Well, that’s disappointing. There goes my idea of plasma that can freeze water .

It’s not a stupidly easy question. It is extremely hard. Calculating the average temperature of all molecules on Earth is extremely hard due to the vast range of temperatures across different environments, from the Earth’s core to the atmosphere. Calculation isn’t the problem but collecting data is. You need to collect all the data that distributed widely on earth. Factors like altitude, latitude, and specific conditions in different regions making collecting an accurate data even harder.