Friday, 17 July 2015

The Seasons – Why summers are hot and winters are cold in the N-hemisphere, and CO2 levels oscillate over the course of a year?

 “Seasons, more than anything else, influence human, animal and
vegetation activities, over the course of a year”
I am often asked about the seasons and what causes them? I thought it might be a good idea to have a separate blog entry for this interesting subject.  What I found particularly exciting that one can easily calculate the maximum angle that the Sun will reach during each season – I shall calculate it for Glasgow, Scotland but the recipe does it for any latitude.  Seasons in the Southern hemisphere are exact opposite to those in the Northern hemisphere but the science is the same.
Our Earth rotates round the Sun in an elliptical orbit, and its orbit describes a plane -  the ecliptic plane. The Earth is spherical – a bit flattened at the poles due to the large centrifugal forces during its cooling down a few billion years ago and it spins about an axis passing through the North and South poles.  Seasons happen because the Earth’s spin axis is not perpendicular to the ecliptic plane but is tilted at 23.5 degrees to the Earth’s plane of rotation around the Sun.  This means that during the summer, the N-hemisphere is tilted towards the Sun and receives more light at a steeper angle while in winters, after six months, the situation is opposite to that in the summers. The days are longer during the summer while in winters the days are shorter.  At Equinox, in March and September, the days are 12 hours long.  This is best explained through pictures; the slides follow below.
The lowest elevation of the Sun at winter solstice is (90 – latitude – 23.5) degrees
The highest elevation of the Sun at summer solstice is (90 – latitude + 23.5) degrees
Glasgow at 56 degrees has elevations of 10.5 and 57.5 degrees at winter and summer solstice times. That is why it always seems dark in Scotland during December!
London is at 51.5 degrees, Sun elevations will be 15 and 62 degrees at winter and summer solstices.
For New York (latitude = 40.7), they are 25.8 and 72.8 respectively.
Notice that the difference in Sun’s elevation at the solstices is 47 degrees at all places
At tropic of Cancer (latitudes +23.5), the Sun will be at 90 degrees (straight overhead and very warm) at summer solstice.
At 66.5 degrees latitude the Sun elevation is zero at winter solstice and it is below horizon on that day.
The Sun’s elevation determines the length of shadows cast on the ground.  Higher elevation gives a shorter shadow (see slides below) and sunlight falls on a smaller area making the Sun feel hotter during the summer.  Reverse is true in the winter.  
Additionally, at higher latitudes (see slide 3) sunlight travels through a greater length of the atmosphere than at lower latitudes.  This causes more absorption of energy and the sunlight carries less energy. 
Another  interesting observation is the variation of measured CO2 levels over the year.  In the N-hemishpere, CO2 levels are about 4% lower at the end of the summer than at the start of spring.  This is simply because during the summer, the vegetation grows rapidly and uses up lot of CO2 present in the atmosphere.  During the winter, photosynthesis is much lower and uptake of CO2 is corresponding reduced.  Dead vegetation also returns some of the CO2 back to the atmosphere giving a peak in its levels. This is shown clearly in the slides.  Another interesting observation is that the difference in CO2 levels during the summer and winter in the southern hemisphere is much less pronounced than in the northern hemisphere because much more of the northern hemisphere has forests and green area than the southern. 

(Click on a picture to view its bigger image)

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