Climatology is not a forecast
This calculator answers a planning question: in this week of the year, at this time of day, how often have the airports along this route been at or above my personal minimums? It does that by counting archived routine METARs. A 70% historical go rate for mid-June mornings is a useful prior — coastal marine layers, afternoon thunderstorms, and winter icing seasons are real patterns — but it is not a prediction for next Tuesday. Obtain an official briefing and read the METARs, TAFs, AIRMETs, SIGMETs, and convective outlooks that actually apply to the flight.
How stations along the route are chosen
Given a departure and destination, the page looks up each site in a compact U.S. ASOS/AWOS list (identifiers, coordinates, and time zones from the Iowa Environmental Mesonet state networks). It then finds other stations whose cross-track distance to the great-circle is at most the corridor width (25 NM by default) and whose along-track position sits between the endpoints. Those candidates are thinned to a capped, evenly spaced set — departure and destination first, then the largest remaining gaps along the route — so a 200 NM hop does not fire a request for every AWOS on the coast. You can remove any of them, add an alternate or a site the auto-picker skipped, and widen or narrow the corridor. A route go requires every selected station to meet the minimums in the same hour, so the table also shows each station alone — the lowest one is the historically weakest segment. Auto-picked sites with a short or patchy archive (for example a station that only came online in the last year or two) are still listed, but they are left out of that combined route score so they do not erase earlier years of data.
How a go hour and a go day are counted
For the chosen month and day, the tool takes a window of ±3 days (a seven-day week; you can shorten or lengthen it) in each of the past five years by default, or 3, 10, or 15 years if you ask. Times are converted from the archive’s UTC stamps into one time zone — the departure zone unless you change it — and only observations in your local hour window are scored. The end hour is exclusive (07–10 is 07:00–09:59). A single observation is a go when:
- The ceiling is at least your minimum, or there is no ceiling (clear, few, or scattered only). Ceiling is the lowest BKN, OVC, or VV layer, as in AIM Chapter 7. Heights come from the archive in feet AGL.
- Visibility is at least your minimum, in statute miles.
- If you excluded rain, snow, freezing precipitation, or thunderstorms/severe weather, none of those present-weather codes appear. Severe here follows the list on the form: thunderstorms (including vicinity), heavy rain (
+RA), hail, funnel clouds, and squalls — from AIM 7-1 contractions. - Optional wind and gust caps, if you set them, are not exceeded. A missing gust is not treated as a gust-limit failure.
Observations that are missing a field you are actually checking (no sky condition, or no visibility) are dropped from the denominator rather than scored as a no-go. A route-go hour is one in which every selected station has an evaluable observation and all of them are go. The headline figure is the share of historical days in the window that had at least one route-go hour — “a go window existed that morning.” The page also reports the stricter share of hours that were simultaneously go, and the share of days on which every hour in the window was a go (“a solid morning”).
The archive, sampling, and request limits
Data are fetched in your browser from the Iowa Environmental Mesonet ASOS/AWOS download service (mesonet.agron.iastate.edu), which allows cross-origin requests. The URL asks only for sky cover and height, visibility, weather codes, and wind, for the week window in each year (padded by one UTC day so local midnights are not clipped), and only for routine METARs (report_type=3). Special observations cluster in deteriorating weather and would bias the climatology toward no-go if they were included. Several stations are combined in one request per year; responses are cached in sessionStorage so a rerun with the same stations and dates does not hit the archive again. Please do not script heavy downloads against IEM from this idea — the page is intentionally light. IEM station identifiers for the lower 48 are usually the three-letter id (KSBA is SBA in the archive); Alaska, Hawaii, and many non-CONUS sites use the four-letter ICAO.
VFR minimums and flight category
The VFR preset (3,000 ft and 5 SM), MVFR-or-better preset (1,000 and 3), and IFR personal preset (500 and 1, plus freezing precipitation and severe weather excluded) follow the familiar AIM flight-category breakpoints as a starting point for personal minimums. Comparisons are inclusive. They are not a restatement of 14 CFR 91.155, whose basic VFR visibility and cloud-clearance values depend on airspace class and altitude and are sometimes lower than “flight category VFR,” sometimes not. Use the legal rule that applies to the airspace, then apply your own tighter minimums here if you wish.
Sources
- Iowa Environmental Mesonet, Iowa State University — ASOS/AWOS/METAR archive and station metadata.
- FAA Aeronautical Information Manual (AIM) Chapter 7, Meteorology — METAR present-weather contractions, sky-condition and ceiling definitions, and flight-category guidance.
- 14 CFR 91.155 — Basic VFR weather minimums (legal minima by airspace, not this page’s presets).
- FAA Advisory Circular AC 00-45, Aviation Weather Services — how METARs, TAFs, and briefings are produced and should be used.
Frequently asked questions
What does this route weather go / no-go calculator tell me?
How often, in the same week of the year and time of day, archived hourly METARs at the selected airports along your route met the personal minimums you entered. A route go means every selected station was a go in the same hour.
Is this a weather forecast?
No. It is climatology and says nothing reliable about the actual day. Get an official briefing.
How is a route go decided?
All selected stations must have an evaluable routine METAR that meets every limit in that local hour. The headline day percentage is the share of historical sessions with at least one such hour. Per-station columns show which airport is the usual weak point.
Where does the historical weather come from?
The Iowa Environmental Mesonet ASOS/AWOS archive, requested from your browser, routine hourly METARs only, with light, cached year-window downloads.
How is ceiling defined, and how do the VFR presets relate to 14 CFR 91.155?
Ceiling is the lowest BKN, OVC, or VV layer (AIM). The presets are AIM-style flight-category starting points for personal minimums, not a substitute for 91.155.