Brassica napus is a cool season crop and, as such, can face considerable yield loss when hot weather occurs during flowering. Various studies have shown this over the years. The gist is that temperatures over 28°C to 30°C at flowering can cause yield loss, and the combination of progressively hotter temperatures and more hot days will make yield loss steadily worse.
Heat stress does not seem to affect flower production, but it can have a severe effect on the fertility of those flowers and subsequent pod and seed production. The result is missing pods or misshapen empty pods. It can take a week for hormone balance and regular pod formation to return.