The pronunciation of "long" in American English is typically [lɒŋ]. This vowel sound is a diphthong, combining the vowel sounds of "o" and "a," and is characterized by a vowel movement from [o] to [ɑ]. The "o" sound is produced with the tongue positioned more forward and the lips rounded, while the "a" sound involves a more open mouth and a less rounded lip position. This diphthong is a common feature in English and is also found in words like "song," "bong," and "pong.

The reason for this specific pronunciation lies in the phonetic evolution of English vowels. Over time, the long "o" sound in Middle English has shifted to the diphthong [ɒː] in Early Modern English, and further to the modern [ɒ] in General American. However, the vowel in "long" has not followed this trajectory but has instead merged with the "a" vowel sound, resulting in the [lɒŋ] pronunciation. This change is attributed to the influence of other English vowels and the natural phonetic development of the language.

Data supporting this pronunciation can be found in phonetic transcriptions and dictionaries. For instance, the American Heritage Dictionary of the English Language (5th ed.) transcribes "long" as [lɒŋ], and the International Phonetic Alphabet (IPA) symbol for this sound is [ɒː]. Additionally, linguistic studies have shown that the diphthong [ɒː] is a common vowel realization in American English, particularly in the midwestern and northeastern regions.

In conclusion, the pronunciation of "long" in American English is [lɒŋ], a diphthong resulting from the phonetic evolution of English vowels. This pronunciation is well-documented in linguistic resources and reflects the natural development of the language.