A comfortable driver is a safe and efficient driver. Having learned to drive before air conditioning was standard, I still have vivid memories of driving in hot, steamy climates. It was not fun.
By the 1970s, when most cars could be ordered with air conditioning, some truck drivers refused to use it. One told me that it was for “wimpy four-wheelers,” though I think his decision not to get it had more to do with cost and reliability than his manhood.
Wimpy or not, air conditioning is a major contributor to safety — even in winter. It comes on with defrosters to remove moisture that would otherwise condense on cold windshields. Thus, a properly maintained system is as necessary for good vision in bad weather as it is for comfort.
During rain or snow storms, relative humidity is often close to 100%. By cooling the air, the absolute amount of moisture is reduced; colder air cannot hold as much for a given volume. Absolute moisture varies with temperature, which is why we refer to relative humidity.
To a scientist, cool or cold merely define lack of heat. Cab air conditioning removes heat and, as a result, the air cannot hold as much water. It drains away, which is why we see clear water on the ground when air conditioners operate.
FROM THE INSIDE OUT
Truck air conditioners are both rugged and delicate. They tolerate a great deal of vibration and they require minimal maintenance.
But they're made of soft materials that, if abused, create problems that stop a system cold (pardon the pun). The major components are the compressor, the condenser, the receiver-dryer, the expansion valve, and the evaporator. They are connected primarily by soft metal tubing, usually aluminum and copper.
The compressor is driven by the engine through a drive belt. It draws a refrigerant fluid from the evaporator as a gas, and forces it under high pressure to a condenser where the gas liquefies.
The liquid refrigerant flows to a receiver-drier where condensed moisture is removed. The liquid refrigerant then goes through an expansion valve into an evaporator, where it “boils” to its gaseous state by drawing heat from cab air through the evaporator. The gaseous refrigerant then returns to the compressor.
For many years, DuPont's Freon, also called R-12, was the standard refrigerant used in vehicles. But in the early 1980s, R-12 was found to attack the protective ozone layer. By international treaty, R-12 was phased out in favor of R-134a, the standard today.