|mb||in Hg||0.030||in Hg||mb||33.864|
|mb||mm Hg||0.750||mm Hg||mb||1.333|
|in Hg||mm Hg||25.400||mm Hg||in Hg||0.039|
|in Hg||atm||0.033||atm||in Hg||29.921|
|mm Hg||atm||0.001||atm||mm Hg||759.999|
|°C||°F||multiply by 9/5 then add 32|
|°F||°C||subtract 32 then multiply by 5/9|
The conversion between observed pressure and relative pressure at sea level requires us to take into account the quantity of air between sea level and the station. As the "weight" of that air also depends on its temperature, that must also be taken into account. The formula is complex, but this website provides a good calculator and explanation.
In 1805, Sir Francis Beaufort, a Royal Navy officer (he later rose to the rank of Admiral), created a system by which seafarers could estimate the force of the wind based on how much sail could be set. There were 13 classes (from 0 to 12). It was standardised and officially adopted by the Royal Navy in the 1830s, and was also converted to use a different set of criteria when landbased. The seagoing criteria were adapted in 1916 to be an appraisal of the state of the sea (rather than quantity of sail) to accommodate the increased use of steam powered ships. The scale was extended to Force 17 in 1946 to cover special conditions such as cyclones.
|0||Calm||< 1||< 0.3||< 1||< 1|
|12||Hurricane||≥ 64||≥ 32.7||≥ 73||≥ 118|