3.1: Air Temperature Measurements
- Page ID
- 46735
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Thermometers
Temperature has been historically measured by a thermometer like the one shown in Figure \(\PageIndex{1}\). A bulb of expandable fluid grows or shrinks depending on the temperature, indicating the temperature by its top. Thermometers of this type are robust and cheap, but not easily automated.
Another type of early thermometer records the minimum and maximum with mercury as shown in Figure \(\PageIndex{2}\). A marker is pushed upward (for the maximum) or downward (for the minimum).
Today, most thermometers are electronic. They use temperature-sensitive components, such as a temperature-sensitive resistor or capacitor, whose electrical current changes with temperature. Based on the instrument's electrical reading, temperature can be automatically recorded. An automated weather station using electronic thermometers is shown in Figure \(\PageIndex{3}\).
Figure \(\PageIndex{4}\) shows a thermistor, which is commonly used in radiosondes. Radiosondes are attached to weather balloons for measuring temperature throughout the entire atmospheric column. We can observe in situ up to 50 km, halfway through the stratosphere, using a radiosonde balloon.
The World Meteorological Organization (WMO) has guidelines for the accurate placement of thermometers for recordkeeping purposes. The primary guides are that a thermometer must be placed:
- 1.5-2 meters above the ground
- in the shade
- in a white (or high albedo) ventilated shelter.
Such a setup is shown in Figure 3.1.5:
Air Temperature Records
With over a century of standardized weather data collected across the world, we have been able to identify temperature extremes from across the world (along with other weather conditions).
The world's warmest recorded temperature was 134°F (56.7°C), recorded on July 10, 1913, at Furnace Creek, in Death Valley, California, USA.
The world's coldest recorded temperature was -128.6°F (-89.2°C), recorded on July 21, 1983, at the Vostok research base in Antarctica.
Air Temperature and Human Comfort
A temperature reading gives the air temperature, but wind and humidity can affect how the atmosphere feels to the human body. Figure \(\PageIndex{6}\) shows a wind chill graph where the wind speed and the temperature give an additional measure of temperature called the wind chill. When the air temperature is low and the wind is high, the temperature feels even colder and can cause frostbite within minutes (purple area).
The chart includes a grid where temperatures decrease across the top, and wind speeds increase down the side. Each cell shows the resulting wind chill value, illustrating how cold it feels under different combinations. Sections are shaded to represent frostbite risk times for 30, 10, and 5 minutes. A formula for calculating wind chill is shown in Figure 3.1.6, showing how air temperature and wind speed determine perceived temperature.
The same table as in Figure 3.1.6 is shown in Table 3.1.1
|
Wind speed (mph)↓ |
Temperature (°F) | |||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Calm | 40 | 35 | 30 | 25 | 20 | 15 | 10 | 5 | 0 | -5 | -10 | -15 | -20 | -25 | -30 | -35 | -40 | -45 |
| 5 | 36 | 31 | 25 | 19 | 13 | 7 | 1 | -5 | -11 | -16 | -22 | -28 | -34 | -40 | -46 | -52 | -57 | -63 |
| 10 | 34 | 27 | 21 | 15 | 9 | 3 | -4 | -10 | -15 | -22 | -28 | -35 | -41 | -47 | -53 | -59 | -66 | -72 |
| 15 | 32 | 26 | 19 | 13 | 6 | 0 | -7 | -13 | -19 | -26 | -32 | -39 | -45 | -51 | -58 | -64 | -71 | -77 |
| 20 | 30 | 24 | 17 | 11 | 4 | -2 | -9 | -15 | -22 | -29 | -35 | -42 | -48 | -55 | -61 | -68 | -74 | -81 |
| 25 | 29 | 23 | 16 | 9 | 3 | -4 | -11 | -17 | -24 | -31 | -37 | -44 | -51 | -58 | -64 | -71 | -78 | -84 |
| 30 | 28 | 22 | 15 | 8 | 1 | -5 | -12 | -19 | -26 | -33 | -39 | -46 | -53 | -60 | -67 | -73 | -80 | -87 |
| 35 | 28 | 21 | 14 | 7 | 0 | -7 | -14 | -21 | -27 | -34 | -41 | -48 | -55 | -62 | -69 | -76 | -82 | -89 |
| 40 | 27 | 20 | 13 | 6 | -1 | -8 | -15 | -22 | -28 | -36 | -43 | -50 | -57 | -64 | -71 | -78 | -84 | -91 |
| 45 | 26 | 19 | 12 | 5 | -2 | -9 | -16 | -23 | -29 | -37 | -44 | -51 | -58 | -65 | -72 | -79 | -86 | -93 |
| 50 | 26 | 19 | 12 | 4 | -3 | -10 | -17 | -24 | -30 | -38 | -45 | -52 | -60 | -67 | -74 | -81 | -88 | -95 |
| 55 | 25 | 18 | 11 | 4 | -3 | -11 | -18 | -25 | -31 | -39 | -46 | -54 | -61 | -68 | -75 | -82 | -89 | -97 |
| 60 | 25 | 17 | 10 | 3 | -4 | -11 | -19 | -26 | -32 | -40 | -48 | -55 | -62 | -69 | -76 | -84 | -91 | -98 |
On the other hand, when the air is humid, it feels hotter. This is called the heat index, and we'll examine it further in the next chapter.