Summer sunlight
Why summer sunlight at 65°N matters
Big ice sheets grew at high northern latitudes, and whether they grew depended on how much winter snow survived the summer. Midsummer sunlight at 65°N is the standard yardstick for that.
Daily average at the top of the atmosphere, on the summer solstice
High northern latitudes
65°N crosses northern Canada, Scandinavia and Siberia, close to the old ice sheets.
Midsummer
Summer sets how much winter snow melts.
Incoming sunlight
Insolation is energy arriving at the top of the atmosphere. It is not a temperature.
Summer matters more than winter
An ice sheet needs snow, and it needs that snow to last through the summer. A snowy winter followed by a bright summer leaves nothing behind. A cool summer lets snow pile up year after year.
That is why Milanković’s theory looks at the summer energy budget at high northern latitudes. It links the orbit to something that decides whether ice grows: how much snow melts.
Why 65°N
The 65°N line runs through the zone where the large northern ice sheets grew. It gives one number that is easy to compare across times.
Researchers also use other latitudes, other seasons and sums over the whole summer. The solstice value at 65°N is simple to explain and follows the same rhythms.
What the number can’t tell you
It is sunlight at the top of the atmosphere. It does not give air temperature, snowfall, ocean currents, greenhouse gases or ice volume. Those depend on the atmosphere, the oceans, the land, and the climate that came before.
A higher number can encourage melting. How much melts depends on all of that.
Try it yourself
Change the orbit
Move all three motions and watch midsummer sunlight at 65°N change.
Questions
Quick answers
What does insolation mean?
Incoming solar radiation. It can be given for any latitude and day, at the top of the atmosphere or at the ground. Here it is the daily average at the top of the atmosphere.
Why not use the global yearly average?
Melting is seasonal and local. The global yearly average hardly changes with the orbit. What changes is how sunlight is shared between seasons and latitudes.