Antenna Measurement

When I have an antenna connected to an amplifier, why is the noise floor -174 dBm/Hz inside an anechoeic chamber and <-176 dBm/Hz when outside pointed to the sky.
Can someone point me to the right books or papers that can explain this?

Have you hear sometime, somewhere, that the sky is "cold"?
Far from human RF emission, when you point your antenna to zenith (the vertical over the head), you'll see a cold sky.
The basic temperature is the cosmic background (2.7 K) and at some frequency it is moderately warmed by galactic noise or atmospheric absobition.
Practically:
Below 1 Ghz: 100K ... 10 K
1-15 GHz: 3K... 10K
22 GHz: 30 (clear sky, mountain)...200K (cloudy sky, land)
This low noise temperature is also little warmed by antenna spill-over looking the ground. Remember that a spill-over (back lobes) looking at the ground produce a 200K...300K noise temperature but usually the back lobes are many times smaller than main lobe.

suggestion for books? Kraus- Radioastronomy - pag 237.

Note: to convert Kelvins into Power apply :
P=KTB
K: Boltzmann's kostant
T: Operational noise temperature
B: bandwidth
all in SI units