Why Noise figure measures how much noise a receiver adds beyond the unavoidable thermal noise floor, expressed in dB. The theoretical minimum is the thermal noise power kTB (about -174 dBm per hertz at 290 K), so a perfect, noiseless receiver would have a noise figure of 0 dB. Equivalently, noise figure is the decibel ratio of the signal-to-noise ratio at the input to the signal-to-noise ratio at the output: a 3 dB noise figure means the receiver degrades SNR by 3 dB.
Watch out The choice comparing receiver noise to atmospheric noise is wrong because the reference is the theoretical thermal noise minimum, not external noise; atmospheric noise varies enormously with band and time and dominates on HF, which is why a very low noise figure buys little below about 20 MHz.
Noise figure = how many dB worse than a perfect receiver. Perfect = 0 dB, reference is thermal kTB, not the atmosphere.