Two Thermometers, One Town
A first-year class has been recording maximum and minimum temperature for 30 days with a maximum–minimum thermometer fixed 1.5 m above a tarred playground, on a bracket attached to a south-facing concrete wall. For comparison, they use the daily values published by a district observatory 2.2 km away, where the thermometer sits in a louvred screen over open grass. The class average is 2.4°C higher for maximum temperature and 1.1°C lower for minimum temperature than the observatory, so the daily range is much larger. On two thunderstorm afternoons the two sets of readings almost agree. A student concludes that the town has a strong urban heat island. The teacher asks the class to separate instrument artefact from real microclimate before anyone writes that conclusion in the project report.
1. Does the 2.4°C difference prove an urban heat island? Explain what the siting alone can produce.
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It does not prove it. An unscreened bulb on a sunlit concrete wall receives direct solar radiation and radiation reflected from the wall and tar, so the daytime maximum is inflated. At night the same exposed bulb radiates freely to the cold sky, so the minimum is depressed. That combination - higher maximum and lower minimum, a wider range - is the classic signature of bad exposure rather than of an urban climate. A genuine urban heat island usually raises the night minimum most, because buildings and roads release stored heat and reduce radiative loss; here the minimum is lower. The near-agreement on thunderstorm afternoons is also telling, since cloud and rain cut both solar input and night-time radiative loss, partly cancelling the exposure error. So the correct judgement is that the class has measured its own instrument shelter, not the town's climate.
2. Design the minimum changes that would make the class comparison fair, and state what each change controls.
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Three changes matter most. Put the thermometer in a properly louvred screen that is ventilated but shaded from direct sun and from radiation off walls and ground; this removes the largest single error. Move the station to open grass, at least several metres from tar, concrete and buildings, and at the standard height above ground, so that the surface energy balance matches the reference site. Record at fixed hours and keep a calibration check against a known thermometer, so that instrument bias is separated from exposure bias. Beyond the school site, if the aim is a real urban heat island study, run several stations along a transect from the built-up core to open country, all with similar screens, and analyse calm, clear nights separately, because that is when the effect is strongest. Each change targets a specific error rather than adding general caution.
3. Which statements can the class safely make in their report, and which must be avoided?
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Safe statements are narrow and dated: during the 30-day study, the playground station recorded higher daily maxima and lower daily minima than the district observatory, and the difference nearly vanished on cloudy, rainy days. These are descriptions of two particular sites in one period. Unsafe statements include 'the town is 2.4°C hotter than its surroundings', 'the urban heat island is 2.4°C', and any comparison with long-term climate, because one month, one site and one instrument cannot carry that weight. The report should also admit that a single month has no climatological normal and that the observatory itself may not represent open country. The strongest educational conclusion is methodological: exposure and screening can produce a difference larger than many real microclimatic effects, so any temperature claim must begin with how and where the reading was taken.