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It’s Not Rocket Science #5: Night Sky Radiant Cooling

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If ever you’ve noticed cloudless nights are colder than cloudy ones, then you’ve experienced Night Sky Radiant Cooling.  

The temperature of the surface of the earth is relatively constant so the amount of energy lost in the form of heat from the surface of the earth at night must be more or less the same as the amount of energy gained by the sun at the rate of approx. 1.5 x 10^19 kJ per day. Otherwise, the planet would overheat. Of course, one of our problems right now is that Earth is overheating a bit but, for the time being, we can still say that most of the Earth’s heat is radiated back into unheated space at night as thermal infrared radiation. Please allow me to introduce you to Count von Rumford.


Two centuries ago he invented a more efficient and smokeless fireplace. For this, he received instant celebrity and the eternal thanks of ladies in London salons. He was a bit of an expert on heat transfer and, on his travels, noted that “the inhabitants of certain hot countries who sleep at night on the tops of their houses in order to be more cool and comfortable, do wisely in choosing that situation to pass their hours of rest.” He concluded that “frigorific rays” arrive continually on the surface of the earth from “the heavens” to cool the planet.

We know now that heating is the transfer of heat into an object and that cooling is the transfer of heat out of an object. Whether it’s heating or cooling depends upon the direction of the radiation but the effect of cooling is the same as if there actually were incoming “frigorific rays”. It’s not a bad way of thinking about night sky radiant cooling.


A previous post about yakhchal described how, around 400BC, the Persians used night sky radiant cooling to make ice in winter. Yakhchal of the domed type were common in Yazd (smack in the middle of Iran now) where the climate was especially suited.

yazd location

Here’s a year’s worth of hourly temperature reports for Yazd.


FREEZING (blue) 0°C COLD (dark green) 10°C COOL (light green) 18°C COMFORTABLE (yellow) 24°C
WARM (light red) 30°C HOT
 (medium red) 38°C VERY HOT (dark red)  

We know the Yazd ice-makers knew about night sky radiant cooling because

  1. walls to the east, south and west sheltered the ice-freezing areas from the radiant heat of the sun 
  2. these walls also kept the air behind them still to further reduce the temperature of the air immediately above the ice
  3. the ice-freezing pools were covered with straw during the day to minimise heat radiation from the daytime sky
  4. this straw was removed at night to increase heat radiation to the night sky
  5. yakhchal were built on the edges of towns
    (In their paper, “Assessment of Ancient Fridges: A Sustainable Method to Store Ice in Hot-Arid Climates”, Mahdavniejad and Javanrudi report that yakhchal were built in the countryside because of their large land usage and the high cost of land in cities. Possibly, but if the ancient Persians understood about night sky radiant cooling then they would of course site their ice-making facilities away from cities and their heat islands.) This is a sketch of the ice house at the edge of the city of Kashan, from 1677.

ice house chardinIt’s well known that the process could produce ice even when the ambient temperatures were above freezing. Once produced, the ice was then moved to the yakhchal and stored through the summer. However, I don’t think the use of night sky radiant cooling ended once the ice was stored. In “An Overview of Iranian Ice Repositories, An Example of Traditional Indigenous Architecture”, Hosseini and Namazian state that

one of the advantages of these vaults was that they could be built step-like with stairs to help workers to cover the external crust of the vault with thatch [straw] to protect it from rain, snow, sun and atmospheric variations. They built smaller stairs between these stairs to make it possible for workers to ply. So people could maintain or repair them easily.

close up of ice house construction

If this stepped construction allowed the dome to be easily covered with straw during the day for whatever reason, then that straw would also protect the dome from daytime heat buildup. Moreover, and with equal ease, the stepped construction would allow that straw to be easily removed at night and so enable night sky radiant cooling to cool the dome during summer nights. I suspect this is what Hosseini and Nazamian meant by this.

In the heat of summers especially in central regions, the sun heats the Yakhchal dome. This method was also employed in order to prevent the ice stored to melt during hot seasons.

A 250 mm mud-brick wall has a U-value of 0.36 and a 350 mm wall has a U-value of 0.26.  [Ref.] Mud brick walls have low thermal resistance and are good at storing thermal energy. Unfortunately, this is exactly not what was wanted. Covering the domes with straw during the day and removing that straw at night is the sensible thing to do. The same process that worked when making the ice also worked when storing it. dome

Interest in night sky radiant cooling waned with the development of refrigeration.

* * *

The following brief summary of late 20C developments in night sky radiant cooling applications is largely taken from the research “Potentials of Night Sky Radiation to Save Water and Energy in the State of New Mexico” by the architect Mark Chalom practising “the design and construction of environmentally appropriate buildings”.  

In 1967,  Harold Hay and John Yellott built a one-room, single-story building and on its roof was a series of ponds with a total area of 170 square feet and covered by movable insulating panels. This is all there was to it.

036-123-01afirst skytherm test building

During the day, the panels were closed so that the water would not heat up and at night the panels were retracted to expose the water to the night sky and radiate the heat it had gained during the day. In winter, the process was reversed and the water was exposed on sunny days and covered at night or cloudy days. Here’s what happened. It’s good.


In 1973 they built a second house in Atascadero, halfway between Los Angeles and San Jose on the California coast. This time, the house was about 1,100 sqft and the water was contained in plastic waterbed-like bags. The system had no pumps, compressors, piping, or ducts, and could be easily operated by the occupants. Indoor temperatures stayed between 68°F and 72°F degrees while outdoor ambient temperature fluctuated between 32°F and 68°F.


It’s been heating and cooling without electricity for the past 40 years now. Its only recognition was the 1976 Bicentennial awards for the categories of environmental and solar energy. [ Ref. ]

Whilst Hay and Yellott were exploring night sky radiation cooling for roofs with their Skytherm houses, Steve Baer was exploring its use in Water Walls. His 1972 house in New Mexico used a stack of 55-gallon drums filled with water to provide thermal mass. The south walls were glazed with single-pane glass but had insulated, reflective covers that are lowered on sunny winter days and closed at night. [pic from here – thanks Batiactu!]

The inside.

water wall

Since then, Steve Baer and Zomeworks have developed the Double Play Solar Heating and Cooling System which is both a passive solar heating system and a radiant cooling system for buildings. The Double Play system uses one or more absorber panels attached to the south side of a structure. (Mark Chalom’s research has shown there is only a 25% reduction in the cooling effect if the night sky radiators are vertically mounted.)

Double Play test building
Double Play test building

There is water storage in the ceiling and radiator/absorber cooling coils on the roof.


The system works.

double play

Zomeworks has also developed CoolCells which are highly insulated, passively cooled, outdoor enclosures that protect and prolong the life of batteries and low-power electronic equipment. They work too. [Here’s some performance graphs.]

cool cell

* * *

In the climates that allow it, the use of unglazed radiators for cooling can provide large cost savings because NSRC is cheaper than refrigeration in creating coolth, and because pumps are more efficient than fans in moving it around. According to Steve Baer, the radiator plate becomes a “sensor” that reacts to the multiple weather variables surrounding it, like wind and cloudiness, producing a coldness that is the coldest useful temperature available at that moment. 


Thinking about all the factors this way simplifies the calculation of the Night Sky Radiant Cooling Rate but, for us, all we need to understand is that night-sky radiant cooling uses 90-95 % less energy than air conditioners and 65-80 % energy than evaporative coolers to provide the same amount of cooling.