sir, how to i find confidence intervals for filtered data?

sir i have a time series of temperature with varying time, i coverted this data into frequency domain by apply LPF & FFT,at last i got graph i.e., amplitude vs Frequency...now i need to find confidence levels for this data...please help me? i attached the data file...

[kopparthi]

 amp=untitled(:,2);
>> f=untitled(:,1);
>> plot(f,amp)
choose scale 0 to 1 for x-axis,0 to 0.8 for y-axis .,such that you will find the peaks for the data.

[kopparthi]

95% confidence levels...