The “inch of mercury” (inHg) is a unit of pressure commonly used in aviation, meteorology, and refrigeration. It originates from the barometric pressure measurement practice using a column of mercury. This unit became a staple due to its early adoption in weather forecasting and its accurate and consistent measurements. An interesting fact is that an inch of mercury is approximately equal to 33.86 millibars or 3386 Pascals, but what truly distinguishes it is its historical significance in the development of modern atmospheric science.
In the 17th century, Evangelista Torricelli, a student of Galileo, first discovered that atmospheric pressure could sustain a column of mercury, laying the groundwork for this unit. His invention, the barometer, revolutionized how we understand and predict weather, marking the inch of mercury as more than just a unit—it’s a nod to the evolution of scientific inquiry and practice.
The hectopascal (hPa) is a commonly used unit for measuring atmospheric pressure, especially in meteorology. One hPa is equivalent to 100 pascals, making it a preferred measurement over pascals due to the convenient scale. Interestingly, 1 hPa is also equivalent to 1 millibar, a unit that was widely used before the adoption of the hectopascal. This equivalence makes it easy for transitioning instruments and readings in weather maps and reports.
Hectopascals play a crucial role in understanding atmospheric conditions, such as weather forecasting and studying phenomena like hurricanes or cyclones. Typically, sea-level pressure is around 1013 hPa, and deviations from this can indicate changing weather patterns, with lower pressures often associated with storm systems. This connection makes the hPa unit vital for predicting and understanding weather events globally.
Inches of mercury | Hectopascals |
---|---|
0
inHg
|
0
hPa
|
1
inHg
|
33.86389
hPa
|
2
inHg
|
67.72778
hPa
|
3
inHg
|
101.59167
hPa
|
4
inHg
|
135.45556
hPa
|
5
inHg
|
169.31945
hPa
|
6
inHg
|
203.18334
hPa
|
7
inHg
|
237.04723
hPa
|
8
inHg
|
270.91112
hPa
|
9
inHg
|
304.77501
hPa
|
10
inHg
|
338.6389
hPa
|
11
inHg
|
372.50278
hPa
|
12
inHg
|
406.36667
hPa
|
13
inHg
|
440.23056
hPa
|
14
inHg
|
474.09445
hPa
|
15
inHg
|
507.95834
hPa
|
16
inHg
|
541.82223
hPa
|
17
inHg
|
575.68612
hPa
|
18
inHg
|
609.55001
hPa
|
19
inHg
|
643.4139
hPa
|
20
inHg
|
677.27779
hPa
|
21
inHg
|
711.14168
hPa
|
22
inHg
|
745.00557
hPa
|
23
inHg
|
778.86946
hPa
|
24
inHg
|
812.73335
hPa
|
25
inHg
|
846.59724
hPa
|
26
inHg
|
880.46113
hPa
|
27
inHg
|
914.32502
hPa
|
28
inHg
|
948.18891
hPa
|
29
inHg
|
982.0528
hPa
|
30
inHg
|
1015.91669
hPa
|
31
inHg
|
1049.78058
hPa
|
32
inHg
|
1083.64447
hPa
|
33
inHg
|
1117.50835
hPa
|
34
inHg
|
1151.37224
hPa
|
35
inHg
|
1185.23613
hPa
|
36
inHg
|
1219.10002
hPa
|
37
inHg
|
1252.96391
hPa
|
38
inHg
|
1286.8278
hPa
|
39
inHg
|
1320.69169
hPa
|
40
inHg
|
1354.55558
hPa
|
41
inHg
|
1388.41947
hPa
|
42
inHg
|
1422.28336
hPa
|
43
inHg
|
1456.14725
hPa
|
44
inHg
|
1490.01114
hPa
|
45
inHg
|
1523.87503
hPa
|
46
inHg
|
1557.73892
hPa
|
47
inHg
|
1591.60281
hPa
|
48
inHg
|
1625.4667
hPa
|
49
inHg
|
1659.33059
hPa
|
50
inHg
|
1693.19448
hPa
|
51
inHg
|
1727.05837
hPa
|
52
inHg
|
1760.92226
hPa
|
53
inHg
|
1794.78615
hPa
|
54
inHg
|
1828.65003
hPa
|
55
inHg
|
1862.51392
hPa
|
56
inHg
|
1896.37781
hPa
|
57
inHg
|
1930.2417
hPa
|
58
inHg
|
1964.10559
hPa
|
59
inHg
|
1997.96948
hPa
|
60
inHg
|
2031.83337
hPa
|
61
inHg
|
2065.69726
hPa
|
62
inHg
|
2099.56115
hPa
|
63
inHg
|
2133.42504
hPa
|
64
inHg
|
2167.28893
hPa
|
65
inHg
|
2201.15282
hPa
|
66
inHg
|
2235.01671
hPa
|
67
inHg
|
2268.8806
hPa
|
68
inHg
|
2302.74449
hPa
|
69
inHg
|
2336.60838
hPa
|
70
inHg
|
2370.47227
hPa
|
71
inHg
|
2404.33616
hPa
|
72
inHg
|
2438.20005
hPa
|
73
inHg
|
2472.06394
hPa
|
74
inHg
|
2505.92783
hPa
|
75
inHg
|
2539.79171
hPa
|
76
inHg
|
2573.6556
hPa
|
77
inHg
|
2607.51949
hPa
|
78
inHg
|
2641.38338
hPa
|
79
inHg
|
2675.24727
hPa
|
80
inHg
|
2709.11116
hPa
|
81
inHg
|
2742.97505
hPa
|
82
inHg
|
2776.83894
hPa
|
83
inHg
|
2810.70283
hPa
|
84
inHg
|
2844.56672
hPa
|
85
inHg
|
2878.43061
hPa
|
86
inHg
|
2912.2945
hPa
|
87
inHg
|
2946.15839
hPa
|
88
inHg
|
2980.02228
hPa
|
89
inHg
|
3013.88617
hPa
|
90
inHg
|
3047.75006
hPa
|
91
inHg
|
3081.61395
hPa
|
92
inHg
|
3115.47784
hPa
|
93
inHg
|
3149.34173
hPa
|
94
inHg
|
3183.20562
hPa
|
95
inHg
|
3217.06951
hPa
|
96
inHg
|
3250.9334
hPa
|
97
inHg
|
3284.79728
hPa
|
98
inHg
|
3318.66117
hPa
|
99
inHg
|
3352.52506
hPa
|
100
inHg
|
3386.38895
hPa
|
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