Definition
A weather instrument that continuously records the relative humidity of the atmosphere on a moving paper chart. A moisture-sensitive element (often a bundle of human or animal hair) expands and contracts as humidity changes, and that movement is transferred through a linkage to a pen that traces a line on a rotating drum, producing a time-based record of humidity.
Plain English
A device that automatically draws a line on a chart showing how humid the air has been over a period of time.
Context Anchor
Seen in aviation weather observing and meteorology, especially when discussing recorded changes in humidity at a weather station.
Derivation
From the Greek 'hygros' meaning 'wet' or 'moist,' and 'graph' meaning 'to write.' So literally: a 'moisture writer' — an instrument that writes down the moisture in the air.
Why Pilots Care
Humidity helps explain weather changes that matter to pilots, including fog, low clouds, and reduced visibility. A hygrograph record can show whether the air is becoming more or less moist over time.
Intuition Check
A hygrograph is not just a humidity gauge. A humidity gauge shows a reading; a hygrograph records humidity changes over time.
Example Sentence 1
The hygrograph at the weather station showed humidity climbing steadily through the morning before the fog rolled in.
Example Sentence 2
During preflight weather review, the hygrograph trace showed a steady rise in moisture overnight, prompting the pilot to check for possible carburetor icing conditions.