Final answer:
The weather balloon rose a total distance of 37.5 feet, calculated by adding the absolute values of its starting and ending elevations.
Step-by-step explanation:
To calculate the distance the weather balloon rose from an elevation of 18 feet below sea level to an elevation of 19 1/2 feet above sea level, we add the absolute values of these two elevations. Since the balloon was under sea level, we consider its elevation as a negative value when calculating the rise. Therefore, the distance risen is |-18 feet| + |19 1/2 feet| which equals 18 feet + 19.5 feet, giving us a total rise of 37.5 feet.