Conversion Factors

Simple conversions

 mbhPa0 hPamb0
 mbin Hg0.030 in Hgmb33.864
 mbmm Hg0.750 mm Hgmb1.333
 mbatm0.001 atmmb1013.250
 in Hgmm Hg25.400 mm Hgin Hg0.039
 in Hgatm0.033 atmin Hg29.921
 mm Hgatm0.001 atmmm Hg759.999
 ktsm/s0.515 m/skts1.943
 ktsmph1.152 mphkts0.868
 ktskph1.853 kphkts0.540
 m/smph2.237 mphm/s0.447
 m/skph3.600 kphm/s0.278
 mphkph1.609 kphmph0.621
 inmm25.400 mmin0.039
 ftm0.305 mft3.281
 °C°Fmultiply by 9/5 then add 32
 °F°Csubtract 32 then multiply by 5/9

Absolute / Relative Pressure

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 site  provides a good calculator and explanation.

Beaufort Scale

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.

0Calm< 1< 0.3< 1< 1
1Lt air1-20.3-1.51-31.1-5.5
2Lt breeze3-61.6-3.44-75.6-11.0
3Gentle breeze7-103.4-5.48-1212-19
4Mod breeze11-155.5-7.913-1720-28
5Fresh breeze16-208.0-10.718-2429-38
6Strong breeze21-2610.8-13.825-3039-49
7Near gale27-3313.9-17.131-3850-61
9Strong gale41-4720.8-24.447-5475-88
11Violent storm56-6328.5-32.664-72103-117
12Hurricane≥ 64≥ 32.7≥ 73≥ 118




Time: 00:00 UTC
Temp: 15.4°C
Hum: 88 %
Wind: 1.7 kts
Dir: SSE 150°
Press: 1001.5 hPa