Outdoor thermometer
An outdoor thermometer is used to measure the air temperature outside a building. However, it is difficult to get reasonably reliable values without a special weather hut - as used by meteorologists . If neglected, measuring errors of 2–4 ° C are by no means unusual.
Most outdoor thermometers are still expansion thermometers filled with alcohol or mercury . Also bimetal thermometers are common, but somewhat sensitive to inaccurate and often for shock. In most cases, the best versions are those as measuring sensors , the sensors of which work predominantly with semiconductor or thermal effects .
Attachment of the thermometer and measurement accuracy
Exact temperature measurements are only possible in moving air, which is why the outdoor thermometer must be installed in such a way that it is not surrounded by stagnant air - i.e. H. at a suitable distance from the building. Avoiding solar radiation is just as important , which is why it should always be installed in the north of the building. Nevertheless, it could also be in the sun for two 2 hours in summer, which almost rises in the northeast between May and August and almost below in the northwest . In general, therefore, the window reveal of a north window, on which the shade of trees falls early in the morning or in the evening, is best.
In addition to the inherent heat of the building (which can help with a distance of a few centimeters from the window glass ), the reflection from a neighboring building can also falsify the measurement by 1–2 ° C. A comparison with a calibrated instrument, e.g. B. a good centrifugal thermometer . The systematic error of standard outdoor thermometers is only below 1 ° C in expensive models and can easily reach 2–3 ° C.
In the reading itself is to make sure horizontally on the thermometer - scale to look. Otherwise a parallax error of 1 ° C and more can result. A reading accuracy of 0.5 ° C is relatively easy to achieve if one imagines the horizontal or recognizes the reflection of one's own head in the window glass.
All in all, it can be stated that if a few rules are observed, the true air temperature can easily be obtained within ± 1 ° C, and at best 0.5 ° C. If the outdoor thermometer is installed in an unfavorable position and without comparison with a calibrated instrument, the values 3–4 ° C can be incorrect, which would include the possible influence of radiation from the sun.
Others
The adjustment of a thermometer to the air temperature takes a certain amount of time, which can take from a few minutes to half an hour (slightly shorter with a sensor). This time is significantly reduced in the wind .
With a room thermometer , you can easily get around by swinging it back and forth with your outstretched arm. After about 20 seconds, the over- or under-temperature has equalized by 50% to the air, after another 20 s to a quarter (a kind of half-life ).
The estimate of the air temperature can be accurate to 1–3 ° C with no wind and appropriate experience. The perceived temperature in the wind is estimated to be considerably colder due to the wind chill .
The maximum temperature of the day in Germany usually occurs between 1 p.m. and 3 p.m. CET , and in summer between 2 p.m. and 4 p.m. The temperature minimum can fluctuate more strongly over time, because it depends not only on the season but also on the night-time cooling and the cloudiness . On average, it occurs a little before sunrise .
Today, outdoor thermometers are already built into many cars . The value displayed at the start is too high in summer when the sun is shining, but it can also be too low after cold nights due to the radiation. Every larger aircraft is also equipped with one or two external thermometers. Occasionally one can also operate a kind of meteorological navigation by observing the temperature change , for example when locating the jet stream .