And that's where a forgotten NSNumberFormatter can show what it's capable of doing.
After you parsed your input string (using regular expression magic or NSString's componentsSeparatedByString:, I'll not be discussing that step) and obtained the spell-out number, you can use NSNumberFormatter's NSNumberFormatterSpellOutStyle to quickly convert your string into a number:
NSString *obtainedDegreesString = @"thirty-two";
NSNumberFormatter *spellOutFormatter = [[NSNumberFormatter alloc] init];
[spellOutFormatter setLocale:[NSLocale currentLocale]]; // or whatever locale you want
[spellOutFormatter setNumberStyle:NSNumberFormatterSpellOutStyle];
NSNumber *degreesNumber = [spellOutFormatter numberFromString:obtainedDegreesString];
NSLog(@"%d", degreesNumber.intValue); // logs 32
But warning - you have to convert strings like "thirty two" to "thirty-two" (the correct English numerals) for your formatter to work - passing "thirty two" results in 3002! Your users probably don't want to burn in 3002 degrees :P You can achieve that using another regular expression, I suppose.