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Welcome to the CHARM FAQ Page. This is the place to look first for answers to your questions about CHARM. Some videos are available on YouTube. There is also a forum available for asking new questions or to discuss how to solve specific problems using the CHARM Software.

General Questions

Flat Terrain Version

Complex Terrain Version


I get an error about a dll. Often polyfrag.dll. How can it be fixed?

The Microsoft Visual C++ runtime may be missing. It can be downloaded and installed separately.

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Why and when should I use Minutes-Seconds mode?

If a release is of a short duration, especially less than a minute, it is possible that when impacts are analyzed on a minute-by-minute basis the maximum at a location may be missed. For example, if the release is less than a minute, the maximum concentration at a point may occur at some minute (X) plus 30 seconds. In Hours-Minutes mode the concentration will be reported at minute X and minute X+1.

Minutes-Seconds mode is suggested for rapidly varying releases and when the region of interest is very close to the source.

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How can I change the default start-up / new input?

First, change all the input values that you want for your default case. Then in the File or MetFile menu select the "Save as default" option.

If you want to reset the defaults back to their original values, you need to delete one or both of the files named chrmdflt.emg and chrmdflt.met. The first is the release description. The second is the met data. The files will be in the directory where CHARM starts. That is most likely the CHARM installation directory.

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How do I get back to the tabbed dialogs for release description and meteorological input?

The menu bar on the main input screen has an Options selection. In that selection are the items "Description Dialog Input..." and "Met Dialog Input...". Choosing those items will bring the tabbed dialogs back up. When you exit CHARM, it will remember if the tabbed dialogs were visible or not. When CHARM is restarted, the dialogs should reappear or not automatically.

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How do I get help on a menu item?

Press and hold the control key down. Then press the F1 key. The cursor should change so that a small question mark is visible. Then select a menu item. Instead of the menu item being executed, a help screen discussing the menu item should appear.

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How can a change in surface roughness have such a large effect on impacts?

The surface roughness along with the stability class has an effect on the vertical distribution of wind speed. Changing the surface roughness may increase or decrease the wind speed at a given altitude. For example, increasing the surface roughness will generally decrease wind speeds below the stated meteorological data measurement height (default of 10 meters). Decreasing the surface roughness would increase the wind speeds. Since wind speed directly affects pool evaporation rate, even the emission rate can change with a new roughness.

If an evaporating pool is present, its emission rate may change if it is being driven by wind speed. Changes in wind speed can also affect the amount of mixing the released material goes through.

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How can I move the viewport around in the plan view displays?

In the plan view you can press and hold down the control key. Whenever an arrow key is pressed the viewport moves in the direction of the arrow. The impact footprint will appear to move in the opposite direction.

The impact footprint and map can be moved by holding down the control key and dragging the mouse with the left button down.

Double clicking a location will create a dose history at that location.

If you have a mouse wheel you can zoom in or out by turning the wheel.

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I receive a message saying "Access denied" to a file or "an unnamed file was not found." How can I get around that problem?

You may need to have IT support or Administrator rights to solve this problem.

The program uses the directory that it starts in as its work directory. The program shortcut in the Start menu can be modified to have it "Run as administrator" or change the default start directory to some other location, such as c:\temp\charm. The database file (ChemReac.mdb for the complex terrain version and all chmdat8.* files for the flat terrain version) and charm.ini file may also need to be written to. They should be moved (not copied) to the start directory and redefine the chemical database location in Charmed.

Charm network installations work this way as well with only the charm.ini file in the start directory on the client machine and no write access at all on the server. Having charm.ini files allows multiple configurations to be run on one machine.

If the database is not in a writable subdirectory, then you will not be able to add or edit chemicals. If that is okay then just move the charm.ini file to the work directory and CHARM should perform correctly.

Another solution is to uninstall CHARM and reinstall it into a directory that has write permission. Do not install into an subdirectory of the Program Files directory.

Installation was changed on June 13, 2014 to install the ini and data files in the Application Data directory of the user. This problem should be eliminated or, at least, minimized.

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I receive a message a file is missing. How can I solve the problem?

The dot or period character ('.') should not be used in a file name except to designate an extension.

CHARM created files are often actually two or more files. For example, map, Shape, and scenario files are multiples. When the secondary files are created the base name is determined by searching if a dot is present in the name. If so, everything to the right is assumed to be an extension. Since a file name may not have an extension present when the user enters a name, if a dot is present as part of the file's name the base name could be incorrect.

The easiest solution is to not use the '.' character in a name.

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In the 3D display map label text is not clear and ground features are blurred. The map looks fine in 2D. Is there a way to resolve this discrepancy?

To draw a map onto a grid in 3D efficiently the pixel dimensions in the x and y need to be a power of 2. CHARM expands or contracts a map to reach the power of 2 dimension. In the process the image may be stretched or compressed.

The easiest fix is to use the CHARMInetMaps to create the maps. Set the Map Size to a square with dimensions a power of 2. For example, use a map size of 512 X 512 or 1024 X 1024.

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How do I uninstall CHARM?

To uninstall CHARM follow the directions given here. Files saved by CHARM will not be removed.

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During installation I receive a message saying "SketchupReader.dll failed to register" and pressed the Continue button. When running CHARM I get ta Visual C++ Runtime Error. How can I get around these problems?

If SketchupReader.dll does not register, there is a chance that improper Registry entries may be made leading to the Runtime Error. Uninstall CHARM. Go to and download and install Google Sketchup 8. Alternatively, you can go to and download and install vcredist_x86.EXE. Then reinstall CHARM.

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When a SketchUp files was imported, nothing happened. What went wrong?

There are three possible reasons why no solid cells appeared after importing a SketchUp file. First, the SketchUp file may not be geo-referenced so that it lies in the grid. If this is the cause, you can geo-reference the file using SketchUp (Window->Model Info->Geo-location). Second, the grid cell dimensions are too coarse. A cell is set to solid only if half or more of its volume is filled with an object. If this is the cause, you can make the grid cells smaller or create a sub grid with smaller cell dimensions for the area covered by the SketchUp file. Third, CharmEd can only import SketchUp files in version 8 or earlier format.

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Why do I sometimes get spikes on the concentration history plots?

The flat terrain version of CHARM uses puffs to simulate a release. Rather than using overlapping puffs with the dynamics calculated for each puff, the algorithm uses "significant" puffs. The parameters of missing puffs are spatially and temporally interpolated for when it is time to calculate a display or impact. Sometimes, very near the source, the interpolation can be inconsistent and spikes may appear at a set interval in time.

It is possible to have a wavering concentration plot as well. However, this is most likely due to an elevated plume oscillating in the vertical. When the plume is at a low point in the oscillation, ground level impacts will be large. When the plume is at a high point, ground level impacts will decrease. This phenomenon is real.

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How can a change in release height have such a large effect on ground level impacts?

The flat terrain version of CHARM uses puffs to simulate a release. If a puff gets to within a certain vertical distance to the ground, the model forces it to the ground and doesn't allow it to reloft. The vertical distance is one and one-half sigma of the concentration width of a puff.

If a release height is raised high enough the puffs may not get within that vertical distance criteria to be forced onto the surface. Decreasing the release height just a little may result in the forcing to the surface to be invoked. If the puffs are on the ground the ground level impacts are larger than if they are aloft.

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What are the system requirements for the flat terrain version of CHARM?

The basic system requirements are:
  • Windows XT or above (can be 64-bit Windows).
  • 2 GB RAM.
  • 100 MB disk space.
  • 3D graphics card.

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What are the system requirements for the complex terrain version of CHARM?

The basic system requirements are:
  • Windows XT or above (can be 64-bit Windows).
  • 4 GB RAM.
  • 100 GB disk space.
  • 8 GB RAM.
  • 3D graphics card.
  • Multi-core processor for parallel processing (e.g. Intel CORE i7).

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Is there a tutorial or list of steps to take to create a grid and other input files?

There is no tutorial. The basic steps to take to create CHARM input files are:

  1. Create a map using CHARMInetMaps and the coordinates you provided. You can download CHARMInetMaps from the Downloads page at It is free and there is no installation. It just runs. Upon occasion when you start the program it crashes. If that happens, just start it again.
  2. A grid location needs to be set using the map that was created. You can do that in CharmEd’s Grid Editor. Select Edit->Grid from the main window of CharmEd. Once in the Grid Editor, select Setup->Define Grid. On the dialog box that appears press the “Browse a Map to Define SW Corner” button. Open the created map and click on the location to use as the SW corner. Then select Options->Save and Return.
  3. Fill in the boxes for the other items in the dialog box. To check coverage of the map area you can press the “Browse Map to Move/Size” button. Open the map. On the map shown you can move the grid by dragging it. If you hold down the Shift key while dragging the mouse, the grid will be resized.
  4. Download a terrain elevation data file. There are a number of locations on the Internet where such files exist. In the Grid Editor press Shift-F1 and select an item in the Import menu to get more information. When ready, select the appropriate item in the Import menu to read the data. The site has data for the entire globe.
  5. Download a land use file. Once again use Shift-F1 and select items in the Import menu to get more information. When ready, select the appropriate item in the Import menu to read the data. The GLCC file, gusgs2_0g.img, covers the entire world and so only needs to be downloaded once. That file in zipped format can be downloaded at
  6. Buildings can be added by using Solid Cells (View->Parameter->Sold Cells) Select the cells at the cell level desired. You can select by dragging the cursor over cells or by holding down the Ctrl button and selecting individual cells. Once a selection exists go to Setup->Edit Selection, choose whether the cell is solid or not and how many cells up or down should also be set.
  7. Add a title to the grid (Setup->Grid Title) and save the grid. Exit CharmEd.
  8. Start CHARM and open the grid (Grid->Open). Then enter information for the release description and meteorological data. The release and meteorological input can be changed by filling in the boxes on the tabbed dialog or double clicking on any line on the Main Input screen and filling the entries and selecting the appropriate units. Save the release description (File->Save) and met data (MetFile->Save).
  9. To get a plume choose Displays->Plume. Other impacts can also be selected from the Displays menu.

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How do I create odd shaped sources?

The complex terrain version of the CHARM software allows multiple sources. Once a grid and subgrids have been defined, each grid cell can have one or more sources present. Almost any shape of source can be created by placing sources at various locations in the grid(s). If you have selected the "Pool will flow over terrain" option, the source's shape will be determined by CHARM.

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How can the airflow in a vertical chimney be simulated?

Only horizontal wind speeds and directions are allowed as input into the model. To get the model to correctly simulate the air flow upward through the chimney you need to enter a horizontal wind speed coming into the chimney building from one or more windows, doors, or other openings. The input wind speed times the area of the doors and windows should be equal to the exit speed times the area of the chimney exit. The horizontal movement will be transformed to vertical movement.

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How should one determine the best grid spacing to use for a given problem?

The rules of thumb to use are:

  • Have at least five grid cells between the source and the nearest receptor point of interest.
  • The vertical cell dimension should be the same or less than the horizontal dimension. One technique is to use the stability class to determine the vertical dimension. Using A=1, B=2, etc. divide the horizontal dimension by the numerical value of the stability class to arrive at the vertical dimension.
  • If the farthest receptor point of interest is beyond 75 or more grid cells, it probably is more efficient to set up another grid or use an outer grid with an embedded subgrid.
  • Besides the number of grid cells, the grid cell dimensions are an important factor. All material in a grid cell is assumed to be well mixed. If a grid cell is too large, the contained material is too diluted. If a grid cell is too small, the time steps in the model can become excessively small which increases compute time. It is normally a good idea to halve the grid cell, at least in the direction of the wind, for a short duration simulation to verify that the results are not changed significantly by grid cell dimension.
  • The idea is to use as small a grid as possible to do the trick in a reasonable amount of time. Most simulations can probably be done using a grid less than 50x50x20.
  • When a sub grid is created it is recommended that the subdivisioning be no more than 5 in any one dimension. The reason for that is that when material goes into a grid cell it is assumed to be instantly homogeneously mixed throughout the cell. If there are greater than 5 subdivisions, the material may be too greatly diluted.
An indicator that there may be a problem is when the time steps can be seen changing only 0.01 seconds per step. That is the minimum time step allowed in CHARM. This usually means things are changing so rapidly that CHARM is unable to keep up with it. The resolution is to make larger grid cells that can handle more mass coming in and going out.

The boundary conditions in the calculation region can fail when the motion of the emission is in the opposite direction of the wind direction. It is possible that mass can make it back into the region from outside of the grid. The problem can be resolved by extending the grid in the direction the emission is going toward so that there is more time for its concentration and momentum in that direction to be reduced.

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Does the subgrid feature allow a smaller grid to be created for an area of particular interest, such as a release location where concentration gradients are high?

Yes, subgrid allows for a finer grid to handle larger gradient concentration fields. Normally, it has been used for the area around a source. However, it has also been used to include a receptor building with windows, doors, or ventilation systems, to allow a finer analysis of what goes on in the building once the emitted material gets to the exterior of the building.

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Is CHARM a CFD model?

The answer to whether or not CHARM is a CFD model is yes and no.

A pure CFD model calculates all flows using basic equations. Boundary conditions for all the flow at the edge of a calculation grid is required to perform the calculation correctly. CHARM uses the WEST Wind Field (WWF - described in the technical documentation) calculation for the ambient meteorological fields. The WWF calculation can use one or more meteorological sites and allows for the influence of terrain on the flow. The fundamental approach is deterministic and is designed to minimize divergence of the wind field. That is, mass is conserved. If there is meteorological data available at different times, a wind field is calculated for each time. When a field is needed for a specific time, it is interpolated from the calculated fields.

The WWF result is assumed to describe the field without the presence of the source. The emission from a source can influence the ambient field. The handling of the interaction between the emission and ambient field is CFD. The Navier-Stokes equations are calculated for each grid cell and time step. The actual flow speed of the wind field is calculated by using the momentum and mass of the ambient and emitted values in a given grid cell. The ambient mass is allowed to be displaced by the mass emitted into the grid cell at the location of the source.

The actual flow can be thought of as the result of a background flow that is perturbed by adding mass to it. As the emission amount increases it can have more and more influence on the ambient conditions. The flow may even be the reverse of the ambient flow if sufficient emission has occurred.

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Are there suggestions on the number and size of vertical grid cells?

Try to make enough vertical grid cells to extend above either the highest object or the highest significant extent of the plume. If the scenario contains a high speed vertical release, the vertical grid cells can be larger than for a horizontal release. The underlying assumption in a grid cell is that it is uniformly mixed. If a grid cell is too large, the emitted material will be artificially diluted. If the grid cell is too small, so much material can be getting released into a grid cell that the internal time step becomes so small to try to keep track of the detailed dynamics that the compute time becomes excessive. Otherwise the rules are the same as for the horizontal spacing - try to get at least 5 grid cells between the release site and the nearest receptor. Note that the 5 grid cell suggestion is for total number of grid cells in the x, y, and z directions.

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Where are some of the graphic displays available in the stand-alone flat terrain version?

Not all of the stand-alone version displays are included in WebCHARM. However, all the information is included. Some graphical displays in the stand-alone version, for example, concentration history, only show up as tables in WebCHARM. The tables can be selected, copied, and pasted into other programs such as Excel. From there, the data can be plotted.

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Why are some changes not applied when I ask for a print out?

The only time changes that you have made are safely updated is when a tab or other button is pressed. The menu items in the program and in the browser do not always ensure that the input data is updated. Printing is one of the items that this applies to. To ensure that your latest changes take effect you can press the "Apply Changes" button, a tab, or a button on a tabbed dialog.

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