"Although not technically correct, many WiFi vendors define signal quality as Signal-to-Noise Ratio (SNR). The signal-to-noise ratio is simply the difference in decibels between the received signal and the background (noise floor, or most importantly the receiver's threshold)." - Signal-to-Noise Ratio defined by CWNP
WiFi adapters typically do not provide an accurate SNR, because they are usually unable to detect the noise floor accurately. The noise floor is generally set to a static number, like -100 dBm.
It is possible to calculate an approximate SNR with Chanalyzer + Wi-Spy by subtracting the noise from the signal strength.
Disclaimer: The calculation is approximate because the signal strength is pulled from a WiFi adapter, and the noise floor is pulled from Wi-Spy. Since they are two separate pieces of hardware in two physically different locations, what they see in the wireless environment will vary slightly.
Signal strength is found by looking at the Networks Table tab. In this case, the signal strength is -56 dBm.
The noise floor can be found in the Channels Table tab. We can see that the noise floor is at -89 dBm in this example.
Signal Strength - Noise floor = SNR
(-56 dBm) - (-89 dBm) = 33, so the SNR in this example is 33 dB.
Since the WiFi data and spectrum analysis data is pulled from two different sources, they will be a few dB off. The result is an approximate estimation of the SNR.