1 Chapter 1 — What Is the IEM?
The Short Version
The Iowa Environmental Mesonet is a free, public weather and climate data platform operated by Iowa State University. It ingests data from dozens of observing networks, archives it, and makes it available through a collection of web applications, download portals, and data services. The archive goes back in some cases to the 1800s. The tools range from simple current-conditions displays to sophisticated charting systems with hundreds of configurable parameters. It is free to use for any lawful purpose, including commercial use.
That is the functional description. The more interesting question is how something like this came to exist at a land-grant university in Iowa — and why understanding that history helps you use the site better.
Where It Came From
The IEM grew out of the Iowa Environmental Mesonet project, which began in the 1990s as an effort to collect data from a variety of observing networks in Iowa and make them available in a unified form. At the time, weather data from different networks — airport ASOS stations, NWS cooperative observers, agricultural weather stations — lived in separate systems that did not talk to each other. The IEM was built to aggregate them.
Over time, the project expanded well beyond Iowa. The network coverage now includes stations across the entire contiguous United States and internationally. The tools evolved from simple data displays into a full research and analysis platform. The Daily Feature — a chart or analysis published almost every day since February 2002 — became a kind of running meteorological journal that demonstrates what the data can show when you ask it the right questions.
The key figure throughout this history has been Daryl Herzmann, a developer at Iowa State who has been the primary architect and maintainer of the IEM for over two decades. His name appears on the contact page, his GitHub repository contains the source code for the entire site (it is open source), and his Twitter handle is linked from the footer. This is not a faceless institutional project — it is one person’s life work, supported by Iowa State’s infrastructure and a small team of collaborators. Understanding that helps explain both the remarkable depth of the system and the occasional rough edges that come with a project of this scale maintained by a small team.
What “Mesonet” Means
A mesonet — short for mesoscale network — is a network of weather stations designed to observe atmospheric conditions at a scale smaller than the traditional synoptic weather observation network but larger than a single point. The classic example is the Oklahoma Mesonet, a statewide network of automated stations spaced roughly 30 kilometers apart, designed to capture the mesoscale variability in temperature, humidity, and wind that drives severe weather in that region.
The IEM is not itself a physical observing network in that sense — it does not own and operate a set of weather stations across Iowa. What it does is aggregate data from observing networks that do exist: the ASOS airports, the NWS cooperative observers, the Iowa State agricultural weather network, river gauges, roadway sensors, and many others. The “mesonet” in the name reflects the original ambition to build an integrated view of environmental conditions across the state at mesoscale resolution. It has since grown into something considerably larger.
The Homepage
When you navigate to mesonet.agron.iastate.edu, the homepage has a specific structure that is worth understanding because it reveals how the IEM thinks about its own audience.
At the top is a red navigation bar with a search box and a hamburger menu. The search box accepts natural-language queries in a specific format — the placeholder text shows 'DSM' 'autoplot 100' as an example, which means you can search for a station ID, an application name, or a combination. It is a useful shortcut once you know what you are looking for.
Below the header, three audience buttons appear: Ag Weather, NWS Users, and Researchers. These are portal buttons that take you to curated entry points for each audience. Ag Weather leads to the agricultural weather and climate information pages. NWS Users leads to the NWS data mainpage. Researchers leads to the archive mainpage. If you are a weather enthusiast coming to the IEM for the first time, none of these three buttons is exactly right for you — the closest is probably Researchers, because you want the archive and the analytical tools. But do not get too attached to these portal labels. They reflect the IEM’s primary funding audiences and do not define the limits of what the site can do for someone with a general weather interest.
The center of the homepage is dominated by the IEM Daily Feature — a chart and short explanation published most days, tied to whatever is happening meteorologically. On the day this guide was verified, the feature covered an all-time March temperature record set at Cedar Rapids, with a chart showing the monthly maximum temperature records by month for that station. Below the current feature is a list of previous years’ features for the same calendar date, going back to 2002. This is one of the most underrated features of the entire site: you can see what the IEM was tracking on this exact date in every year going back over two decades, which gives you an immediate sense of what the tools can produce and what kinds of questions they answer.
The right column of the homepage has two sections worth noting. The Most Popular sidebar lists the seven most-accessed applications on the site at any given time — currently Automated Data Plotting, Climodat Reports, Iowa Mesonet Plot, IEM One (RADAR + Warnings), IEM GIS Rainfall, Sortable Currents, and Winter Storms of 2025–2026. This is a real-time list and changes with the season and with whatever weather is happening. It is a reasonable proxy for “what do most users actually come here for.”
The Current Website Performance panel is a small but revealing feature. It shows live bandwidth usage and request rates. When this guide was verified, the IEM was handling 2,259 requests per second across the entire site and 224 requests per second specifically to the API and data services, with 100% API success rate. On a busy weather day — a significant severe weather outbreak, a record-breaking heat event — these numbers spike considerably. The fact that this performance data is displayed publicly on the homepage is characteristic of the IEM’s general transparency about its own operations.
At the bottom of the homepage, three map widgets show current soil temperatures (from the Iowa State soil moisture network), a current precipitation analysis, and a current surface observation plot. These are live products that update automatically and give you a quick read on current conditions across the Midwest.
The Daily Feature Archive
The IEM Daily Feature deserves more attention than the one paragraph most visitors give it. Since February 2002, someone at the IEM — almost always Daryl Herzmann — has published a chart or map tied to whatever is currently interesting in the weather record, with a short explanation of what the chart shows and why it matters. These are not press releases. They are the kind of quick analysis a meteorologist does when something catches their attention: an all-time record, an unusual pattern, a new tool worth demonstrating, a climatological pattern that puts the current weather in context.
The archive is at mesonet.agron.iastate.edu/onsite/features/past.php and is organized by month. Every entry links to the chart, includes the short write-up, and — critically — includes a “Generate This Chart on Website” link when the chart was produced by Autoplot. That link takes you directly to the Autoplot configuration that produced the chart, with all the parameters already set, so you can run it yourself, change the station, change the year, and immediately start exploring.
The archive is also searchable by tag at mesonet.agron.iastate.edu/onsite/features/tags/. Tags include things like winter2526 (the current winter storm season), blizzard, thunder, aridity, warmfront, and dozens of others going back years. If you are trying to find examples of a particular type of analysis — say, how the IEM has visualized cold air damming events, or how it handles precipitation type verification — the tag archive is a fast way to find representative examples.
There is a practical tip buried here for new users: if you do not know where to start with the IEM, start with the Daily Feature archive. Read a month’s worth of entries. Every one links to a live chart that you can immediately run for your own station. By the time you have worked through a few dozen entries, you will have a much more concrete sense of what the tools can do than you would get from reading any amount of documentation.
What the IEM Is Not
A few clarifications that save confusion later.
The IEM is not the official government archive. NCEI (the National Centers for Environmental Information) is the official custodian of US climate records. The IEM draws on some of the same underlying data — GHCND stations, ASOS observations, NWS COOP reports — but the IEM’s processed products are explicitly labeled as unofficial. This means the IEM’s period-of-record records for your station may not exactly match what NOAA would cite in an official publication. This is not a flaw; it is an honest acknowledgment that different processing choices produce slightly different results, and the IEM’s choices are optimized for usability and currency rather than archival certification.
The IEM is not primarily a forecast site. You will not find model output, forecast soundings, or ensemble guidance here. What you will find is historical data, climatology, and analysis tools built on the observational record. The one exception is Model Output Statistics (MOS), which is a statistical bias-correction of model forecasts archived at the station level — but even MOS on the IEM is an archive product, not a live forecast display.
The IEM is not a real-time alerting service. It archives NWS warnings and processes them in real time, and it operates iembot — a service that posts warning text to social media — but it is not designed to be the source you rely on for life-safety alerts during an active severe weather event. Use the NWS directly for that. Use the IEM afterward to research what happened.
The IEM is not always Iowa. Despite the name, the data coverage extends nationwide and internationally. The observing networks, the Autoplot charts, the warning archive, the MOS data — all of it works for stations across the contiguous US and in many cases beyond. The Iowa-specific products (the Iowa flood center precipitation maps, the ISU Soil Moisture Network data) are a subset. Do not let the Iowa branding lead you to conclude the site is only useful if you care about Iowa weather.
A Note on the “Unofficial” Label
Throughout the IEM, you will encounter the word “unofficial” applied to climate records and statistics. This is worth understanding precisely, because it means something specific and does not mean what it might first appear to mean.
The IEM computes daily records and climatological statistics directly from the data it ingests. Because its processing pipeline may handle missing data, duplicate reports, and timing offsets differently than NCEI’s official climatological procedures, the numbers do not always agree exactly. The IEM calls its results “unofficial” to clearly signal that they should not be cited in official government reports or legal proceedings as the authoritative value.
What “unofficial” does not mean is inaccurate or unreliable for practical purposes. For the kind of analysis a weather enthusiast does — comparing this year’s precipitation to the historical average, finding out what the coldest temperature on record was for your station on a given date, tracking how many days above 90°F a station has had over the past decade — the IEM’s unofficial numbers are entirely appropriate. The differences from official values, when they exist, are typically small and stem from defensible methodological choices rather than errors.
The label is also a form of legal protection for the university. It keeps the IEM from becoming the de facto cite in every insurance claim or agricultural contract dispute that touches weather data. That is a reasonable position for a free public service to take.
Key URLs — Chapter 1
| Page | URL |
|---|---|
| IEM Homepage | mesonet.agron.iastate.edu |
| Daily Feature Archive | mesonet.agron.iastate.edu/onsite/features/past.php |
| Feature Tags | mesonet.agron.iastate.edu/onsite/features/tags/ |
| Feature Titles (full list) | mesonet.agron.iastate.edu/onsite/features/titles.php |
| Daily Bulletin Signup | groups.google.com/g/iem-dailyb |
| IEM News | mesonet.agron.iastate.edu/onsite/news.phtml |
| RSS Feed | mesonet.agron.iastate.edu/rss.php |
| GitHub Source Code | github.com/akrherz/iem |
| Contact | mesonet.agron.iastate.edu/info/contacts.php |
Next: Chapter 2 — Station Networks: The Foundation of Everything