The colder months of the year have the greatest temperature variability. The difference between a cool summer day and a hot summer day might be 30 degrees whereas winter temperatures can vary by twice that. The primary reason for this is the temperature gradient across the northern hemisphere. The equatorial regions of the Earth have temperatures near 90 degrees all year. At the North Pole, summer temperatures hover near freezing while winter temperatures are often 50 degrees below zero or colder.
This means the temperature difference between the equator and the pole varies from about 60 degrees in summer to 140 degrees in winter. This increased temperature gradient in the winter makes large scale storm systems more robust and causes a much wider variety of weather from day to day. This helps explain how our weather typically goes up and down more frequently as we get further into fall.