Fast Fourier Transform i.e. ffta yields the same unit of the variable as it is the amplitude of the variable for various frequencies. so on y-axis it is celcius to best of my knowledge.
Without knowing Ferret's implementation of FFT, I think Saurabh's answer is most likely to be correct. Here I would like to add a bit of background.
Normally "Fourier transform" (time -> frequency) is defined as