~Optical Thermal observation method further explained, proving Ti<=Ta
~Likely 24 hour bottom melt earliest captured....
Preceding article questioning NCAR calculations can be seen here. The sea ice Horizon would
drop below Astronomical Horizon (AH) if top of sea ice was warmer than surface air. In many years of observations it was never observed doing that, the much lower sea water horizon observations with colder than sst air were never repeated with ice. Instead spring sea ice horizons maintain AH until evening or until under sea ice melting is 24 hours a day. This likely happened yesterday, South Cornwallis Island looking at westward MW Passage.
On a given Arctic spring day, the horizon drops to AH when the air temperature Ta is equal to top of sea ice temperature Ti. When reaching AH, it is highly likely that the bottom of sea ice melts,
but during spring the AH horizon lasts a few minutes when it first shows, in March or early April, so accretion keeps on making net gains. AH horizons gradually become longer, but when AH is maintained more than 12 hours, the bottom of sea ice melts more than forms, net bottom melting occurs. This has happened yesterday, when AH was observed 1 hour before the midnight sun. For the first time I have observed this in May, this makes Spring 2016 fast ice the weakest heat resisting sea ice observed since 2010 when spring observations have started.