The O3 sensors (made by Vaisala, Norway) are enclosed in polystyrene boxes. An ozonesonde consists of an O3 sensor connected by an interface unit to a modified radiosonde. Thus humidity, temperature, pressure and geopotential height can be measured simultaneously with O3 sampling. Preparation of the ozonesonde is done in three steps—one week prior to release, the day before release, and within an hour of release. Three instruments are made ready to ensure that a backup is available in the event of a failure.