Page 183 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 6 atmospheric and Oceanic Circulations 147
Deep low-pressure system
Normal sea-level pressure
1013.2 mb (29.92 in.)
Canadian record low:
940.2 mb (94.02 kPa) Saint Anthony,
Newfoundland (51° N 56° W)
Jan. 1977
Strong high-pressure system
U.S. record low:
882 mb (26.02 in.) Hurricane Wilma (Atlantic/Caribbean) Oct. 2005
123
Earth’s record low:
870 mb (25.69 in.) Typhoon Tip (western Pacific) Oct. 1979
mb 850 860 870 880 890 900 910 920 930 940 950 960 970 980 990 1000 1010 1020 1030 1040 1050 1060 1070 1080
in. 25 25.5 26 26.5 27 27.5 28 28.5 29 29.5 30 30.5 31 31.5 32
Some convenient conversions:
1.0 mb = 0.1 kPa = 0.75 mm (Hg) = 0.0295 in. (Hg) 1.0 in. (Hg) = 25.50 mm (Hg) = 33.87 mb = 3.387 kPa
▲Figure 6.3 Air pressure readings and conversions. Scales express barometric air pressure in millibars and inches of mercury (Hg), with average air pressure values and recorded pressure extremes. note the positions of Hurricanes gilbert, rita, and katrina on the pressure dial (numbers 1–3).
U.S. record high:
1065 mb (31.43 in.) Barrow, AK
(71° N 156° W) Jan. 1970
Canadian record high:
1079.6 mb (107.96 kPa) Dawson, Y.T.
(64° N 139° W)
Feb. 1989
Earth’s record high:
1084 mb (32.01 in.) Agata, Siberia
(67° N 93° E)
Dec. 1968
1. Hurricane Gilbert
888 mb (26.23 in.) Sept. 1988
2. Hurricane Rita
897 mb (26.46 in.) Sept. 2005
3. Hurricane Katrina
902 mb (26.61 in.) Aug. 2005
Earth’s arc in an hour, equivalent to 1.85 km·h−1.) A wind vane determines wind direction; the standard measure­ ment is taken 10 m above the ground to reduce the effects of local topography on wind direction (Figure 6.4).
Winds are named for the direction from which they originate. For example, a wind from the west is a west­ erly wind (it blows eastward); a wind out of the south is a southerly wind (it blows northward). Figure 6.5 illus­ trates a simple wind compass, naming 16 principal wind directions used by meteorologists.
The traditional Beaufort wind scale (named after Ad­ miral Beaufort of the British Navy, who introduced the scale in 1806) is a descriptive scale useful in visually estimating wind speed and is posted on our Mastering- Geography website. Ocean charts still reference the scale (see www.srh.noaa.gov/jetstream/ocean/beaufort_max.htm), en­ abling estimation of wind speed without instruments, even though most ships today use sophisticated equip­ ment to perform such measurements.
▲Figure 6.4 Wind vane and anemometer. instruments used to measure wind direction and speed at a weather station installation. [nOaa Central library Photo Collection.]
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