title: FFT using Python - unexpected low frequencies
body: <p>I'm still trying to get frequency analysis for this <a href="https://drive.google.com/open?id=0B7um2WC4G6FFazF1eXJHRVNqVFU" rel="nofollow noreferrer">data</a> using FFT in Python.
  The sampling rate is 1 data point per minute.</p>
  
  <p>My code is:</p>
  
  <pre><code>from scipy.fftpack import fft
  df3 = pd.read_csv('Pressure - Dates by Minute.csv', sep=",", skiprows=0)
  df3['Pressure FFT'] = df3['ATMOSPHERIC PRESSURE (hPa) mean'] - df3['ATMOSPHERIC PRESSURE (hPa) mean'].mean()
  Pressure = df3['Pressure FFT']
  Fs = 1/60
  Ts = 1.0/Fs
  n = len(Pressure)
  k = np.arange(n)
  T = n/Fs
  t = np.arange(0,1,1/n) # time vector
  frq = k/T # two sides frequency range
  frq = frq[range(int(n/2))] # one side frequency range
  
  Y = np.fft.fft(Pressure)/n # fft computing and normalization
  Y = Y[range(int(n/2))]
  
  fig, ax = plt.subplots(2, 1)
  ax[0].plot(t,Pressure)
  ax[0].set_xlabel('Time')
  ax[0].set_ylabel('Amplitude')
  ax[1].plot(frq,abs(Y),'r') # plotting the spectrum
  ax[1].set_xlabel('Freq (Hz)')
  ax[1].set_ylabel('|Y(freq)|')
  </code></pre>
  
  <p>But the result gives:</p>
  
  <p><a href="https://i.stack.imgur.com/FDYQn.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FDYQn.jpg" alt="enter image description here"></a></p>
  
  <p>So my problems are:</p>
  
  <p>1) Why there are no frequencies at all ? The data is clearly periodic.</p>
  
  <p>2) Why the frequency spectrum is so low ? (0 - 0.009)</p>
  
  <p>3) Maybe I should try different filtering technique?</p>
  
  <p>Any insights ?</p>
  
  <p>Thanks !!!  </p>