~Even when some part of sea ice column is always warmer than air (during winter).
~Sea ice horizon never been observed below Astronomical Horizon has now an explanation.
The best way to sum up horizon refraction throughout the Arctic Ocean year is by this sketch once posted here:
Strictly by several years of observations, a visual correlation was made with temperature profiles from sea water to upper air related to physical conditions of the horizon. Notice top of sea ice temperature was never observed warmer than the air immediately above. Only with the presence of sea water does the horizon elevation drop below the "Astronomical Horizon" ( orange line). The Astronomical Horizon of any planet Earth location would permanently remain at the same unchangeable altitude if our planet did not have an atmosphere.
Many years of sea ice horizon observations gave a proposed theory written here and here.
The biggest feature of sea ice horizons happens when the sea ice horizon stops going down, does not go below the Astronomical Horizon, settles there until the sun lowers in the sky to set, only to spring up higher again. Its the spring time great steady "LAN" horizon which may happen daily for a while after Local Apparent Noon. When so, the temperature profile at interface between ice and air temperature is isothermal. This feature also makes it possible for well informed sea Navigators to recognize the presence of sea ice without radars.
Sea ice Buoys offer proof, despite their near or above ice thermistor problems. Selecting a thermistor embedded in top of ice usually should give good results, so without further a do, lets use 2015 F at thermistor T5 (50 cm down from top of thermistor string):
|Buoy 2015F August 13, 2015 to April 19 2016, 4 hour interval surface temperature (in blue) Thermistor 5 (in red 50 cm down).
Temperature of sea ice was always Greater or Equal to surface air, except for a few very rare interesting occasions.
If top of sea ice was always warmer than air, there should be a permanently very low sea ice horizon. This does not happen, not only because of upward thermal heat flux from sea which warms the lowest atmosphere causing a near surface inversion or a well above upper air temperature profile maxima. The dark season thermal flux is stronger nearer to ice, but there is an inversion right above, something cools the surface to air interface. It is wonderfully complicated. Heat Capacity of sea ice and snow is twice more than air. Thermal Capacitance plays a role, Heat Conductivity and especially insulation properties of sea ice are very important. Eventually, the combination of properties cause top of the ice always colder than air, as may be seen on your own house:
Physics replicates itself with different matter, in this case house insulation. Note in blue, top of insulation is always colder or equal than air except when too sunny. Consider sea ice as insulation, the same happens over the frozen sea. But also again identical with middle of sea ice column as with buoy 2015f graph above, center of insulation layer is almost always warmer than air, this is a good model proxy presentation for sea ice.
As observed optically following Local Apparent Noon (LAN) with the sun present, the temperature lapse rate of the surface to air interface appears to become isothermal over sea ice, the horizon is at the Astronomical Horizon. Considering an hypothetical, if top of sea ice would remain cold, unaffected by shortwave radiation, the horizon would remain higher than Astronomical Horizon.
Finding a Buoy replicating the house insulation graph would be great. However, there is a problem with sea ice buoys, they seem affected by sun rays, and there is few other considerations to take, the exact position of the thermistor matters, the coldest layer may be at a certain height not always placed with a thermistor. Lets try to idealize a true measurement of top of sea ice as much as possible (or in the snow layer next to it). The only way around is to find measurements in darkness, away from sun rays affecting the thermistor, lets try close to the North Pole Buoy 2015l:
In Darkness 4 hr interval readings 2015l November 1 to 8 2015 1st thermistor called 31 (in blue) is always colder than surface air (in red).
2015l December 1-7 2015, always colder or equal. Top thermistor wonderfully matches optical physics observations well in darkness or spring. Likewise, I have filmed in darkness no ice horizons very close to astronomical horizon as in spring, the warming of surface to air interface occurs rarely during the long Polar night.
2015l January 1-7 2016, the data is overwhelming, top of sea ice is always colder or equal to surface air. [Or perhaps inside snow next to ice, but snow sensor did not seem operational].
Sun presence might have affected a few readings, 2015l September 22-29 2015
Top of sea ice is always colder or equal than surface air, this is a profound conclusion from refraction observations. Adding a better view of the complexity of sea ice thermal physics. WD April 28 ,2016.