Abbreviation for Instrument Flight Rules, but commonly used to refer to the weather and/or flight conditions to which these rules apply, i.e. low visibility.

A type of cistern barometer in which the level of mercury in the cistern is adjusted to the zero point of the scale before each reading.

The difference between the air temperature and the dew-point. Also called dew-point deficit, dew-point depression.

Apparatus from which the nature and time of precipitation may be determined.

Same as contact anemometer.

Same as an upper air observation, but commonly used to refer to a single complete radiosonde observation.