(Chicago, IL) -- United Airlines is being slapped with a $1.1 million fine by the U.S. Department of Transportation because of flight delays at O'Hare.

The fine stems from a stormy day in July of last year, when severe thunderstorms and lightning struck the Chicago area.

Thirteen flights by United and United Express were stuck on the runways for three to four hours. It's the largest fine assessed by the feds since the rule against long delays went into effect three years ago.

The airline was fined for not having an adequate contingency plan and for failing to ask the airport or other airlines for assistance.