Happy Solstice, everyone!
The previous post on melting momentum was running long, so here's an addition dealing with compactness, the final piece of information we have to assess the amount of melting momentum. Just before the start of July, the month of big melt. First, a couple of paragraphs to explain what compactness is and what it can tell us.
Not only melt ponds, but also open water between ice floes can absorb solar radiation, heating up the water, and then speeding up the melt process. We can get an idea of how compact an ice pack is by dividing sea ice area numbers (SIA) with sea ice extent numbers (SIE), the products of two different ways of measuring total sea ice cover.
The idea is simple, really. The Arctic is divided into grid cells, and each grid cell has a certain sea ice concentration. On the right there's an example of one such grid cell, with white representing ice and blue representing open water/melt ponds. This grid cell measures 10 x 10 km, and thus has a total area of 100 km2. Sea ice concentration is 80% in this grid cell, which means that 80 km2 is covered with ice. This is the number that represents sea ice area.
However, for sea ice extent there's a so-called threshold of 15%, meaning that if there's 15% of sea ice concentration or more, the entire grid cell is considered ice-covered. In other words sea ice extent for this particular grid cell is 100 km2. The reason for this different way of assessing total ice cover is to reduce the influence of melt ponds, that can fool satellite sensors into 'thinking' they're seeing open water.