Dr. Celsius is a meteorologist unicorn; working from their lab to assist Ponyville in all things, but not limited to, weather. While ridiculously clumsy and accident-prone they remain a happy and agreeable pony that likes nothing more than a good chat. Especially about their latest experiments.
Raised in Vanhoover with the name Smoke Plume, they were convinced that it was their calling to present the weather. In nursery they often put on little shows where they would report on the current state of the sky with their favourite toy microphone, replicating the voices they heard on TV and radio every day, and often making bold predictions that would typically fail to transpire.
This continued for quite some time throughout their schooling, but with building frustration at their inaccuracy and the lack of a weatherpony cutie mark they decided to take matters into their own hands. In a makeshift laboratory in the corner of their room they attempted and succeeded in creating a minature weather balloon fit with radio transmitters and parachutes, sending readings to their family computer. Just as the first successful results were sent from the device to their machine did they finally gain their cutie mark.
Meteorology was their calling!
Vanhoover to Canterlot to Ponyville
In their late adolescence they moved to Canterlot in order to pursue their studies of meteorology. It was there that they gained their doctorate and decided to adopt the moniker of Celsius; partly based on a school nick-name but also due to their, perhaps misguided, idea that it would make them sound more professional. Even cool!
They considered returning home to Vanhoover but decided that the quiet pastoral life of Ponyville suited them better. There would be plenty of space for an independent lab and various experiments, helping everypony understand the weather and advising weather-producing factories on when, where, and why weather events should occur. When they did arrive they made friends quickly in Blazing Flame and Makaar, even finding romance with Marekaar. Everything was going swimmingly, until in a failed chemical experiment to see how Everfree soil would react under certain conditions, they accidentally made an intensely sleep-inducing serum which put them into a deep slumber for 6 years.
Now they are awake and find many of their former friends have moved or disappeared, leaving Celsius having to rediscover friendship all over again.