The term ‘hoar frost’ comes from the Old English term ‘hoar’ which can mean old, aged, venerable or grey – although it’s most commonly used to mean ‘grey-haired with age’. As a weather description, it refers to a type of frost that resembles an old man’s grey, feathery beard.
One of the volunteers on our current Zooniverse project, “BeardyBrian”, found reference to ‘Hoar frost’ in our records on July 7, 1899. By checking the original Perth Gardens weather records closely, we also found two more consecutive days of ‘hoar frost’ on August 3 and 4, 1899.
Hoar frost is formed by water vapour that skips the liquid stage and forms an intricate crystalline structure on surfaces like trees, grasses, poles and wires. It’s essentially a type of light, fluffy ice that forms directly from the atmosphere. It can blanket surfaces in a range of crystal formations, and it’s transparent or sometimes even grey in colour.
The kinds of weather conditions needed to create hoar frost are cold, calm nights, with clear skies. The night also needs to be fairly humid; air that might become dew if it were a little warmer, instead becomes a delicate hoar frost on surfaces that are colder than the air.
Although it doesn’t have the most beautiful name, hoar frost itself can be quite beautiful!
Cross-checking our record with documentary sources
To cross-check the observation remark of a hoar frost in the Perth Gardens record, we checked the newspapers at the time on the National Library of Australia’s Trove historical database. There were plenty of references to hoar frost in South Australia and even Queensland in 1899. However, we couldn’t find any references to a hoar frost in Perth in the newspapers at the time.
Our research team recently acquired a great resource, the ‘Results of Rainfall Observations made in Western Australia (WA)’, published in 1929. The document, written under the direction of Commonwealth Meteorologist H. A. Hunt, isn’t just about rainfall, but in fact collates huge amounts of weather material up to 1929, including a range of extreme weather events like heatwaves, floods, snow, cyclones, and frosts.
Hunt’s document summarises nine examples of ‘severe hoar frost’ across WA between 1830 and 1923. Overall, there were 28 severe frosts, fogs and exceptionally cold weather events documented in WA prior to 1900. There is also a summary of the associated societal impacts of severe frosts, such as ruining crops, seedings and stock feed. For example, in a particularly severe frost in Perth in 1914, “even sturdy acacias and eucalyptus were affected”. However Hunt’s document doesn’t mention any notable severe frosts in 1899, which we found in the Perth Gardens record.
However the nearby Perth Observatory does note that there were frosts in Perth in 1899, with the following observations:
- June 1899: “frosts were occasionally experienced at night” with the lowest ground surface temperature recorded of 1.4 °C.
- July 1899: “frosts were frequent at night time” with the lowest minimum temperature on the surface of the ground of -0.7 °C.
Given the minimal data available to cross-check the 1899 ‘hoar’ frost events, it might suggest that our volunteer has uncovered a previously unknown severe weather event. However there are a range of factors at play, which further justifies the need to collect more historical weather data for the Perth region. By gathering, collating and analysing continuous, instrumental, daily weather observations, we can have more confidence in historical extreme weather events that are uncovered.
Help us to better understand the past climate of Perth
Right now, you can become a citizen scientist to help us fill a gap in Perth’s historical climate records. By recovering handwritten weather observations for the period 1880 to 1900, you’ll help connect historical data beginning in the 1830s with modern observations kept by the Australian Bureau of Meteorology. With your help, we can create one of the longest continuous weather records in the Southern Hemisphere.
Want to help uncover more of Australia’s climate history? Join our project, here: https://www.zooniverse.org/projects/caitlinhowlett/climate-history-australia.