Cirrus clouds are wispy, high-altitude clouds made of ice crystals. They often appear in fair weather conditions, but can also precede a change in weather, such as a warm front or approaching storm. Here's why:

* Fair Weather: When cirrus clouds are thin and scattered, it usually indicates stable atmospheric conditions and good weather. They often form when the air is dry and the temperature is below freezing.

* Approaching Change: As a warm front approaches, cirrus clouds can thicken and spread, eventually becoming cirrostratus clouds which can cover the entire sky. This often signals that precipitation is on the way. Cirrus clouds can also form ahead of thunderstorms, but usually a few hours before the actual storm arrives.

In short, cirrus clouds alone don't necessarily mean bad weather, but they can be a sign of a change in weather conditions, especially when they become thicker and more widespread.