It is caused by the glass's surface temperature being much lower than the air temperature inside the car. Humans are warm. The air we breath is warm. We're heating up the interior of the vehicle. When that warm air contacts the cold surface of the glass, the water droplets migrate from the air onto the surface.

In order to detect it you'd need to know the glass's surface temperature ideally in the middle (away from the car's body) and also know the interior temperature. The interior temp they already have. But figuring out the glass's temp is non-trivial. Infrared camera is the only thing I can imagine working (since a sensor wouldn't be transparent or wouldn't be replaced when the glass is) but that would likely give inaccurate readings due to the outside temperature.

It doesn't have to be perfect -- we can have it just turn on the defrost whenever the probability of window fogging is >10% or something.

When it's sort of cold, but not "ice on the car" cold, it works great as defogging. Way faster than waiting for the car to heat up. When there is ice on the car, you turn on the heaters and within a minute or two you can simply use your wipers to clear the window of ice.

Or, perhaps more simply, have an optical sensor that detects and reacts to the fogging itself sooner than a human would.

Not really. Many windshields already have elements embedded in them like the ultra-thin wire used for the AM radio antenna. Sensing temperature via the resistance of a wire like that is about as trivial as it gets.

It ain't broke, so stop trying to 'fix' it.

I honestly don't remember and my solution is "try both for a little bit while maintaining control of the car".

now in the winter you can sometimes do better than this by fixing the temperature differential itself. you can either lower the windows or cool the entire interior by blowing air with no heat. in reality, most people don't actually want to drive around in the winter with windows down or no heat, so the best tolerable option is usually to do as above: turn on AC and heat.

in the summer you can also get fogging on the outside of the windows. just use your wipers for this.

After reading this I am still not sure which one to use. They seem to both work.

When I get in the car in the morning, the air is already cold. No amount of more cold air from the vents will clear that. What's making it fog up, is the moisture I'm exhaling. More cold air is not going to fix that unless I drive with all windows down. So the only reasonable option since AC don't work at low temperature, is waiting for the car to heat up.

(Warm dry air evaporates water that's condensing on cold surfaces. That's very far from rocket science.)

Two: You are making a false choice: Automatic settings need not replace manual ones. Essentially what we are talking about is a spot on the dial that says "auto" which you are free to not use, much like headlight controls on higher end cars.

Well, they worked fine, but my A/C just went out, so getting dehumidified air is a problem these days.

Sure, but the marketing department had to fill a few blanks in the "bad weather options pack" (optional but casually installed on all cars in production that will be available in the next six-eight months) for a mere US$ 3,000:

1) self-learning wipers (using AI to set automatically an appropriate wiping speed, including economode, that only wipes the right side of the windscreen when you make a free turn on right at a traffic light)

2) intelligent defrost (computing the correct defrost temperature through analysis of real-time satellite heat maps)

3) heated and ventilated mats (that can dry your wet shoes and lower half of trousers independently from heating/confitioning settings)

Dry hot air to heat the windshield above the dew point avoids the "damned if you do, damned if you don't" choice of cold dry air to keep the inside from fogging up versus hot humid air to keep the outside from fogging up.

It's not a matter of temperature sensing on the windshield, but humidity sensing in the cabin air.

There is a max-demist button, which for me is the perfect automatic function - it says what I want, and lets the car get on with selecting max fan/heat/AC/flow to the demist vents. And I can find it with muscle memory when I need it.

Heat randomly turns on -> human is confused, maybe uncomfortable, and might think the car is broken/haunted.

Heat randomly turns on, car displays "Preventing windshield fogging" -> human is thankful car is thinking ahead.

It’s certainly not important enough to distract someone performing any delicate maneuver.

This is a false dichotomy.

Probably because it costs more money and nobody will pay for it. That's the best guess I have.

Personally I solved the problem by buying a lithium ion car starter for both of our cars. Works great, but I really would prefer the cars just had some redundancy built in.

I have been contemplating using the second one as an auxiliary battery after installing a fridge in the boot, though. Basically a matter of fitting a voltage monitor and a hefty relay.

Leaving the headlights on (~10A) as you leave the car in the evening will still let you start it in the morning, for example.

there are bad touch screen systems and there are good ones. just as there are badly laid out physical controls and good ones.

even muscle memory works with touch screens, to say otherwise is just a lie. however the biggest oversight people tend to ignore is, how little they actually interact with controls of their car other than the turn signals and such. Most modern cars have full climate controls which does include humidity and such, many have automatic lights and wipers, and some go as far as doing the driving.

nineteen to twenty four buttons on steering wheel is just fine and intuitive to some just like the same number on a center console yet these same people will complain about the simplicity of a properly design touch UI and fewer physical buttons as being too complex.