From Glossary of Meteorology
In a radiosonde observation, a level (other than a mandatory level) for which values of pressure, temperature, and humidity are reported because temperature and/or moisture- content data at that level are sufficiently important or unusual to warrant the attention of the forecaster, or they are required for the reasonably accurate reproduction of the radiosonde observation.
There are definite rules governing the selection of significant levels, set forth in the Manual of Radiosonde Observations, WBAN Circular P, 7th ed. rev., 1957.
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