It is one of the most common questions weather hobbyists ask, usually with a note of frustration: why do two thermometers in the same garden, sometimes only a few metres apart, show different temperatures? The instinct is to assume one of them is broken. Almost always, both are working perfectly, and the disagreement is telling you something real about how local temperature works.
The first and biggest factor is sun versus shade. A thermometer catching even a little direct or reflected sunlight will read substantially higher than one in full shade, because it is absorbing radiant heat rather than measuring the air. Two thermometers a couple of metres apart, one nicked by afternoon sun and one not, can easily differ by five degrees or more. This is exactly why a true air temperature is always a shaded measurement.
The second factor is the surface underneath and nearby. A thermometer above a lawn sits over a cool, moist, living surface that holds little heat. One above paving, gravel, decking or near a wall sits over materials that bake in the day and radiate warmth back out, especially in the late afternoon and after sunset. The ground beneath an instrument quietly shifts its reading.
Height matters too, more than people expect. On a clear, still night, cold air pools near the ground, so a thermometer at ankle height can read several degrees colder than one at head height. This is the same effect that lets frost form on the grass while the air a metre up stays just above freezing, and it is why gardeners and growers care so much about exactly where their sensor sits.
Airflow is the fourth piece. A thermometer in a sheltered, stagnant corner warms up and cools down differently from one in a spot with free air movement. Trapped air around a sensor exaggerates both heating and cooling, while a well-ventilated location tracks the true air temperature far more faithfully.
None of this means measurement is hopeless. It means that to get a reading you can trust and compare day to day, you place the sensor thoughtfully: shaded, well ventilated, over grass rather than paving, at a standard height of about a metre and a quarter, and away from walls and heat sources. Do that and the disagreements largely vanish.
The practical takeaway is reassuring. If your thermometers disagree, you are not looking at broken equipment, you are looking at microclimate, the very real way temperature varies over small distances. Understanding it makes you a sharper weather watcher, and checking your instrument against a live, location-specific reading is the quickest way to know whether a difference is microclimate or a genuine calibration drift.