pert:
Note this is measuring the temperature inside the ATmega328P, which will be significantly higher than ambient temperature due to the heat produced by the IC.
Unless you sleep to let it cool down before measuring... In fact any sensor needs lot of care to measure the true temperature. Many are self-heating. IIRC DS18B20 increases its temperature by 1 or 2 °C if you measure continuously and constantly poll for a new result. OTOH connecting wires to a thermocouple leads some heat away so it is a bit cooler than the measured something. Measuring any temperature is very interesting topic with a lot of surprises.