dB scale on spectrum analyzer

[kumar]

I am new to spectrum analyzer and want to know more about the dB scale displayed on spectrum analyzer.
As I understand, the dB is the log of ratio of two similar values ( voltage,power etc.) On the spectrum analyzer the dBm scale is zero on the top of Y axis and becomes negative as we come down. Why is it so?

[Bored@Work]

Negative dB values indicate the actual value is smaller than the reference value. For dBm the reference is 1 mW. So, negative dBm values indicate the power is less than 1 mW.