Here's why:
* Short vowel sounds are typically quick and sharp. Think of the sound of a "a" in "cat" or an "i" in "bit."
* Long vowel sounds are typically drawn out and sound like the name of the letter. Think of the sound of an "a" in "cake" or an "i" in "bike."
The "a" sound in "rat" is short and quick, similar to the "a" sound in "cat."