The Power of Weather Ball is initially 50, however, when used in any Weather such as Sunlight, then it will be Doubled to 100.
However, Weather Ball's Type can also change to Fire, Ice, Rock, or Water, making it possible for an Ice Type Pokemon such as Froslass to receive STAB from Weather Ball, totaling to 150 instead of 100.
Source: This Site
The power and type of Weather Ball changes with the weather. With no weather condition it is Normal type and has a base power of 50. If a weather condition is in effect, its power doubles to 100 and the move becomes a different type: