"The dog is weird."
But it is even not spoken. As far as I learned is "آ" the same like " أَ + ا " and is pronounced like "Aa" (long, like in "Ana aakulu"). But I never heared the article "Al" pronounced in this way. In contrary, the syllable is even lost, when "Al" follows on a vocal ("...wa-lkalb ghariib"). I learned that the syllable of an "آ" never gets lost. Is there a grammatical explanation, why there should be this symbol at article in that case?
Well, it's fatHa actually. Kasrah goes below the letter.
I thought though it was not fatHa. I was thinking that it was صـ sign. Sometimes this character is placed on Alif to mark it as Hamzat-Wassl (as you explained already, an Alif that get consumed in the vowel before it). Anyway, even this fatHa above Alif here is not necessary. It's known that it is the definite article (AL).