Yes, apologies to grobhead, I did not emphasise one important part of my last post. Buys Ballot's law is generally applied at the surface, and is then related to the pressure system - i.e. with the wind at your back the depression/trough is on the left, the anti-cyclone/ridge is on the right. These are of course determined by QFF - the pressure corrected to sea level.
You are absolutely right that the aircraft is descending, but it is keeping a constant pressure altitude. You are also right that, in general, while descending the temperature increases. However this is related to the increase of pressure - but the pressure is not increasing because the aircraft remains at FL180, a pressure altitude!
So we then have to think, if the pressure is not changing, why are we descending? Either the surface pressure is reducing (which will not consistently affect the temperature, so won't answer the question) or the column of air below is becoming denser, so increasing the rate of pressure reduction with altitude, so the pressure for FL180 is reached at a lower altitude. This denser air must be colder. As already stated the temperature effect is likely to be dominant by FL180, have more effect than any likely surface-pressure variation.
This site is great - 2 well-respected Met instructors plus another ATPL instructor helping out. Keeps my mind working too, on the obscure questions outside my subjects, ready for any interviews!