It's still cold whether °C or °F

One of the most challenging aspects of adopting a new measurement system is relearning the "feel" or "look" of things and estimating their measure. In the case of temperature, many people are able to estimate based on a lifelong association of what the number on a thermometer feels like. For example, 4.5°C (40°F) is "cold" while 40°C (104°F) is "hot".

We first learn the arbitrary word association with the feeling when we are children. We feel the sensation, then we learn to verbalize that sensation as "hot" or "cold". Later we assign a personal range of numbers to that sensation/verbalization.

Your brain has already formed its association between temperature scale and sensation; so we need to establish an overlay using new numbers. I use a method I call "key conversion points" to help myself quickly estimate equivalent values without the use of mental math or a calculator.

Key Temperature Conversion Points (Celsius benchmarks):

50°C = 122°F
40°C = 104°F
* 37°C = 98.6°F (Normal human body temperature)
* 35°C = 95°F
25°C = 77°F
* 21°C = 70°F (Room temperature)
15°C = 59°F
* 10°C = 50°F
0°C = 32°F (Water freezes)
-15°C = 5°F

Key Temperature Conversion Points (Fahrenheit benchmarks):

* 95°F = 35°C
80°F = 26.67°C
70°F = 21°C (Room temperature)
60°F = 15.5°C
* 50°F = 10°C
32°F = 0°C (Water freezes)
23°F = -5°C
* -4°F = -20°C
* -40°F = -40°C

Memorizing some (or all) of these conversion points will help you start estimating right away. (I put asterisks next to my favorites.) Eventually, the SI numbers will start to have the same impact as the customary unit values; you'll still think "cold" whether the weather is given in Celsius or Fahrenheit.