Fig 1 shows the background sky noise temperature as a function of frequency as seen by an antenna outside the earth's atmosphere (in free space).
Fig 2 shows the sky temperature as seen from the earth's surface. Note the significant increase in noise level above 10 GHz, especially in the peaks labeled O2 and H2O. Oxygen has a particularly strong resonance at 60 GHz, so not only does the earth's atmosphere radiate significant radio noise at this frequency, but it also absorbs RF at about 10 dB/km at the earth's surface. This makes the 60 GHz band particularly useful for low-probability-of-intercept (LPI) military radios and local area computer networks.
The H2O peaks obviously depend strongly on atmospheric water content (humidity and temperature). Heavy rainstorms can result in severe attenuation at the higher microwave frequencies, as any DSS dish owner has probably noticed.
Phil Karn, 16 Aug 1997