Or at least they did until very recently. There might now be some active-airflow alternatives.
Anyway, if it isn't measured with a (calibrated) standard apparatus, it isn't an official measurement.
[1] https://en.wikipedia.org/wiki/Stevenson_screen
https://www.nist.gov/traceability/supplementary-materials-re...
Or at least they did until very recently. There might now be some active-airflow alternatives.
Anyway, if it isn't measured with a (calibrated) standard apparatus, it isn't an official measurement.
[1] https://en.wikipedia.org/wiki/Stevenson_screen