’ of the unit system you want. English is the default, so you only have to do this if you want to use SI units or have been using SI units and want to switch back to English.

You may also want to establish a nominal speed and length. The Flow menu item allows you to input a nominal speed and chord length which are used to compute dynamic pressure, Reynolds number, and Mach number information to go with the atmosphere data. If you don’t change this the computations use a default value of 3 meters per second speed and one meter chord.

When input of a specific number is required in any of the dialog boxes it is done from keyboard, but you first have to click on the edit window (a window with a short line of text) to make it active. This window should contain the present value the program is using. When the window is ‘active’, edit the present value or enter a new value from the keyboard.

The Atmosphere menu has all of the items that you may wish to change for the dry or moist atmosphere analysis (pressure, temperature, density, humidity). One can start by getting the standard atmosphere quantities at any altitude (up to 80 km or so) and/or modify individual items. There are many ways of setting these values and in each case you can select by radio button what you want to hold constant. For example, if you change the temperature you can keep pressure constant and the density is recalculated or the other way around.

For example, if you want to set a given density altitude and temperature, under Atmosphere you could select Atmosphere|Density Altitude. When the dialog box appears type in the altitude value you wish, the radio button selection won’t matter at this point. The recomputed atmosphere values show up. Then select Atmosphere|Temperature. When the dialog box appears, click on "hold density" radio button and input the new temperature.

Under the file menu you can select File|Print this Page and the whole screen contents (including hidden areas that may not be scrolled into view) are sent to the printer. The output is a little fancy so, depending on your printer, this may be slow.

At any point you can save all the results you can see on the screen by selecting the File|Save this Data menu item. This adds a block of data to a save area in memory. When you are ready you can select File|Write Saved Data to File and all the information you have saved is written to a text file that you can name. After the data are written the saved data are flushed from memory, ready to start a new set.

Output and Examples

The output for each calculation is displayed in a window. When the program comes up the quantities in the window are for the standard atmosphere at sea level.

You may scroll the window up and down to see all the results, or if you have a larger resolution than 480 by 640, you can see more by expanding the window. This display is updated each time any input value is changed, but saved, printed, or written to file only when you use selections from the File menu, discussed above.

The output of the program is often explained by the titles and units printed next to the values. There are, in addition, a number of dimensionless ratios. The elementary ones are the ratio of the atmospheric quantity in either the dry atmosphere or moist atmosphere to the corresponding sea level quantity. The density and pressure ratios are usually given by the symbols = and / , respectively. There are also square roots of density ratio for scaling speeds, the ratio of density of the humid air to the dry air at the input conditions, and the ratio of the dry part of the density of the humid air to sea level standard density. This latter figure is a density ratio for the oxygen-bearing part of the humid atmosphere.

An example may be useful. Suppose you are going to fly in Reno in the summer. Temperature 80° F, barometer of 26.0 in Hg, and 75% Relative Humidity. Estimated glide speed is 28 feet/sec and chord is 0.67 feet.

1. From the Atmosphere menu choose Temperature and enter 80.0.
2. From the Atmosphere menu chose Pressure: In. Hg and enter 26.0. The radio buttons are preset correctly so temperature is held constant.
3. From the Atmosphere menu choose Relative Humidity % and enter 75.0.
4. From the Flow menu choose Velocity and enter 28.0.
5. From the Flow menu choose Length and enter 0.67.

Each entry will produce a new set of calculations. At the end of this sequence the results on the screen should show the density altitude (moist) is 6483 feet (quite a bit higher than the ‘standard’, even for Denver), with an air density of 0.001963 slugs/ft³ . This amounts to 0.8257 of the sea level standard value.

The kinematic viscosity (dry) is 1.941E-4 (i.e. 0.0001941) and the R_e (dry) is 96600. The R_e for the moist air must be increased further by dividing by the density ratio (wet/dry), however this results in only a 1% increase. The resulting kinematic viscosity is 1.25 times the sea level value. The glide speed should be increased from the sea level value by dividing by the square root of the appropriate density ratio. This is the (moist air)/(sea level standard) square root density ratio, shown as 0.9087.

To compute Reynolds number for some other wing geometry or speed you can use the reciprocal of kinematic viscosity to get a formula that does not involve numbers with exponential notation. In the example above for a 0.55 foot chord and 25.2 feet/second velocity one gets R_e = 0.55 * 25.2 * 5093 = 71200

In this example, as a general result the flight speeds should be up by about 10% from sea level standard, while the kinematic viscosity and Re at a given speed are up 25% from standard conditions.

As a second example, you may know the local temperature and relative humidity but only have the barometric pressure at a nearby location at a different altitude. Unless there are strong winds, pressure changes very slowly with distance. To make an approximate correction for altitude difference, first set up the temperature and humidity at your location. Hold the temperature constant and change the pressure to the value given for the remote location. Note the pressure altitude. If you are 600 feet higher than the station altitude for the barometer, for example, select Pressure Altitude. The current pressure altitude will be displayed. Add the 600 feet to it and enter that value. The new pressure will be reduced by approximately the amount that the pressure will drop with increased altitude.

Some Closing Remarks

Well, that’s how it should work. The on-line Help will provide more information on the controls, program operation, and references. Generally, experimentation is the best way to learn.

If you have comments you can email me at    microair3@cox.net.