Introduction
For more than two decades, a website run by the Agronomy Department at Iowa State University has been quietly doing something remarkable: making a genuinely professional-grade weather and climate data system freely available to anyone who wants to use it.
If you have spent time on weather forums, you have almost certainly seen its charts. Someone posts a plot showing sixty years of daily high temperatures at their local airport, with the period-of-record average overlaid and the current year highlighted in red. Someone else shares a map of every tornado warning polygon issued over a five-state region across the past twenty years. Another person pulls up a chart showing that last Tuesday’s dew point reading at their station was the highest ever recorded for that date in March. All of those charts came from the same place: the Iowa Environmental Mesonet, known as the IEM.
The problem with the IEM is not that it is hard to use — most of it is point-and-click. The problem is that it is enormous, and the sheer scale of it means most people only ever use a small corner of what is available. The navigation menu has eight top-level categories. Behind those eight categories are hundreds of individual tools, download portals, data viewers, and applications. Some of the most useful features on the entire site are not in any menu at all — they exist at direct URLs that you only find if someone points you to them.
This guide is an attempt to fix that. It covers the IEM from the outside in: what the site is, how it is organized, what each section actually contains, and how to get the most out of the tools that matter most for serious weather hobbyists. It does not assume you are a beginner. It does assume you have not spent years mapping every corner of the IEM the way a researcher or developer might. Somewhere in between is exactly the reader this guide is written for.
A few things this guide is not: it is not a programming tutorial, it is not a manual for scraping data in bulk with Python scripts, and it is not a guide for forecasters or agricultural scientists, even though the IEM serves those audiences too. For those topics, the IEM’s own documentation is actually quite good. This guide is for the enthusiast who wants to understand the weather record at their station, research past events, analyze patterns, and know where to look for whatever question they happen to be asking on any given day.
Throughout the guide, URLs are provided as direct links. In the HTML and EPUB versions, every URL is live — click it and you go directly to that page on the IEM website. The IEM is an actively maintained site and things occasionally move, but the core structure has been stable for years. The URLs verified for this guide reflect the site as of March 2026.
One last note before getting started. The IEM is operated by a small team at Iowa State, led by developer Daryl Herzmann, who has been building and maintaining the system for over twenty years. It runs on public university resources. When you use the IEM, use it thoughtfully — the server handles over 2,000 requests per second from users worldwide, and it keeps running because the people behind it care about keeping it available. If you download data in bulk, be polite about it. If you find a bug or a data quality issue, report it. The IEM’s own QC ticket system is public, and the team actually reads and responds to it.
With that, let’s start at the beginning.