Nimbus clouds don't form at a specific altitude. Instead, they are classified by their type of precipitation.

Here's why:

* Nimbus means "rain cloud" in Latin.

* Cumulonimbus clouds are the most common type of nimbus cloud, and they can form from very low altitudes to over 10 km (6.2 miles).

* Stratonimbus clouds are a more horizontally spread out type of nimbus cloud, usually forming at lower altitudes.

So, to answer your question directly:

* Cumulonimbus clouds can form anywhere from low altitudes (like 1 km / 0.62 miles) to very high altitudes (over 10 km / 6.2 miles).

* Stratonimbus clouds generally form at lower altitudes, typically below 2 km (1.2 miles).

It's important to note that these are just general guidelines, and the altitude at which a nimbus cloud forms can vary depending on factors like location, temperature, and atmospheric conditions.