A weather warning was in place on the day that a bouncy castle blew across a fairground with a little girl inside it, a court heard.
Seven-year-old Summer Grant died in hospital after she was rescued from the inflatable, Chelmsford Crown Court was told.
Fairground worker William Thurston, 29, and his wife Shelby Thurston, 26, both deny manslaughter by gross negligence and a health and safety offence following the incident at Harlow Town Park in Essex.
Meteorologist Dr Richard Wild told jurors on Thursday that a Met Office yellow weather warning had been in place since two days before the incident, on Easter Saturday in 2016.
He said a yellow warning is the lowest of the Met Office's three types - yellow, amber and red.
The expert witness, who is credited on Harry Potter films as their weather consultant, said he analysed forecast data from the time.
"I concluded that the gusts in the area at that time were, I believe, 35-40mph," he said. "That was within the hour of the actual incident."
David Kerr-Sheppard, the air ambulance pilot who flew to the incident, said conditions were "squally".
Asked what he meant by this, the chief pilot of Essex Air Ambulance said: "My interpretation would be sudden, sometimes violent bursts of wind that could easily change direction and it's generally affected by strong, very active cloud activity."
He described it as a "pretty miserable day".
"It was wet, windy, total cloud cover and there had been some quite strong gusts going through the airbase prior to us taking off," he said.
He said the weather was not suitable to fly Summer to a London hospital, and she was instead taken by land ambulance to Princess Alexandra Hospital in Harlow.
Nicola Peckham, digital portfolio manager for the Met Office, described the Met Office weather app that smartphone users could download at the time.
"The forecast is updated every hour and the app has to receive the forecast within 15 minutes of it being released," she said, appearing by video-link. "The app would automatically update with the latest forecast."
She said it needed a GPS or WiFi signal to work, and weather warnings would display in the app, and would also display on the front screen of a phone if push notifications were switched on.
The app used at the time is no longer available, with a new app introduced in May 2016, Ms Peckham said.
Prosecutors say that the Thurstons, of Whitecross Road, Wilburton, near Ely, Cambridgeshire, failed to ensure that the bouncy castle was "adequately anchored" to the ground and failed to monitor weather conditions to ensure it was safe to use on Easter Saturday in 2016.
Summer, from Norwich, had been at the fair with her father Lee Grant and other family members.
The trial continues.