Defrost, defog and visibility systems
Why defrost mode forces the air conditioner on, even in winter.
You'll learn
- Explain defrost, defog and visibility systems at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real vehicle
Overview
A foggy or iced windscreen is a safety problem, and the system responds by prioritising visibility over efficiency or even comfort.
In depth
Why dehumidifying matters more than heating
Fog forms when moist cabin air contacts a cold windscreen. Blowing warm air alone does not remove the moisture that caused it — the air conditioner must run to dehumidify the air before the heater warms it, which is why defrost mode locks the AC on even below freezing outside.
Don't skip the wait
Driving with a partially cleared windscreen to save time or energy significantly reduces visibility and reaction time — let the defrost cycle finish.
Key takeaways
Remember this
- Fog requires dehumidification, not just heat, to clear.
- Defrost mode forces the air conditioner on regardless of outside temperature.
- Visibility is prioritised over efficiency by design.
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