It seems simple enough to hold a thermometer out in the sun in order to get the temperature “in the sun.” But this idea is all wrong.
Air isn’t heated up very much by sunlight. The rays pass right through. Air warms up in sunlight mainly because the sun’s rays heat the ground and that heat then rises up into the air via conduction and convection. A thermometer bulb in the sun will heat up dramatically past the ambient air temperature because it is a solid object and is warmed directly by the sun’s rays.
You always have to measure air temperature in the shade. If you want to measure the temperature of a soccer field, then you have to stick a thermometer into the ground and out of the sun. A thermometer in the sun is only measuring the temperature of the thermometer bulb and nothing else.