2 Standard atmospheric pressure and units

The standard atmospheric pressure is the atmospheric pressure at sea level at 0 °C, equal to

1 atm = 1.013 × 10⁵ N/m² (Pa) = 76 cm Hg = 760 mm Hg = 1013 millibar (mbar)

The millibar is used to express atmospheric pressure on weather maps: 1 mbar = 100 N/m² (1 hPa). (So 1.013 × 10⁵ N/m² = 1013 mbar.) 1 mmHg = 133.3 N/m².

Isobars

• are the lines drawn on weather maps to connect places with equal atmospheric pressure.
• Closely spaced isobars mean a steep pressure difference and therefore strong wind; closed isobars with low values mark a depression (storm centre) and high values an anticyclone (fine weather).

Problem. (a) The pressure at the top of a mountain is 750 mm Hg. Express it in N/m² (given 1.013 × 10⁵ N/m² = 76 cm Hg). (b) Express 980 mbar in N/m². (c) Normal blood pressure is 120/80 mm Hg. Express each value in kPa.

Solution. (a) P = (750/760) × 1.013 × 10⁵ = 9.9967 × 10⁴ N/m² (99 967 N/m²). (b) 980 × 100 = 9.8 × 10⁴ N/m². (c) 120 × 133.3 = 15 996 Pa ≈ 16.0 kPa; 80 × 133.3 = 10 664 Pa ≈ 10.7 kPa.