Final answer:
The first sign of an approaching warm front is typically cirrus or cirrostratus clouds, not nimbostratus clouds. Nimbostratus appears when the front is nearby or has just passed, which means the statement is false.
Step-by-step explanation:
The statement that the first sign of an approaching warm front is the appearance of nimbostratus clouds is false. Usually, the initial sign of an approaching warm front is the appearance of high-level clouds, such as cirrus or cirrostratus, which are thin and wispy.
As the warm front gets closer, the cloud types typically change from high to lower levels, and then you might see altostratus and finally nimbostratus clouds, which can bring steady rain or snow depending on the temperature. Nimbostratus clouds generally appear when the warm front is directly overhead or has just passed, rather than being the first sign. Therefore, cirrus or cirrostratus clouds would be the initial indicators of an approaching warm front.