FAA has published a new edition of AC 00-6 – Aviation Weather (PDF), the 1975 handbook that explains weather theory for pilots.
This AC and several other weather-related advisory circulars have been supplanted by the Aviation Weather Handbook (FAA-H-8083-28), published in December 2022.
The new edition of the companion handbook, AC 00-45 Aviation Weather Services, which explains aviation weather reports and forecasts and the briefings available to pilots, is also available at the FAA website.
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