Page 156 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

120 part I The energy–atmosphere System
Kelvin Degrees °F °C 373 210 100
Boiling point of water (at normal air pressure at sea level)
100°C (212°F)
Highest air temperature recorded:
360
90 80
200
190
180
170
160 70
340 150
140 60
130 Death Valley, California, USA 50 (37° N 117° W)
120 57°C (134°F) July 10, 1913 320 110
100 40 90 30
Average normal
Degrees °F °C
5 C°
300
50 10 280 40
260 0 –20 –10
240
220
200 –110 –80 –120
–130 –90
February 7, 1892
World
Vostok, Antarctica (78° S 106° E) –89°C (–129°F) July 21, 1983
Sublimation point of dry ice
(solid carbon dioxide)
body temperature 80 36.8°C (98.2°F)
70 9 F°
20 Normal
60 room temperature
–10 ice
10 0°C (32°F)
–20
–30
–40 –40 –50
–60
–30
20°C (68°F) Melting
(a) (b)
▲Figure 5.4 Instrument shelter. (a) This standard thermistor
shelter is white and louvered, installed above a turf surface. (b) The Stephenson Screen, a louvered wooden box that houses
a minimum and maximum thermometer or a wet bulb thermometer apparatus, provides protection from direct insolation.
[(a) Bobbé Christopherson. (b) Dick Hemingway.]
Measuring Temperature
A familiar instrument for measuring temperature is a thermometer, a sealed glass tube containing a fluid that expands and contracts according to whether heat is added or removed—when the fluid is heated, it expands; upon cooling, it contracts. Both mercury thermometers and alcohol thermometers are used to measure outdoor temperatures—however, mercury thermometers have the limitation that mercury freezes at –39°C. Therefore, in Earth’s colder climates, alcohol thermometers are pre- ferred (alcohol freezes at a much lower temperature of –112°C). The principle of these thermometers is simple: A thermometer stores fluid in a small reservoir at one end and is marked with calibrations to measure the ex- pansion or contraction of the fluid, which reflects the temperature of the thermometer’s environment.
Devices for taking standardized official temperature readings are placed outdoors in small shelters that are white (for high albedo) and louvered (for ventilation) to avoid overheating of the instruments (Figure 5.4). They are placed at least 1.2–1.8 m above the ground surface, usually on turf. Official temperature measurements are made in the shade to prevent the effect of direct insolation. Stan- dard instrument shelters contain a thermistor, which mea- sures temperature by sensing the electrical resistance of a semiconducting material. Since resistance changes at a rate of 4% per C°, the measure can be converted to temper- ature and reported electronically to the weather station. Staffed weather stations are typically equipped with a Stevenson Screen and thermometers from which tempera- tures have to be read and recorded manually. Geosystems in Action, Figure GIA 5, shows some of the hottest tem- peratures ever recorded on Earth.
Note the distinction between °C and C°. Actual temperature measurements give results in degrees Celsius, or °C: "It
is 15 degrees Celsius (°C) outside." But when you are describing a change in temperature, you give the number of degrees of change in Celsius degrees, or C°: "The temperature went up by 3 Celsius degrees (C°)" (also applies to °F and F°).
–70 –60
–80 Northern Hemisphere
–90 Verkhoyansk, Russia –70 (67° N 133° E)
–100 –68°C (–90°F)
30 0
20 point of
Freezing point of mercury –39°C (–38°F)
–50 Lowest air temperatures
recorded:
▲Figure 5.3 Temperature scales. Scales for expressing temperature in kelvin (k) and degrees Celsius (°C) and Fahrenheit (°F), including significant temperatures and temperature records. note the distinc- tion between temperature (indicated by the colour gradation on the scale) and units of temperature expression, and the placement of the degree symbol.
British physicist Lord Kelvin (born William Thomson, 1824–1907) proposed the Kelvin scale in 1848. Science uses this scale because it starts at absolute zero, making temperature readings proportional to the actual kinetic en- ergy in a material. The Kelvin scale’s melting point for ice is 273 K, and its boiling point of water is 373 K, 100 units higher. Therefore, the size of one Kelvin unit is the same size as one Celsius degree.
Most countries use the Celsius scale to express temperature—the United States is an exception. Continu- ing pressure from the international scientific community and other organizations makes adoption of Celsius and SI units inevitable in the United States.
–78.5°C (–109.3°F)