Here's why:

* Low Pressure System: Middle-latitude cyclones are low-pressure systems, meaning the air pressure is lower at the center of the storm than in the surrounding areas.

* Air Flow: Air flows inward and counterclockwise around the low pressure in the Northern Hemisphere (clockwise in the Southern Hemisphere).

* Cold Front: The cold air mass associated with the cyclone pushes into the warmer air, creating a cold front that appears as a sharp line on weather maps.

* Warm Front: Ahead of the cold front, a warm front forms, where warm air rises over the cold air. This front appears as a more gradual line.

* Comma Shape: The combination of these features, with the cold front and warm front curving around the low pressure center, creates a comma-like shape.

You can often see this comma shape depicted on weather maps and satellite imagery.