Annual VMC/IMC statistics for any ICAO airport — data-driven VFR planning powered by Iowa Environmental Mesonet.
Criteria
Visual Meteorological Conditions (VMC) allow operations under Visual Flight Rules (VFR). For a day to be classified as VMC, at least 75% of its METAR observations must simultaneously satisfy all of the following criteria, per Class E airspace below 10,000 ft.
≥ 1,500 ft AGL. The base of the lowest BKN or OVC cloud layer (or vertical visibility into an obscured sky) must exceed 1,500 feet above aerodrome level.
≥ 5 km (3 SM) below 10,000 ft. The prevailing meteorological visibility reported in the METAR must be at least 5 kilometres to allow visual navigation.
≥ 1,500 m from clouds. The aircraft must maintain a minimum horizontal distance of 1,500 metres from any cloud to avoid inadvertent IMC entry.
≥ 1,000 ft from clouds. Both above and below, the aircraft must remain at least 1,000 feet clear of any cloud in the vertical plane.
Methodology
Each METAR observation is classified individually. Values are never interpolated and conditions are never assumed for periods without data. The daily classification directly reflects the actual observations available.
Data source
Historical METAR data from the IEM ASOS network. Up to 12 months of hourly observations retrieved in a single request per airport.
Classification
Each observation is evaluated against the thresholds: visibility ≥ 3 SM and ceiling ≥ 1,500 ft. If both conditions are met the observation is VMC. Observations with missing critical fields are recorded as "no data".
VMC Day
A day is classified as VMC if at least 75% of its observations meet the criteria. Between 25% and 74% → Mixed. Below 25% → IMC. All times are UTC.
Note: all data is expressed in UTC. Periods without METAR observations are excluded from the daily percentage calculation.