Page 182 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

146 part I The energy–atmosphere System
Suction pump
Vacuum
Air pressure pushes drainage water
up pipe
Water drainage in mine
Air pressure
Glass tube sealed at one end
Normal sea-level pressure
Column of mercury
Mercury in dish
(b) Idealized sketch of a mercury barometer.
◀Figure 6.2 Developing the barometer. Have you used a barometer? if so, what type was it? Did you try to reset it using a local weather information source? [(c) Stuart aylmer/alamy.]
the pressure on the chamber—in both cases causing changes in the chamber that move the needle. An aircraft al­ timeter is a type of aneroid barometer.
Today, atmospheric pressure is measured at weather stations by elec­ tronic sensors that provide continuous measurement over time using mil­ libars (mb, which express force per square metre of a surface area) or hec­ topascals (1 millibar = 1 hectopascal). To compare pressure conditions from one place to another, pressure mea­ surements are adjusted to a standard of normal sea­level pressure, which is 1013.2 mb. In Canada and certain other countries, normal sea­level pressure is expressed as 101.32 kilopascals, or kPa (1 kPa = 10 mb). The adjusted pressure is known as barometric pressure.
Figure 6.3 shows comparative scales in millibars and inches of mer­ cury for air pressure. Note that the normal range of Earth’s atmospheric pressure from strong high pressure to deep low pressure is about 1050 to
Vacuum
760 mm of Hg 1013.2 mb 101.32 kPa
(a) While trying to solve a mine-drainage problem, Torricelli developed the barometer to measure air pressure.
Air pressure
(c) An aneroid barometer.
the tube’s closed end (Figure 6.2b). Torricelli found that the average height of the column of mercury remaining in the tube was 760 mm, depending on the weather. He concluded that the mass of surrounding air was exerting pressure on the mercury in the dish and thus counterbal­ ancing the weight of the column of mercury in the tube.
Any instrument that measures air pressure is a barometer (from the Greek baros, meaning “weight”). Tor­ ricelli developed a mercury barometer. A more compact barometer design, which works without a metre­long tube of mercury, is the aneroid barometer (Figure 6.2c). Aneroid means “using no liquid.” The aneroid barometer principle is simple: Imagine a small chamber, partially emptied of air, which is sealed and connected to a mech­ anism attached to a needle on a dial. As the air pressure outside the chamber increases, it presses inward on the chamber; as the outside air pressure decreases, it relieves
980 mb (31.00 to 29.00 in.). The figure also indicates pres­ sure extremes recorded for Canada, the United States and worldwide.
Wind: Description and Measurement
Simply stated, wind is generally the horizontal motion of air across Earth’s surface. Within the boundary layer at the surface, turbulence adds wind updrafts and down­ drafts and thus a vertical component to this definition. Differences in air pressure between one location and an­ other produce wind.
Wind’s two principal properties are speed and di­ rection, and instruments measure each. An anemometer measures wind speed in kilometres per hour (km·h−1), miles per hour (mph), metres per second (m·s−1), or knots. (A knot is a nautical mile per hour, covering 1 minute of
Georeport 6.2 Pressure Changes in an Airplane Cabin
as an airplane descends to the landing strip and air pressure in the cabin returns to normal, passengers commonly feel a “popping” in their ears. This same effect can happen in an elevator, or even during a drive down a steep mountain road.
The “popping” generally follows a plugged-up sensation produced when a change of air pressure affects the pressure balance of the middle and outer ear, distorting the eardrum and causing sound to be muffled. actions such as yawning, swallowing, or chewing gum that open the eustachian tube connecting the middle ear to the upper throat can equalize the pressure in the middle ear and “pop” the eardrum back to its normal shape. (See www.mayoclinic.com/health/airplane-ear/DS00472 for more.)