Here's why:

* Tropopause: The tropopause is the boundary between the troposphere and the stratosphere. It's characterized by a temperature inversion, meaning the temperature stops decreasing with altitude and starts increasing.

* Cirrus Formation: Cirrus clouds form at very high altitudes, typically around 16,000 to 45,000 feet (5 to 13.5 km), which is within the upper troposphere and often close to the tropopause.

* Ice Crystals: Cirrus clouds are composed of ice crystals, which form due to the extremely low temperatures found at these high altitudes.

Therefore, the presence of cirrus clouds often signifies that you're looking at the top of the troposphere or very close to it.