The system that you use for temperature is stupid. I don’t care if you use Celsius or Fahrenheit, it’s dumb. It’s illogical (I will concede that C is more logical than F, but that only means you failed the test by less).
Why is it dumb? Because they are both so arbitrary! Why is zero degrees for both set where it is? The freezing point of water kind of makes sense, until you stop to think about it (and 0°F just doesn’t make sense).
As an analogy, let’s look at mass. Say we’re trying to invent a nice, standardized unit of mass in some bizarro-dimension where the kilogram doesn’t exist. Let’s use water as a reference, because it’s a pretty common compound, and important to us. We’ll take something logical, like a cubic meter, and we’ll use that to set our system up.
Only, instead of having that be a reference point, we’re going to make that our zero point. So the amount of mass in a cubic meter of water is zero. Anything less is negative, and anything more is positive.
Sound dumb? That’s what Celsius and Fahrenheit do! Temperature is a measure of thermal energy (sort of – it’s actually more of a measure of the kinetic energy of the particles (as a simplified explanation), but what exactly it measures isn’t that important here), and we’ve set our “zero” point where the property we are measuring isn’t zero! Just like the mass example, only we have a problem with something having negative mass and no problem with something having negative temperature, even if neither make much sense.
So why do we use °C and °F? Because we are bad at numbers. 273 and 300 don’t seem that different to us as numbers, but they are (approximately) the freezing point of water and the temperature of a warm spring day (in Kelvin). They feel a lot different, but they don’t register as very different, so we adjust scales and zero points until it makes sense to us. We want cold to be really small numbers, and hot to be really big numbers, not for the numbers to actually make sense scientifically.
So that’s my rant. The system of temperature that you use to determine if you need a jacket or not is dumb, no matter which you use, and it’s because we’re bad with numbers. Simple as that.