This data is logged using a cheap tipping bucket rain gauge connected to a programmable Spark Core WiFi module. According to the eBay listing it takes 5mL of rain to cause the bucket to tip. However, I've measured it a bunch of times and on average its about 2mL. Sometimes some water sticks to a tipped which sets the reading off a bit. I may looking into a more accurate way of measuring rainfall. More details in the blog post.