system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How on earth is the "Chance of Rain" calculated?
gpt
Meteorologists collect lots of information about weather. They measure wind speed, cloud clover, how much it rains, the barometric pressure, etc. With all this information, they notice patterns and can use those patterns to predict the weather to some extent.  

For example, if in the past, it rained 40% of the time when the clouds formed like they are today, and other weather factors were also similar, they would say there is a 40% chance of rain.