Finally, leaves once again are appearing on trees, gleaming like a still-wet painting, and the sun is high enough in the sky that it’s warming the atmosphere, not just throwing flat shadows.

Car interiors get uncomfortably warm with passive solar radiation, and it’s time to crank up the air conditioner.

Soon that warm air, laden with winter dust, will give way to the refreshing breeze of manufactured coolness. Anytime now. Just another minute or two. Maybe if I fiddle with that recirculation button. Is that supposed to be on or off? Does it matter which vents it comes out of? Why is it so hot in here?

The first hot day of the year brings the same response when driving. Turn on the air conditioner or lower the temperature of the climate control, but when that action yields no relief from the heat, what are your options? Air conditioning, or A/C, was invented by Willis Carrier in 1902, and his name, for a time, was synonymous with commercial units. How A/C works is actually fairly complicated so I’ll review the operation of a typical system.

Technically, air conditioning doesn't provide cool air; it removes heat and humidity from the air. It sounds like the same thing, but it's an important distinction. Air-conditioning systems are closed pressurized systems and function through several major components, including the compressor located under the hood, mounted to the engine and driven by an accessory or serpentine belt.

The compressor’s job is to compress the refrigerant, thereby building pressure in the system. The refrigerant, once commonly called by its brand name, Freon or R12, has been largely replaced by R134a, a more environmentally friendly material, and auto manufacturers were required to switch to this less harmful refrigerant by 1996.

As the refrigerant builds pressure, it heats up and flows into the condenser, a thin radiator-like component mounted adjacent to the car's engine radiator, where it utilizes airflow and the engine fan to cool down the refrigerant.

Gases give off heat when changed from gas to liquid, and as it cools, it condenses into a liquid state and flows to the receiver dryer, which stores and dries the liquid refrigerant by absorbing any moisture contamination before it travels to the expansion valve.

Moisture in the system can form ice crystals and cause damage. The expansion valve regulates the flow and lowers the pressure of the refrigerant from a high of 200 psi to a low of 35 psi or about the same pressure in your car's tires. The now low pressure R134a flows to the evaporator where it starts to boil at the extremely low temperature of -15 degrees F. (water boils at 212 degrees F.) so that the heat of the cabin will start this reaction.

Liquids absorb heat when changing from liquid to gas and this intense boiling takes place inside the evaporator, which is mounted inside the car usually behind the dashboard. Heat is absorbed into the boiling refrigerant, which causes the evaporator itself to become very cold as the heat around it is sucked into the boiling refrigerant. The car’s interior fan blowing on the evaporator circulates this cold air.

Simultaneously, humidity is removed from the car by condensing it on the evaporator coil and this is the water you see under the car when stopped with the A/C running. The removal of humidity is also why the defroster works more effectively when run through the A/C system. The refrigerant then leaves the evaporator as a low pressure gas taking the heat with it and returning to the compressor to begin again.

When the A/C stops blowing cold, what could be the reason? The usual go-to issue is lack of refrigerant. Some people dismiss the leak and just keep topping off the system. Depending on the size of the leak, it might stay cold all season before needing another charge.

Pressurized cans of refrigerant are available from local auto part stores and automotive departments at larger department stores, and installing them is fairly straightforward, although this isn’t always the cure and it isn’t very good for the environment to be venting refrigerant as you drive.

Commercial service and repair shops have specialized machines that evacuate the sealed A/C system, vacuum out any contaminants, and recycle and install the correct quantity of refrigerant along with a small amount of lubricating oil. Air-conditioning-rated auto technicians are certified to work on the system, operate the equipment and purchase refrigerant in bulk.

A common way to detect a small leak is to service the system and inject a small amount of dye, which shows up under special lighting to pinpoint a leak. Remember, this is a sealed, pressurized system and should not be leaking. If any amount of refrigerant is absent, it’s not normal.

Other problems can be caused by a defective pressure switch that’s meant to detect appropriate pressure and quantity of refrigerant, a compressor that isn’t functioning, or a compressor clutch that doesn’t operate correctly, and previously mentioned leaks from nearly any component or line.

Troubleshooting an automotive A/C begins with the basics: Make sure it’s actually on. Some systems have a separate switch to engage the compressor. Be sure the system is set to recirculate for maximum cool air. That allows the cabin air to continue circulating, cooling inside air rather than trying to cool off warm outside air.

Because the compressor runs off the engine, it’s not as effective at idle speed as it is at higher RPM, and you may find it cools better as you drive.

Sitting in the hot sun makes the A/C work harder; park in the shade when possible or consider tinting your windows to help cut down on passive solar heating.

A sensitive subject with pet owners and, really, any human being with a conscience, is leaving dogs or pets or even children in a hot car.

The interior of a car can reach unlivable temperatures in short order when a closed car is exposed to the sun, and good Samaritans and first responders alike have been know to break a car window to provide relief to a suffering animal. Many states have laws that allow such an action.

With the proliferation of fully electric vehicles, it’s now difficult if not impossible to tell from the outside whether a car’s interior is habitable. In response, Tesla has unveiled “Dog Mode” that keeps the car’s cabin at the perfect temperature and displays a message on the vehicles huge center display screen to let passersby know that your furry friend is comfortable. There’s even a cellphone app to warn the driver if the battery charge gets low. I expect we’ll be seeing similar features from other EV manufacturers.

Air-conditioner season doesn’t last long around here, but it’s coming fast, so stay cool.

Eric and Michelle Meltzer own and operate Fryeburg Motors, a licensed, full-service automotive sales and service facility at 299 Main St. in Fryeburg, Maine. More than a business, cars are a passion, and they appreciate anything that drives, rides, floats or flies.

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