Glossary of terms used in the Pilot Balloon Theodolite Web Site
Ascent Rate. The rate at which a balloon will climb (gain altitude) through air free of up-drafts or down-drafts usually expressed in meters per minute.
Bent Axis Telescope. A telescope sighting device where the optical path has a 90 degree bend in it so the operator's eye position does not change when the elevation is changed through a complete 180 degrees. This is usually accomplished with the use of a three or five sided prism.
Free Lift. The total lift of the balloon minus the weight of the balloon. It does not include any attachments such as a lighting unit. The lift is caused by dense air in the altitude floating the less dense - lighter gas (helium or hydrogen) inside the balloon.
Pibal. Meteorological jargon for a Pilot-Balloon itself or the determination of upper winds by releasing and tracking a freely ascending balloon with a theodolite. (In some low altitude-short duration applications a compass and clinometer are used in place of the theodolite.)
Radiosonde. A electronics package attached to a balloon and carried aloft. It contains set of instruments that measure pressure temperature, and humidity and a radio transmitter that transmits a data stream containing the information from the onboard sensors. See also NOAA's Web site for more information.
Rawinsonde. A type of upper-air observation that determines the wind speed, direction, pressure, temperature, and relative humidity by utilizing a radiosonde tracked by a radio direction-finding device such as a radio theodolite. For more information on Rawinsondes follow this link to http://www.atirus.com/wwa_text3.html, also the NOAA's Site.
Tail Method. In a single theodolite observation the height of the balloon is approximated by multiplying a assumed rage of ascent by the duration of the ascent. This method assumes an accurate ascent rate assumption as well as a lack of up or down-drafts. A tail of a known length can be attached to the balloon, the apparent length of the tail as viewed through a divided gradicule on a theodolite can be determined. The measurement of the angle subtended by the length of tail enables the distance to the balloon and hence the height of the balloon the be calculated without making any assumptions about the rate of ascent of the balloon. (This method is useful in conditions where the wind speed is great enough such that the observed elevation angle of the balloon is 40 degrees of less).
True North. The direction heading towards the geographical north pole. This is as apposed to Magnetic North, the direction that a compass indicates as north - the magnetic north pole (some place in Northern Canada). The difference between Magnetic North and True North is called magnetic declination.