Wes Stewart wrote:

...The clear night sky has a black body temperature very near
absolute zero (0 Kelvin)...

It's not that cold. There are approximate formulas for "effective sky
temperature." Here's what Duffie and Beckman say on pp 157-8 of the
1991 edition of "Solar Engineering of Thermal Processes":

To predict the performance of solar collectors, it will be necessary
to evaluate the radiation exchange between a surface and the sky.
The sky can be considered as a black body at some equivalent sky
temperature Ts... The net radiation from a surface with emittance E
and temperature T to the sky at Ts is Q = EAs(T^4-Ts^4).

The equivalent blackbody sky temperature... accounts for the facts
that the atmosphere is not at a uniform temperature and that the
atmosphere radiates only in certain wavelength bands. The atmosphere
is essentially transparent in the wavelength region from 8 to 14 um,
but outside of this "window" the atmosphere has absorbing bands
covering much of the IR spectrum. Several relations have been proposed
to relate Ts for clear skies to measured meterological variables.
Swinbank (1963) relates sky temperature to the local air temperature,
Brunt (1932) relates sky temperature to the water vapor pressure,
and Bliss (1961) relates sky temperature to the dew point temperature.
Berdahl and Martin (1984) used extensive data from the United States
to relate the effective sky temperature to the dew point temperature,
dry bulb temperture, and hour from midnight t...

Ts = Ta[0.711+0.0056Tdp+0.000073Tdp^2+0.013cos(15t)]^0.25,

where Ts and Ta are in degrees Kelvin and Tdp is the dew point
temperature in degrees Celsius. The experimental data covered a
dew point range from -20 C to 30 C. The range of the difference
between sky and air temperatures is from 5 C in a hot, moist
climate to 30 C in a cold dry climate.

Clouds will tend to increase the sky temperature over that for a
clear sky... the sky temperature is critical in evaluating
radiative cooling as a passive cooling method.

Let's see. Philadelphia has an average daily minimum Ta = 67.2 F
(= (67.2-32)/1.8 + 273 = 293 K) in July, with an average humidity ratio
W = 0.0133 pounds of water per pound of dry air, which corresponds to
a dew point of about 65 F (18 C), so at 6 AM, when cos(15x6) = 0,

Ts = 293[0.711+0.0056(18)+0.000073(18^2)]^0.25 = 280 K or 45 F.

Albuquerque, NM has a min Ta of 64.4 F (291 K) in July with W = 0.0092
corresponding to Tdp = 55 F or 12.8 C, so at 6 AM on an average day,

Ts = 291[0.711+0.0056(12.8)+0.000073(12.8^2)]^0.25 = 275 K or 35 F.

Nick