Thunderstorms: the #1 delay maker
Convective weather (thunderstorms) causes more delays than any other weather type. They're most common June-August, 2-8 PM, in the Southeast and Midwest. A single storm cell over Atlanta can cascade delays across the entire US system.
Snow and ice
Snow itself doesn't cause many delays — airports like Denver and Minneapolis handle it well. The problem is ice, especially freezing rain. De-icing adds 30-60 minutes per departure. The worst ice delays hit airports not equipped for it: Dallas, Atlanta, Charlotte.
Fog
Fog reduces airport capacity by 50% or more because planes need more spacing on approach. SFO is famous for summer fog delays. Most fog burns off by 10 AM — another reason early morning flights aren't always the best at fog-prone airports.
💡 Weather planning
Check the weather at BOTH your departure and arrival airports. A sunny departure doesn't help if your destination is socked in.