There are two reasons. First the engine is running faster. The AC works by compressing the coolant gas (which heats it up), letting it cool outside the car, then letting it expand (which makes it cool) and letting it absorb heat from inside the car. The compression of the gas takes a lot of force, and that force is coming from your engine itself. Running the engine at a higher RPM gives more available energy to push that "heat cycle" along. The second reason is this: The coolent, once it is compressed and hot, must give up it's heat to the outside air. (There is nowhere else to give it!) The coolent goes through coils to help this, but it's just like any other hot object--it will cool faster if there is air blowing over it. The motion of the car helps provide this aitflow. This is why you AC works best when the car is running. It will work even better when the air is below zero--although you probably won't need it then. :)
Do you have the recycle air button turned off? If you do you would be sucking in hot air when you are moving but when you stop the outside air stops flowing into the air conditioner and it gets a chance to get cooler air from inside the cab and cool it more. Make sure your recycle button is lit on the dash. This closes off the outside air and just recycles the cooler air from inside the cab.
An "air cooler" or "evaporative cooler" cools air by evaporating water. The evaporation process is endothermic and therefore the temperature of the air drops. However, the relative humidity level of the air goes up. This is a common method of cooling air in very dry climates. An air conditioner cools air through a heat exchanger and does not increase the relative humidity of the air.
In the humid warm climate an air conditioner is the solution. In dry hot climate and air cooler (aka evaporative cooler) works best since it cools by evaporation of water and introduces a lot of humidity in the space. An air cooler is not usually recommended (even in dry climate) for rooms where electrical/electronic equipments (such as TV, PC etc) are placed since too much humidity mixed with already deposited dust makes a mess and results in failure. Also air cooler is cheaper in cost as well as operation.
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