This thread has a post suggesting that the majority of car a/c’s cycle on and off (or more or less on and off) according to the pressure of the refrigerant…
So, my question is, if I idle the engine and turn on the a/c on a hot summer day (say, 95 degrees), does that impose more drag on the engine than idling with the a/c on on a winter day (say, 35 degrees)? Or does the system just cycle off longer when the temp is lower?
The answer is it depends.
(Great Rick, what other words of wisdom do you have today)
No really it does.
First off there are two major types of A/C systems in cars. The first is called a CCOT (Cycling Clutch Orfice Tube) Some car makers use a different term, but the same idea. The other major type is an expansion valve system.
CCOT systems cycle the electric clutch on the compressor on and off based on the pressure / temp. of the evaporator. (the part of the system inside the car that gets cold) The evaporator is in front of the heater core, so any mixing of hot and cold air occurs after the evap. Turning the temp dial will not effect the temp of the air crossing the evap, therefore will not make the sytem cycle any more often or any less often then full cold will.
Expansion valve system have a valve that opens and closes based on temp / pressure of the evaporator. Typically the compressor does not cycle on this type of system. (There may be a safety shutdown to prevent the evap from freezing) Again the evap is beofre the heater core, so any heat introduced into the airstream has no effect on the temp of the evap.
Now a small exception would be if you were running the in car thermostat at say 90 degrees, and the outside temp is 75, and you have the system in recirculation, then you would be supplying warmer air from inside the car to the A/C system. This would cause the system to work somewhat harder than it would otherwise. I cannot understand why anyone would set their A/C to 90, & recirculation, but I did want to hit all the bases. 
Well yes, but not for the reason you think. If you go out on a 35F day and start your car cold, and turn on the A/C nothing will happen. Yeah I know that the little blue light comes on and the fan blows, but the compressor will NOT be running. Why not? Simple it is too cold. As the temp drops, the pressure in the A/C system drops. At about 45 degrees (about 45PSI) there is not enough pressure in the system to close the low pressure switch and allow the comprssor to start. As the engine warms up, heat will be transferred into the A/C system from the engine (the compressor is bolted to a very warm engine) and when the pressure in the system gets above about 45 PSI the system will start to cycle. The on time of the compressor will be very short, and the off time long as the evap will be very cold from the very cold outside air blowing across it. Now if you would like to change your temps to 70 and 100, the answer is of course, there is no such thing as a free lunch. You are removing more heat from the 100 degree air, more energy will be required to do this.