<s>
In	O
the	O
study	O
of	O
heat	O
transfer	O
,	O
radiative	B-Algorithm
cooling	I-Algorithm
is	O
the	O
process	O
by	O
which	O
a	O
body	O
loses	O
heat	O
by	O
thermal	O
radiation	O
.	O
</s>
<s>
Radiative	B-Algorithm
cooling	I-Algorithm
has	O
been	O
applied	O
in	O
various	O
contexts	O
throughout	O
human	O
history	O
,	O
including	O
ice	O
making	O
in	O
India	O
and	O
Iran	O
,	O
heat	O
shields	O
for	O
spacecraft	O
,	O
and	O
in	O
architecture	O
.	O
</s>
<s>
In	O
2014	O
,	O
a	O
scientific	O
breakthrough	O
in	O
the	O
use	O
of	O
photonic	O
metamaterials	O
made	O
daytime	O
radiative	B-Algorithm
cooling	I-Algorithm
possible	O
.	O
</s>
<s>
It	O
has	O
since	O
been	O
proposed	O
as	O
a	O
strategy	O
to	O
mitigate	O
local	O
and	O
global	O
warming	O
caused	O
by	O
greenhouse	O
gas	O
emissions	O
known	O
as	O
passive	O
daytime	O
radiative	B-Algorithm
cooling	I-Algorithm
.	O
</s>
<s>
The	O
Earth-atmosphere	O
system	O
uses	O
radiative	B-Algorithm
cooling	I-Algorithm
to	O
emit	O
long-wave	O
(	O
infrared	O
)	O
radiation	O
to	O
balance	O
the	O
absorption	O
of	O
short-wave	O
(	O
visible	O
light	O
)	O
energy	O
from	O
the	O
sun	O
.	O
</s>
<s>
The	O
atmospheric	O
and	O
oceanic	O
circulation	O
redistributes	O
some	O
of	O
this	O
energy	O
as	O
sensible	O
heat	O
and	O
latent	O
heat	O
partly	O
via	O
the	O
mean	O
flow	O
and	O
partly	O
via	O
eddies	O
,	O
known	O
as	O
cyclones	B-Device
in	O
the	O
atmosphere	O
.	O
</s>
<s>
Radiative	B-Algorithm
cooling	I-Algorithm
is	O
commonly	O
experienced	O
on	O
cloudless	O
nights	O
,	O
when	O
heat	O
is	O
radiated	O
into	O
outer	O
space	O
from	O
Earth	O
's	O
surface	O
,	O
or	O
from	O
the	O
skin	O
of	O
a	O
human	O
observer	O
.	O
</s>
<s>
The	O
effect	O
can	O
be	O
experienced	O
by	O
comparing	O
skin	O
temperature	O
from	O
looking	O
straight	O
up	O
into	O
a	O
cloudless	O
night	B-Application
sky	I-Application
for	O
several	O
seconds	O
,	O
to	O
that	O
after	O
placing	O
a	O
sheet	O
of	O
paper	O
between	O
the	O
face	O
and	O
the	O
sky	O
.	O
</s>
<s>
The	O
same	O
radiative	B-Algorithm
cooling	I-Algorithm
mechanism	O
can	O
cause	O
frost	O
or	O
black	O
ice	O
to	O
form	O
on	O
surfaces	O
exposed	O
to	O
the	O
clear	O
night	B-Application
sky	I-Application
,	O
even	O
when	O
the	O
ambient	O
temperature	O
does	O
not	O
fall	O
below	O
freezing	O
.	O
</s>
<s>
The	O
term	O
radiative	B-Algorithm
cooling	I-Algorithm
is	O
generally	O
used	O
for	O
local	O
processes	O
,	O
though	O
the	O
same	O
principles	O
apply	O
to	O
cooling	O
over	O
geological	O
time	O
,	O
which	O
was	O
first	O
used	O
by	O
Kelvin	O
to	O
estimate	O
the	O
age	O
of	O
the	O
Earth	O
(	O
although	O
his	O
estimate	O
ignored	O
the	O
substantial	O
heat	O
released	O
by	O
radioisotope	O
decay	O
,	O
not	O
known	O
at	O
the	O
time	O
,	O
and	O
the	O
effects	O
of	O
convection	O
in	O
the	O
mantle	O
)	O
.	O
</s>
<s>
Radiative	B-Algorithm
cooling	I-Algorithm
is	O
one	O
of	O
the	O
few	O
ways	O
an	O
object	O
in	O
space	O
can	O
give	O
off	O
energy	O
.	O
</s>
<s>
So	O
the	O
only	O
way	O
their	O
temperature	O
changes	O
is	O
by	O
radiative	B-Algorithm
cooling	I-Algorithm
.	O
</s>
<s>
The	O
widespread	O
application	O
of	O
passive	O
daytime	O
radiative	B-Algorithm
cooling	I-Algorithm
(	O
PDRC	O
)	O
technologies	O
that	O
use	O
the	O
infrared	O
window	O
(	O
8	O
–	O
13	O
µm	O
)	O
to	O
dissipate	O
heat	O
through	O
longwave	O
infrared	O
(	O
LWIR	O
)	O
thermal	O
radiation	O
heat	O
transfer	O
with	O
outer	O
space	O
,	O
has	O
been	O
proposed	O
as	O
a	O
method	O
of	O
reducing	O
temperature	O
increases	O
caused	O
by	O
climate	O
change	O
.	O
</s>
<s>
By	O
covering	O
the	O
Earth	O
with	O
a	O
small	O
fraction	O
of	O
thermally	O
emitting	O
materials	O
,	O
the	O
heat	O
flow	O
away	O
from	O
the	O
Earth	O
can	O
be	O
increased	O
,	O
and	O
the	O
net	O
radiative	O
flux	O
can	O
be	O
reduced	O
to	O
zero	O
(	O
or	O
even	O
made	O
negative	O
)	O
,	O
thus	O
stabilizing	O
(	O
or	O
cooling	O
)	O
the	O
Earth	O
(	O
...	O
)	O
If	O
only	O
1%	O
–	O
2%	O
of	O
the	O
Earth	O
’s	O
surface	O
were	O
instead	O
made	O
to	O
radiate	O
at	O
this	O
rate	O
rather	O
than	O
its	O
current	O
average	O
value	O
,	O
the	O
total	O
heat	O
fluxes	O
into	O
and	O
away	O
from	O
the	O
entire	O
Earth	O
would	O
be	O
balanced	O
and	O
warming	O
would	O
cease.PDRCs	O
mimic	O
the	O
natural	O
process	O
of	O
radiative	B-Algorithm
cooling	I-Algorithm
,	O
in	O
which	O
the	O
Earth	O
cools	O
itself	O
by	O
releasing	O
heat	O
to	O
outer	O
space	O
(	O
Earth	O
's	O
energy	O
budget	O
)	O
,	O
although	O
during	O
the	O
daytime	O
,	O
lowering	O
ambient	O
temperatures	O
under	O
direct	O
solar	O
intensity	O
.	O
</s>
<s>
Radiative	B-Algorithm
cooling	I-Algorithm
thus	O
offers	O
potential	O
for	O
passive	O
cooling	O
for	O
residential	O
and	O
commercial	O
buildings	O
.	O
</s>
<s>
If	O
made	O
sufficiently	O
reflective	O
to	O
sunlight	O
,	O
these	O
materials	O
can	O
also	O
achieve	O
radiative	B-Algorithm
cooling	I-Algorithm
during	O
the	O
day	O
.	O
</s>
<s>
Other	O
notable	O
radiative	B-Algorithm
cooling	I-Algorithm
strategies	O
include	O
dielectric	O
films	O
on	O
metal	O
mirrors	O
,	O
and	O
polymer	O
or	O
polymer	O
composites	O
on	O
silver	O
or	O
aluminum	O
films	O
.	O
</s>
<s>
When	O
backed	O
with	O
silver	O
coating	O
,	O
the	O
material	O
achieved	O
a	O
midday	O
radiative	B-Algorithm
cooling	I-Algorithm
power	O
of	O
93	O
W/m2	O
under	O
direct	O
sunshine	O
along	O
with	O
high-throughput	O
,	O
economical	O
roll-to-roll	O
manufacturing	O
.	O
</s>
<s>
High	O
emissivity	O
coatings	O
that	O
facilitate	O
radiative	B-Algorithm
cooling	I-Algorithm
may	O
be	O
used	O
in	O
reusable	O
thermal	O
protection	O
systems	O
(	O
RTPS	O
)	O
in	O
spacecraft	O
and	O
hypersonic	O
aircraft	O
.	O
</s>
<s>
In	O
addition	O
to	O
effective	O
radiative	B-Algorithm
cooling	I-Algorithm
,	O
radiative	O
thermal	O
protection	O
systems	O
should	O
provide	O
damage	O
tolerance	O
and	O
may	O
incorporate	O
self-healing	O
functions	O
through	O
the	O
formation	O
of	O
a	O
viscous	O
glass	O
at	O
high	O
temperatures	O
.	O
</s>
<s>
The	O
James	O
Webb	O
Space	O
Telescope	O
uses	O
radiative	B-Algorithm
cooling	I-Algorithm
to	O
reach	O
its	O
operation	O
temperature	O
of	O
about	O
50	O
degrees	O
K	O
.	O
To	O
do	O
this	O
,	O
its	O
large	O
reflective	O
sunshield	O
blocks	O
radiation	O
from	O
the	O
Sun	O
,	O
Earth	O
,	O
and	O
Moon	O
.	O
</s>
<s>
Before	O
the	O
invention	O
of	O
artificial	O
refrigeration	O
technology	O
,	O
ice	O
making	O
by	O
nocturnal	B-Algorithm
cooling	I-Algorithm
was	O
common	O
in	O
both	O
India	O
and	O
Iran	O
.	O
</s>
<s>
Such	O
apparatus	O
consisted	O
of	O
a	O
shallow	O
ceramic	O
tray	O
with	O
a	O
thin	O
layer	O
of	O
water	O
,	O
placed	O
outdoors	O
with	O
a	O
clear	O
exposure	O
to	O
the	O
night	B-Application
sky	I-Application
.	O
</s>
