You're likely thinking of the eye of a hurricane. It's not actually a circle in the sky, but a calm, clear area in the center of the storm. Here's why it appears that way:

* Low Pressure: The eye is a region of extremely low air pressure. This causes the air around it to spiral inward, forming the powerful winds and heavy rainfall that characterize a hurricane.

* Descending Air: Within the eye, the air descends from the upper atmosphere. This descending air is dry and warm, resulting in clear skies and calm conditions.

* Circular Shape: As the air spirals inwards, the eye appears to be a circle in satellite images and from ground level, as it represents the calm center of the storm.

It's important to remember that while the eye might seem peaceful, it's surrounded by the most dangerous part of the hurricane. The powerful winds and heavy rainfall can cause catastrophic damage.