A School Weather Station Reads Hot
A middle school weather station reports a temperature of 41 degrees C at 2:00 p.m. on a clear, calm day. The official airport station 6 km away reports 33 degrees C. The school sensor is mounted 1.5 m above a black asphalt parking lot, 3 m from a brick wall, and inside a passive shield that is cracked and missing two louver plates. A second sensor on the grassy field reads 34 degrees C. The relative humidity at the parking-lot site is 24%, while the field site reads 51%. Students want to announce a record heat wave on the school website. The teacher has one week of data: parking-lot sensor daily maxima are 6 to 9 degrees C higher than the field sensor, and the largest differences occur on calm, sunny afternoons. The class has a calibration thermometer accurate to plus or minus 0.2 degrees C.
1. Evaluate whether the 41 degrees C reading is a valid air temperature for the school area. Identify the main siting and instrument errors and explain the physics.
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The 41 degrees C reading is not a valid representative air temperature. The sensor sits over asphalt, which absorbs solar radiation and heats the air immediately above it; a brick wall adds longwave radiation; the cracked shield admits direct sunlight and reduces ventilation. On calm, sunny afternoons, poor mixing lets a shallow hot layer build up, producing the 6 to 9 degrees C excess over the field sensor. The lower relative humidity at the parking lot is consistent with warmer air, not with a different air mass, because relative humidity falls as temperature rises if moisture is unchanged. The field sensor's 34 degrees C is close to the airport's 33 degrees C, which supports the field site as more representative. The 41 degrees C should be labeled a microclimate or exposure artifact, not a school record.
2. Design a corrected comparison using the available sensors and calibration thermometer. Include placement, timing, replicates, and what data to report.
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Move a calibrated sensor to the grassy field, 1.5 m above ground, at least 10 m from pavement and buildings, in an intact radiation shield. Keep the parking-lot sensor in place only as a comparison, after repairing its shield. Calibrate both against the plus or minus 0.2 degrees C thermometer in an ice-water bath and at room temperature, then operate them at the same logging interval for seven days. Report paired 10-minute differences, daily maxima and minima, and the calibration correction. Report the field site as the school's official temperature and the parking-lot site as an urban-surface comparison. If the field and airport differ by more than 1 degree C consistently, state that local siting affects the comparison.
3. A student argues that because the parking-lot sensor is precise to 0.1 degrees C, its 41 degrees C should be trusted. How should the teacher respond?
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Precision is repeatability; accuracy is closeness to the true value. A sensor that reads 41.0 degrees C every time is precise, but if it is heated by asphalt and sunlight, it is consistently wrong. Calibration checks can detect and correct a scale offset, but they cannot correct a bad exposure. The teacher should ask the student to compare the parking-lot readings with the field and airport readings and to identify the systematic difference on calm, sunny days. The correct response is to fix the siting and shield, then recalibrate, not to trust the number because it has one decimal place.