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"Thank you for actually penning these books. They are a wonderful review of the concepts without the excessive (but necessary) amount of math behind them. I wish they had been recommended as supplements in both my undergrad and graduate years."

— Josh Tobias, 2017

"I have used Digital Atmosphere for quite sometime and consider it to be one of the most straightforward, comprehensive, and operationally useful programs on my server. I have found it to be invaluable for my day-to-day analyses and forecasting as well as my ongoing research. Tim is also extremely helpful and absolutely indispensable when it comes to locating scripts, the best datasets, and even professional contacts to help make any organization as productive as possible."

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— Simon Keeling
Weather Consultancy Services, UK

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"The archive data arrived today. WOW! What an amazing set of data! Thanks again for all the extras that you included as well!"

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"This new version is even better than the older version which was awesome! Your programming skills and met knowledge amaze me!"

— Chris Kincaid

WX-SIM Professional
Adds GFS bias learning, advanced parameters, and enhanced customization for your site. See the FAQ below for specific differences.
Electronic delivery

WX-SIM Standard
Suitable for most users. See the FAQ below for specific differences. You can upgrade at any time for the difference in price.

Electronic delivery

WX-SIM is a desktop meteorological forecasting model developed over a span of 25 years by Tom Ehrensperger, a teacher in Atlanta who holds an M.S. in physics. It is a dynamic forecast model operating at one location, forecasting the weather for that spot. WX-SIM exploits observed conditions and centralized forecast data from NOAA and combines it with its own sophisticated physics model of the local atmosphere. The result? A singularly accurate forecast for the location of your choice. This makes it the tool of choice for the agricultural industry, aviation forecasters, and sporting venues.

To try out a fully working copy of WX-SIM (runs Atlanta, Georgia only) click here:

    wxsimdem.exe (WX-SIM Version 12.6 Limited / 1.3 MB) (Windows Vista/XP/2000)
The main screen for WX-SIM is the data entry screen. This is where you set up the "starting conditions" for the virtual atmosphere. You can adjust the time and date (solar positions are automatically handled), temperature, humidity, wind, pressure, cloud layers, and more. To automate this whole process you can use the WXSIMATE module to import data from your home weather station or via the Internet from the nearest observation site. In this example we'll take a look at the forecast for Atlanta.
The synoptic map for October 8, 2009 showed a warm front just south of Atlanta. During the next 24 hours or so, therefore, we can expect unsettled weather, an increase in clouds, and a slight warming trend.

NOTE: This graphic is NOT created with WXSIM and is only shown here for comparison purposes.
WXSIMATE comes with WXSIM and is a much-recommended enhancement that automates much of the data entry. It connects with either your home weather station or the Internet, automatically populating all of the windows with conditions from the nearest observation site. It's also capable of downloading gridded model fields, statistical model output, and upper air conditions, making it almost essential for the serious WX-SIM user. If you have a home weather station, WXSIMATE does not connect directly to the station but looks through the logs generated by one of three weather station programs, which you must be running. These three programs are Davis Instruments WeatherLink, Brian Hamilton's Weather Display, or Virtual Weather Station.
The data import window in WXSIMATE shows you the results of any data that's been imported. Here we can see that one METAR observation close to our site has been found, along with a graph of NAM forecast output to help provide a first guess dataset for WX-SIM. Forecast model output from dynamic models run at NOAA/NCEP is important because it dictates the large-scale changes that will impact the station's forecast. WX-SIM uses this to refine and adjust its output.
The FOUS import panel lets us fine-tune the exact details of the forecast model data that we want to ingest. The cloud, precipitation, and wind data is particularly helpful since these represent parcels brought in from other areas which WX-SIM is not capable of forecasting for.
The Upper Air panel displays the latest radiosonde data along with the model output. All of the functions except for automated 1-Click are only available in the Professional version, but the panel allows adjustments to be made to help reach the exact sounding profile desired. Note that stability indices and cloud layers are automatically calculated and displayed.
Advection indicates air mass properties that will be brought in from elsewhere. While WX-SIM is capable of estimating changes without knowing such information, it can help make the forecast much more accurate. As seen here, WX-SIM can examine all of the data upwind and use it to figure out how the air mass will change over time (graph at lower right).
The interrupt screen is used to force manual changes in clouds, wind, and other parameters during the forecast period. This is useful if, for example, you know when storms are likely to develop or exactly when a cold front will arrive. You can even add in an expected eclipse and factor in the reduction in solar heating.

The interrupt planner takes advantage of free, automatically (through WXSIMATE) downloaded GFS data out to 180 hours. This now even includes use of GFS forecast data for advection after wind shifts that occur at any time during the forecast run.  This means model data to support WXSIM is available worldwide, out to at least a week.  Most people run forecasts between 4 and 7 days.
The Output window shows the progression of the forecast computation, which takes about 20 to 30 seconds on a fast Pentium, and produces an output of the forecast in graphic and text format. The information is also exported to CSV and TXT files (see below). The forecast here calls for slow warming and cloudy weather, with drizzle arriving the next evening. In fact, here is the forecast it generates:

WXSIM text forecast for Atlanta, initialized at 11:52 Oct 8, 2009

This afternoon: Partly cloudy. High 82. UV index up to 6. Wind southeast around 7 mph.

Tonight: Partly cloudy. Low 62. Wind southeast around 7 mph in the evening, becoming 11 mph after midnight.

Friday: Mostly cloudy. High 84. Wind southeast around 12 mph.

Friday night: Mostly cloudy to cloudy in the evening, becoming cloudy after midnight. A slight chance of rain. Low 68. Wind southeast around 11 mph in the evening, becoming 14 mph after midnight. Chance of precipitation less than 20 percent. Precipitation mostly less than a tenth of an inch.

Contrast it with the official NWS forecast shown here.
915 AM EDT THU OCT 8 2009

915 AM EDT THU OCT 8 2009


For those wondering, Atlanta reached 80 degrees this afternoon. WX-SIM correctly anticipated slightly warm temperatures, and that with just a bare minimum of setup and without any gridded model data for this particular run.
Data is automatically exported in CSV and TXT format so you can view the information in Excel, ingest them into a database, or use custom scripts to display the forecast. Examples can be downloaded here: latest1daily.csv (daily), latest1.csv (15 min interval), and latest1.txt (text version with detailed summaries).
WRET is a separate program that allows you to view detailed displays of the WX-SIM output. It is included free with all copies of WX-SIM. With WRET you can mix and match different kinds of graphed fields and control the period of the display window.
The Great Sun Outage is an experiment we like to do with WX-SIM. Here we have taken real data but switched the sun off at 12 noon on Friday. This is represented by the drop in the light blue line on the graph As you can see, there is an immediate dropoff from the midday temperature of 80 degrees, and by midnight it's 58. The temperature fall during the weekend and the rest of the week is very gradual due to the high specific heat of the earth's surface and the lack of an insulating blanket of snow. A week later, it's in the 40s.
The Great Sun Outage displayed in WRET shows plots of temperature, dewpoint, and valley temperatures (valleys tending to accumulate cold air drainage at night). There is also a text plot showing the amount of solar radiation received in watt-hours per meter squared and inches of accumulated rain. The lower graph displays cloud cover, relative humidity, solar radiation, and thundershower activity. By clicking anywhere on the graph, a sounding for that time will pop up. Each of the weather parameters can be changed to something else using the options in WRET.

Frequently asked questions

Q. Can I run WX-SIM for any location on Earth?
A. No, at least not at this time. This is a technical limitation since Tom Ehrensperger customizes each weather site by hand, using climatology, vegetation, and topography. If you move and need to run WX-SIM for another location, you can order a new configuration package for a fraction of the purchase price.

Q. Why is WX-SIM useful if the centralized forecast models already give us hour-by-hour forecasts?
A. The centralized models only give one solution. There's no way of making adjustments to the conditions based on your own knowledge and intuition and tweaking the forecast. Not only does WX-SIM allow this interactive control but it also uses its own physics model of the atmosphere at your site to provide a more precise look at the forecast weather.

Q. In what niche is WX-SIM most accurate?
A. If used strictly by itself, WX-SIM is most accurate during the 0 to 12 hour time frame. During this time the centralized models are still resolving the noise in the initialized fields, which does not smooth out for a number of hours, and since WX-SIM has a highly accurate picture of current conditions it can use this to produce a very accurate forecast. Beyond 12 to 24 hours centralized models have a much greater grasp on the large-scale weather patterns, but WX-SIM exploits this data to take into account localized effects and microclimatology specific to your location. Another example is that it models special weather and phenomena. If the temperature is at a critical level for a planned weather event and an eclipse is expected, you can either hope that the centralized models are considering the small temperature drop with the eclipse, or you can run WX-SIM and find out for sure.

Q. What is this internal physics model?
A. The internal physics model in WX-SIM looks at the incoming short-wave radiation, the outgoing long-wave radiation, the angle of the sun, and the various cloud layers and lithometers that absorb, reflect, and radiate energy. It also considers the atmospheric temperature and moisture profile, evaporational cooling, latent heat, and their effect on wind, cloud, and weather. And of course it also takes into account advection, which is the movement of moisture and temperature properties by the winds.

Q. What are the quantities that WX-SIM can forecast?
A. WX-SIM is capable of predicting temperature, dewpoint, wet-bulb temperature, heat index, wind chill factor, temperatures in valleys (drainage) and on hilltops (above the nocturnal inversion), sky cover, transmission of visible sunlight, solar radiation and UV index, wind direction, wind speed, precipitation total, probability of precipitation, snow depth, thickness, snow levels, upper level temperatures and dewpoint, severe weather indices, sun altitude, visibility, and more. Many of these parameters are simply not available from centralized model products found on the Internet.

Q. Can WX-SIM learn from its mistakes?
A. Yes, this is a capability built into the Professional version. Automatic archiving of forecasts allows for quite a collection of forecasts (hundreds within a few months, for example) to be built up, and then the WRET module can be used to compare various forecast variables to actuals, and then determine bias correction data that can (optionally) be used by the program.  "Artificial intelligence" may not be quite the right term, but the ability to use feedback from past results is a pretty powerful feature on any forecast system.  In summary, specific corrections are to overall temperature, diurnal temperature range, temperature tendency towards climatology, and overall dew point.  It's also possible to make bias corrections to cloud cover and precipitation, based on use of these comparisons. 

Q. What is the difference between the Standard and Professional version?
A. The Standard version is suitable for most users and includes the vast majority of the features described above. Tom Ehrensperger states that most users should be fine with the Standard Version unless they are interested in monitoring model biases or using the more advanced, experimental parameters offered by WX-SIM. A complete list of differences follow:

  Standard Version Professional Version
Customization Tom Ehrensperger accomplishes a standard configuration of variables for your site. Tom Ehrensperger completes a very detailed configuration of variables for your site.
Model Bias adjustment Not available. Included.
Data Entry Form Auto stratus, auto cumulus, auto haze, urban heat island adjustment, cull/append, and test for midpoints are not available. All items are available.
Upper Air Form Only "1-click" adjustment can be used to reconcile the sounding data with model data. All items are available.
Advection Form "Two Upwind Sites", "Direct Click", "Use Previous", and "change wind direction" slider are not available. All items are available.
Interrupt Planner Clearing, saving, recall, 850 mb RH, and "reduce superadiabatic" options are not available. All items are available.

How to try it and buy it
To try out the Professional WX-SIM version (Windows Vista/XP/2000) click here:
    wxsimdem.exe (WX-SIM Version 12.6 Limited / 1.3 MB)

This is a fully unrestricted version, however it only operates for Atlanta, Georgia. You can freely download live data, test out WXSIMATE and WRET, and play with all the bells and whistles. Once you're satisfied and want to order for your own locale, simply scroll to the top of this page and click one of the "order" buttons.

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