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I know that water behaves strangely once the temperature drops below 4 (celsius) degrees. In a nutshell, its volume starts to expand (see wiki for further details) below 4 degrees - when temperature reaches zero it freezes. Again, as far as I know and understand, this anomaly has only to do with the density of the water - ice forms when temperature reaches zero.

So my question why in many (all?) cars the temperature display starts to blink (sometimes also give sound warning) when the temperature reaches 4 degrees? Is it just a general warning that it is cold, or does it have to do with some properties of the water?

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When the temperature is 0 Celsius (32°F), ice is likely to be visible and drivers will be aware. Typically they will be driving carefully, and may even be on winter tyres.

Where the temperature is falling through 4 degrees (40°F), most road conditions will be fine, but there may be parts of roads (eg in the shade) that are at 0 or below and could be icy. The risk here is that the driver who isn't expecting ice may be driving at a speed which is dangerous when they hit an ice patch. This is the cause of a lot of accidents - the surprise black ice on a shady corner.

I don't know why it is 4 degrees rather than 3 or 5, but the core premise makes a lot of sense. Twice I have been on a long overnight drive (over 8 hours) and making good progress when the ice alarm has alerted me, so I have dropped my speed by a significant fraction before reaching a potentially dangerous skid.

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  • I've actually wondered why my car tells me about ice when the temp his 40°F, but never thought to ask.
    – JPhi1618
    Mar 17, 2016 at 21:51
  • Also, I believe snow can fall at temperatures above zero. On my cars (VW) the snow warning appears at 4 degrees and once illuminated, stays lit until the temperature climbs to 6 degrees. Mar 18, 2016 at 10:54
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The outside air temperature sensor is often located in a part of the vehicle that will receive some warmth from the engine in certain parts of the operating envelope. Reading as much as 5 degrees high is something it may do. When it's indicating 4 degrees, it warns you because the actual temperature may well be below freezing.

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As you refer to in the question, water starts to expand going below 4 (=3.98) degrees Celsius. This causes a turn in the diffusion of temperatue in water puddles where the temperature differs slightly, below 4C the colder water will diffuse toward the top, which in turn increases the risk of planing to some minor degree.

I have no idea if this is taken into consideration in terms of the warning flash, or even how large this effect can be in the worst of scenarios.

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The Low Temperature Warning is activated because the sensed ambient temperature is low. The sensor will not know whether the ambient temperature is warming up or getting colder. It will not take into account any 'wind chill' factor making any temperature effect lower.

When the LTS is on it means there is a high probability of ice on the roads. Drive carefully and reduce your speed in anticipation of ice.

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It may be 4 degrees ambient, but 0 or less degrees on the road surface.

Think about the morning when it has been sub-zero overnight, the sun has warmed the air but the ground hasn't caught up.

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The sensor in your car measures the ambient air temperature, but on a clear morning before sunrise the road surface will be cooler than the air. During the night the road will be radiating heat into the cold sky and receiving very little back. The ground loses heat quickly like this, and it is actually the cold ground that cools down the air.

If you have a black car (black radiates heat better) and park it somewhere that it has a clear view of the sky above, you will find frost on the roof on mornings that the air temperature is still above freezing for the same reason.

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