Upper air technician Dana Zalys readies a weather balloon for a noon release during a month-long intensive program that saw four balloons released by the Alert Weather Station each day. Balloon releases are synchronized around the world, with hundreds of upper air stations releasing them twice a day, every day. These balloons carry compact little packages of sensors, known as sondes, measuring temperature, pressure, GPS location, and sometimes ozone concentration as they rise into the upper atmosphere. The data is transmitted by radio back to the weather station where it was launched, and the results are published and used to generate weather models that are crucial to air travel. Eventually the balloon bursts and the sonde falls back to Earth, sometimes more than a hundred kilometres away from where it began its journey.