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Graphical interpretation of magnitude (derived from fft)

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Pedro Mateus
Pedro Mateus on 4 Mar 2014
Closed: MATLAB Answer Bot on 20 Aug 2021 at 16:49
Hello to all users of Matlab.
I have 20 years of temperature measurements. Twice a day (max and min) with 12 hours between measurements. To understand which frequencies are more pronounced, I applied the following code:
x = detrend(x);
n = length(x);
nfft = 2^nextpow2(n);
Fs = 1/(12*60*60);
f = 0 : Fs/(nfft-1) : Fs/2;
Y = fft(x, nfft);
Y = Y(1:((nfft+1)/2));
Y = Y/(nfft/2);
Ya = abs(Y);
plot(f, Ya); xlabel('Frequency (Hz)'); ylabel('|Y(f)|'); grid on
I get:
The energy is concentrated in the last frequency (Nyquist, if I'm not mistaken). Is there any error in the code? Not the first time I do this, but never for this type of data and with a sample rate (Fs) so small.
Thanks for the help

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