More solar energy falls on the North Pole during each summer day than ever falls at the Equator
Insolation – as opposed to insulation – is the the ‘amount’ of sunlight falling on a planet. Thinking about planet Earth, the variation of insolation across the planet surface is responsible for the main features of the Earth’s climate:namely that it is cold at the poles and the warm at the equator. However, on reading my favourite newsletter from NASA’s Earth Observatory the other day I read an article about the albedo of Greenland which said:
The top-of-the-atmosphere insolation at the North Pole peaks in June at about 520 watts per square meter. By contrast, the insolation at the equator peaks in March at about 439 watts per square meter.
In other words the article claimed that the peak insolation at the North Pole was greater than peak insolation at the Equator. My first thought was that this was obviously wrong, and I began to write a note to them. But then I began to ask if it really could be possible? And amazingly it is not just possible but a fact. It took me an hour or two to convince myself, but I will try to summarise.
The calculation: moderately complex: sorry.
Insolation is expressed as watts per square metre – which is a measure of irradiance or what we colloquially call the intensity or brightness of sunlight. However insolation is generally averaged over a period of time, typically 1 day, one month or one year. At the top of the atmosphere above the Equator the intensity of sunlight is around 1365 Watts per square metre. But the intensity of this sunlight falling on a particular part of the Earth’s atmosphere at the Equator varies through the day. It is 1365 Watts per square metre at midday, but falls to zero after the sun has set. This makes the daily average insolation only 432 Watts per square metre according to my rough calculation. This ties up with the 439 Watts per square metre quoted by NASA’s more accurate calculations.
As one moves away from the Equator to the Poles, the peak insolation falls because of the shallow angle with which the sunlight intercepts the Earth. At the height of northern hemispheres summer the Earth axis is tilted by 23° towards the Sun. This means the intensity of sunlight at the North pole at height of its summer is reduced to just 39% of the peak value at the Equator (cosine(90° – 23°) for those interested) i.e. the maximum insolation throughout one day is not 1365 Watts per square metre, but only 533 Watts per square metre. However at the North pole in summer, the angle of Sun in the sky does not change for 24 hours. So the daily average insolation is the same as the peak insolation. This rough figure of 533 watts per square metre ties up with the figure of more accurate NASA figure of 522 Watts per square metre. NASA provide a link to a site with more data in which you can make these calculations yourself. I made graph showing how monthly averaged insolation varies with latitude and it is clear that that even at my latitude in the south of the United Kingdom (53° N) more solar energy falls on the UK during each summer day than ever falls on the Equator at any time of year.
What this means.
I was amazed by this fact because I had always imagined the summers at the polar extremes on Earth were somehow very ‘weak’. I hadn’t quantified in my mind exactly what I meant by ‘weak’ but I think I just imagined that not much warming would result from this sunlight no matter what. This fact – that more solar energy falls on the North Pole during each summer day than ever falls at the Equator – has shocked me. As the original article said, it makes the albedo the northern hemisphere critically important for climate stability. As more lands and oceans become ice free in the summer and change from reflecting sunlight to absorbing it – lots of it I now realise – the amount of energy absorbed by the land and oceans will increase significantly.