Here's why:
* Short vowel sounds are typically quick and sharp. They are often found in words with one syllable and are followed by a consonant.
* Long vowel sounds are typically longer and drawn out. They are often found in words with two syllables or when the vowel is followed by a silent "e".
Since "bat" is a one-syllable word and the vowel sound is short and sharp, it is a short vowel sound.