The changes vary between brain portions and male versus female brains. The data are from thousands of MRIs taken over 15 years at a neurology practice. Brace for a big brain summer: Your brain swells to its largest size in the warmest months, says a new study in Scientists from Hartford Hospital’s Olin Neuropsychiatry Research Center studied the brain scans of more than 3,000 healthy people over a 15-year period. The researchers found the sizes of different brain portions were consistently larger during the northern hemisphere’s warm summer months and smaller during the cold winter months. Hartford, Connecticut, which averages a high of 84 degrees Fahrenheit in July and a high of 36 degrees in January, is “an ideal location to test weather and seasonal effects because it is near sea-level, experiences four distinct seasons, and a wide range of weather conditions,” the scientists say. They continue: