What is the Polar Vortex?

A powerful winter storm “Fern” swept across large parts of the United States on Sunday (January 26), bringing heavy snow, sleet, and freezing rain from the Ohio Valley and mid-South to New England. The extreme weather disrupted transport and daily life, leaving at least seven people dead.

Role of the Polar Vortex
Meteorologists attribute the intensity of the storm to the activity of the polar vortex, a vast region of low pressure and frigid air that surrounds the Earth’s poles. During winter, disturbances in the polar vortex can allow cold Arctic air to spill southward into mid-latitude regions.

What is the Polar Vortex?
The polar vortex is a large-scale atmospheric circulation pattern that always exists near the poles, though its strength varies seasonally—weakening in summer and strengthening in winter. The term “vortex” refers to the counter-clockwise flow of air that helps confine extremely cold air near the polar regions.

Types of Polar Vortex

• Tropospheric Polar Vortex:
  Occurs in the lowest layer of the atmosphere, extending from the Earth’s surface up to about 10–15 km, where most weather phenomena take place.
• Stratospheric Polar Vortex:
  Found higher in the atmosphere, roughly between 15 km and 50 km. Unlike its tropospheric counterpart, it disappears during summer and is strongest during autumn and winter.

Interaction with the Jet Stream
When the polar vortex is strong and stable, it keeps the jet stream—a narrow band of powerful winds high in the atmosphere—moving in a relatively circular path around the globe. This helps keep cold air locked in the Arctic and warmer air farther south.
However, when the polar vortex weakens or becomes distorted, the jet stream can dip southward, allowing bitterly cold air to move into regions such as the United States, Europe, and parts of Asia.

Not a Surface Phenomenon
The polar vortex does not exist at the Earth’s surface. Forecasters monitor it by analyzing atmospheric conditions tens of thousands of feet above ground. Nevertheless, extreme cold spells felt at the surface are often linked to shifts or weakening of the polar vortex aloft.

Global Implications
While the current storm has impacted the United States, polar vortex-related cold surges are not limited to North America. Similar patterns can trigger severe winter conditions across Europe and Asia, highlighting the polar vortex’s global influence on winter weather.