A few practical things to consider. Fundamentally temperature is a measure of the average(random) atomic kinetic energy of a substance. There is a sensor involved in this measurement- usually a Pt 100 RTD. It produces a predictable change in resistance with a change in temperature. Errors associated with this measurement include self heating(electrical current flowing through the resistance of the sensor = heat). Minor effect. Absorbing solar energy in direct sunlight has a similar effect- but it produces a more significant error(due to sensor heating) while measurement of actual air temperature is the objective. That can't be accomplished in direct sunlight. The 2 meter/in shade 'requirement' allows consistency in the measurement regardless of location. That became the standard. It could have easily been at some other level, but 2 meters off the ground is where we live. One of the topics I teach is temperature measurement for industrial processes- so I am a bit geeky about the topic lol.