~ Ice formation is not strictly the result of colder temperatures
Unlike surface water, sea ice has rapid thermal variation characteristics which enables an instant evaluation of its thickness and age.
Lets go back in time, a March 2004 sunset comes to mind:
Apparent high Arctic sunset disappears in mid air. That was not so. The sunset was much raised with the ice horizon. By 11 arc minutes. The horizon was higher, the entire sun disk transforms in turn. From ovoid to becoming saucer like, eventually morphing to a sun line which was highly compressed sun disk image. Arctic haze, more common then, masked the raised ice horizon. At the time, sea ice over the NW passage was a mix of old and new. Multi-year ice made the onset of freeze-up earlier in autumn 2003. The end result was thicker ice many months later in March when this sequence was taken.
Refraction effects can be even more complex. Horizontal refraction permeated the lower horizon, seeming to dissect the air
layer by layer, making the sun's upper limb wearing a green flash hat, or at the end revealing gravity waves streaming.
Fortunately sun disk effects are much more complicated than the rising horizon. There is so much air thickness between sun and camera which absolutely demands a greater knowledge of great chunks of the atmosphere.
Inspiration to study something apparently simpler came about. To make sense of all these mirages a steadier subject was needed. Elevation wise, sea ice varies throughout the day. Can be studied at any time for days especially with good visibility.
From this visual aspect, using a baseline value for sea horizon height having water and surface air with the same temperature, a retrospect evaluation of sea ice thickness is possible. On this March 2004 day, there was likely a sea ice mean thickness of 3.2 meters over a huge area of the NW passage.
1- Instant Identification of sea ice status, freezing, melting or steady state
Long wave radiation has a way to be revealed, if a body emits a great deal of it, the immediate area adjoined will be warmed. In the case of sea ice, sun rays heats sea ice surface, but warming comes from below as well, Arctic sea surface temperatures are almost always higher than air during winter, this continuously tends to soften the ice bottom making it become brittle or fragile. If the sun is strong enough, the sea ice to air interface looses its boundary layer, or structure.
2- Determining ice thickness by refraction
There can be several methods of determining sea ice thickness optically, which means immediately. During the long sunless night, the horizon was highest when radiation escaped to space directly without interference by clouds, which may affect the radiation balance at sea to air interface. On a mostly clear evening the ice horizon peaks after sunset:
Studying instantly whether sea ice is growing adds a better understanding on how accretion works, in the case of winter 2012- 2013 freezing season, there was melting as early as March 18, at about sea ice maxima peak. It was short lived by a few hours, subsequently melting grew longer day by day, eventually the melting period of any given day will exceed the freezing once the horizon is lower than a steady state level more than 12 hours a day. Freezing is not a continuous process dependent on degree days, but rather whether the radiation budget of sea ice is in a loss or gain mode. Remote sensing merely calculates the gain or loss of sea ice volume or extent, but horizon measuring explains what is happening directly. The two methods of visualizing sea ice complement each other, and should be utilized in improving sea ice models. WD April 14, 2013
High Arctic Earliest melt, West of Southwest Cornwallis Island
Cryosphere is not a coincidence. The returning sun stops accretion and causes the melting of sea ice, very early on particularly on its bottom. Surface Sea ice may appear cold, normal and well set, but that is not the case on its underside. It may be very cold outside, but sun rays on a clear sunny day have a huge impact over lower accretion action. Above is last 4 seasons spring collage. 2011 was the coldest year sea ice wise. The first column displays earliest bottom melts. 2012 and 2013 had very low horizons way earlier, although 2010 data was scarce in February. Second column displays first extreme low horizons, all within March 11 and March 19, sea ice Maxima period. Third column sets the date when continuous low horizons were seen afterwards. 2011 was particularly good for sea ice preservation. Late February shots were at sun elevations 5 to 7 degrees during Local Apparent Noon. Indicating a sun merely 6 degrees high has an impact over the energy balance of ice. Thus 11.8 to 15 degrees elevation sun causes certain melting on the underside, while 5 to 7 degrees elevation noon sun can do likewise but the conditions must be right, thinner ice and likely less snow insulation may be major factors. 2012 and 2013 were noted as having earliest underside melts, this is likely because of the disappearance of multi year-ice. WDApril 21,2013
Current NOAA North Pole Cam :
Using fix point reference the radiometer box,
Lets go Surfing
This method applies for any horizon, including Redondo beach L.A.
May 6, 2013:
What about Scotland?
and at evening end. WD May 1,2013
Scotland never disappoints look carefully:
Fraserburgh astronomical horizon.