I notice in a website taling about jetstream https://www.weather.gov/jetstream/jet
The winds blow from west to east in jet streams but the flow often shifts to the north and south. Jet streams follow the boundaries between hot and cold air. Since these hot and cold air boundaries are most pronounced in winter, jet streams are the strongest for both the northern and southern hemisphere winters.
Why these hot and cold air boundaries are most pronounced in winter? I dont get it.