Source. Automated Surface Observing System (ASOS) routine hourly observations,
retrieved from the Iowa Environmental Mesonet archive, which republishes NOAA/NWS
observations. Station data pulled 16 August 2026.
Period. 1 August 2025 to 1 August 2026 inclusive — twelve full months.
Observations used. Routine (METAR) reports only, between 07:00 and 18:00 local
time at each station. Local time is applied per station, so daylight-saving shifts
are handled correctly and stations in different time zones are compared like for like.
Ceiling. The lowest reported layer with coverage BKN, OVC or VV. Layers reported
FEW or SCT are not ceilings and are excluded, consistent with FAA definitions.
Two Thresholds.
Basic VFR — ceiling at or above 1,000 ft and visibility at or above 3 statute miles. This is the legal floor for VFR flight in Class D airspace.
Training minima — ceiling at or above 3,000 ft and visibility at or above 5 statute miles. This is the more conservative standard most flight schools apply to student
solo and early dual instruction. It is the number that matters to a student pilot.
Flyable day. A calendar day counts as flyable when at least four daytime hourly
observations met the threshold, and the day had at least six valid observations.
Four hours is roughly two lesson slots including preflight and debrief.
Hourly tables. For each month and each hour, the percentage of observations at
that hour meeting the training minima across the whole period.
Known Limitations.
Ceiling and visibility only. Wind, turbulence, density altitude and crosswind component are not modeled, and any of them can scrub a lesson on a clear day.
ASOS is automated. It occasionally misreports thin layers and blowing dust.One year of data. Marine layer behavior varies year to year with sea surface temperature, so treat single-month figures as indicative rather than climatological.
Station outages produce missing days, which is why the observed-day count is under 365 at several fields.
Raw pulls and the analysis code are reproducible from the URLs in this repository.