Here's why:
* The vowel 'u' in "umbrella" has the short 'u' sound, similar to the 'u' in "cup" or "fun".
* Long vowel sounds are typically represented by a vowel followed by a silent 'e' at the end of the word (like 'ate' or 'time') or by a vowel digraph (like 'ee' in 'see' or 'ai' in 'rain').
Since "umbrella" doesn't have either of these patterns, the vowel 'u' has the short sound.