"Cut a slit in a tennis ball and fill it with beans and small rocks." That's Jessica Lundquist's first step to creating a $30 version of a $10,000 scientific weather station. Think of snowmelt as the Brita pitcher for 40% of the world's population, and it's trickling dry in more places given climate change. Sophisticated temperature data collected in rugged terrain are needed to model what will happen to water supplies. Lundquist, 33, who grew up hiking in California's Sierra Nevadas, brings a sense of fun to this serious work. Her hack—sticking a grocery-store temperature sensor into a tennis ball and launching it high into trees with the doohickey that people use to toss balls to golden retrievers—provides valuable data for up to 11 months, saves money, and captures the imagination.