The polar vortex is a phenomena where the jet stream keeps cold arctic air near the poles. A weakening jet stream causes the polar vortex to become unstable, allowing the arctic air to escape the poles.
Many climate alarmists claim that global warming is leading to an unstable polar vortex, leading to more cold weather.
Read more: https://climateataglance.com/climate-at-a-glance-cold-spells/