You’d think it would be the other way around, wouldn’t you? The top surface steaming away happily, losing all its heat to the air. The bottom half is insulated by your mug, which should mean the last bits are still warm by the time you get to them.
It’s not even the issue that you leave it and come back to find it cold. You can drink the first few gulps at a perfectly nice temperature and, contrary to all the laws of thermodynamics, the second half is magically colder. Joseph Fourier would be turning in his grave… At least he would be, if he was a tea-drinker.
The solution to this one lies in figuring out when the phenomenon happens.
Think carefully – when was the last time you put your cuppa down on a surface that had nothing electronic on it? Not your office desk with telephone, computer and peripherals. Not your coffee table with your mobile phone and teevee remotes. Not your bedside table with your alarm clock and lamp and radio. Because if you did choose a surface not shared with electrical items, you would find your tea a nice, even temperature all the way down and still warm at the bottom.
The reason for this is that tea is in fact a very strong electrical conductor. It encourages the electricity from any powered items near it to travel in order to create a circuit through the tea and back. The electricity’s path will take it the shortest route via the nearest surface which in most cases will be along the table, in at the bottom of the mug and back out again. This process uses energy, and takes some of the heat energy out of the tea as the circuit runs. The heat is sapped away from the bottom up, leaving the top too hot to drink but secretly cooling the underneath layers.
In related news, this wasn’t the question asked but it is this phenomenon that makes everything seem better after a cup of tea.
When you’re stressed, or thinking too much, or just a bit tense, most of these are caused by your body passing too many electrical signals around between your muscles and neurons. Nerves are run by electrical impulses and so is thought. When you drink tea it pulls these electrical signals away from bad tasks like making your muscles tense up or making you overthink something and puts the electrical energy out of harm’s way in your belly. That’s why you feel better.