Searing heat.
Freezing ice.
Monsoon rains and dusty droughts.
Spring.
Summer.
Autumn.
Winter.
For hundreds of millions of years they have they have defined the rhythm of nature on
our planet.
They underpin the daily existence of billions of us today.
A product of the clockwork of the heavens.
These are the seasons of Planet Earth.
When talking about climate, one cannot ignore the concept of the seasons.
It’s the variation in temperature and precipitation throughout the year that defines each climate
zone on our planet.
Each zone has a unique combination of seasonal patterns.
And these patterns determine the biome – the characteristic of vegetation – of the land
in that zone, what agriculture can be sustained there, and ultimately whether it can support
human population.
But what are the seasons?
What is their origin?
Why do they always repeat year after year?
Why are they different in the tropics than in the higher latitudes?
Why do they reverse in the southern hemisphere?
All these questions are answered in the relationship between our planet and its parent star -
the sun.
When a planet orbits a star, it traces an elliptical path that is always in the same
horizontal plane – the orbital plane.
But the planet itself will rotate on its own axis at some other angle – what is known
as axial tilt.
The result of this is that as the planet moves around the star, the angle at which the star’s
light strikes any particular point on that planet will change.
As surface and atmospheric heating is directly related to the angle of the star’s light,
the position of the orbit will therefore influence temperature for that region of the planet.
So every planet in the galaxy with an atmosphere will have some combination of seasons, and
so the phenomenon shouldn’t be thought of as unique to our planet.
In fact the planets Mars, Saturn, Uranus, and Neptune and Saturn’s moon Titan all
have distinct seasons that have been observed.
There are four significant points in any planet’s orbit in this regard – the first two being
when each of the planet’s poles is pointing closest toward the star, and the second two
being when the poles are exactly at right angles to the star.
In the case of our planet, our axial tilt is just over 23°.
So on the day when the North Pole is facing closest toward the sun, June 21st to be precise,
every point on a line encircling the earth 23° north of the equator will experience
a midday sun that is exactly overhead.
This is midsummer in the northern hemisphere, when heating from sunlight is at its greatest.
Exactly 6 months later, on December 21st, when the South Pole is facing closest toward
the sun, every point on Earth 23° south of the equator will have a noonday sun overhead.
This is midsummer in the southern hemisphere.
These two lines are respectively the Tropics of Cancer and Capricorn, called such because
the sun is in those zodiac constellations on those special days, which are known as
the summer and winter solstices.
The word solstice comes from Latin [sol + stit], meaning “sun stationary”, and literally
describes the stop and reversal of the sun’s highest or lowest daily point in its seasonal
progression.
Exactly in between the solstices, on March 21 and September 21, the Earth’s poles are
at an exact right angle to the sun, which means that on these days, the sun at noon
at the equator is directly overhead.
These days are called equinoxes - from the Latin [aequus + nox], meaning “equal night”.
The hours of day and night across every point on earth are the same at 12 hours each on
these special days.
In practice, midsummer temperature peaks lag behind midsummers day by a month or so – most
will experience that July and August are the hottest months in the Northern Hemisphere,
and not June.
And the same goes for winter, with January and February being the coldest, and not December.
This is called “seasonal lag” and occurs because it takes time for the earth and oceans
particularly to warm up in that area.
This cycle repeats itself near-exactly, year in year out, because the Earth returns back
to the same point exactly one year later, as regular as clockwork (if human-made clocks
could ever be so precise).
I say near exactly, because the Earth is in fact wobbling by a few degrees on its axis,
in the same way a spinning top might wobble.
But this wobble has a cycle of almost 26 thousand years, so… you’re not going to notice
anything in your lifetime.
Ok, so that explains the origin of the seasons, but the effects of this on daily weather depend
upon whether you live inside or outside of the Tropics of Cancer and Capricorn.
Within the Tropics, the changes in the angle of sunlight from one solstice to the other
has little or no influence on surface heating and so the tropics see very little change
in temperatures across the year.
Seasons in the tropics are in fact determined by the presence or absence of rain.
When the sun is at right angles to the earth, heating is at its greatest, leading to thermals
of hot moist air, which, when rising into the upper atmosphere, cool, losing their moisture,
and depositing rain in the form of thunderstorms.
As the sun tracks back and forth across the equator throughout the year, this band of
rain follows it.
This band is called the doldrums, or Inter-Tropical Convergence Zone (ITCZ), as it is the convergence
of tropical winds, known as trade winds, being sucked into it from surrounding areas.
When these winds carry moisture, they also bring rain, and so this complex pattern determines
what areas will be wet or dry during what season.
Only where the ITCZ is stable, or where trade winds carry moisture regardless of direction
do we have the Tropical Rainforest, where rainfall and temperature are near constant
– the only climate on earth without seasons.
Outside the tropics though, these sunlight angles become important.
If you know anything about trigonometry, then you’ll get the idea that as the angle of
sunlight to the surface becomes shallower, there is an acceleration of the effect of
that heat spreading out across the surface and being diluted in power.
So that by the time you reach the poles, the heating effect of the sunlight is only a tiny
fraction of what it is at the equator.
In the mid latitudes, the change in seasonal angles is significant enough to produce large
changes in temperature.
In summer the sun is high above the horizon, concentrating heat and delivering warm or
hot days.
In winter, it’s much closer to the horizon and so surface heating is much less, with
the result of significantly colder days.
This variation in temperature in the mid-latitudes is diluted, however, by the proximity of oceans.
Being a large fluid mass, kilometres deep, the oceans do not heat as much in summer as
the land, and do not cool as much in winter.
So land areas near an ocean have much smaller winter/summer temperature ranges than those
in a continental interior.
This difference in coastal and interior continental temperatures is at its greatest in the mid-latitudes
and is the essence of why we have more climate types in this region of earth than any other.
In the polar regions, the effect of axial tilt is at its extreme.
Above the polar circle at 67°N, there is at least one day each winter when the sun
does not even climb above the horizon, and at least one day each summer when the sun
never sets.
As one approaches the pole, these number of days increase until at the North Pole, the
sun doesn’t rise for 6 months in winter, and stays above the horizon for 6 months in
summer.
The same occurs at the opposite time of year in Antarctica.
In the Koppen climate classification system, we have categorised patterns of temperature
and rainfall determined by the seasons.
You can learn more about this in my intro video linked top right, or by watching my
Secrets of World Climate series which covers each zone in detail.
What you almost certainly haven’t seen before, is a representation of seasons across all
Koppen zones in one image.
And here it is.
For readability, I have grouped similar zones together or omitted rare or isolated ones.
This list shows the most important and distinct zones, starting at the equator at the top
and finishing at the pole at the bottom of the page.
For each zone I have depicted the change in temperature across the year in a colour spectrum,
from red as the hottest to purple as the coldest.
Below each temperature progression is a progression of precipitation, with rain in blue, snow
in white, and intensity of such precipitation as proportional shading.
These patterns are for the northern hemisphere and will naturally be reversed for the southern.
I have provided a link to a high resolution version of this image in the notes accompanying
this video so you can pore over it at your leisure, but for now, I will highlight the
most interesting features.
Up top we see the unchanging single hot and wet season of the Tropical Rainforest.
Next we see the change in rainfall across the year in the Tropical Monsoon and Savannah
climates, and a slight bump in temperature just before the wet season.
After this we see the unique Subtropical Highland climate, where temperatures are reduced due
to altitude.
Note that there is a form of this climate where the rain is constant year round, but
is not shown here.
Then we come to the Hot Desert, where there is little to no rain, but intensely hot conditions
in summer.
We then have the unique three season climate of the Subtropical Monsoon - a mild dry winter,
then a short and very hot dry summer, followed by hot wet season.
As we move into the mid latitudes, we get our first cool winter temperatures in the
Humid Subtropical, followed by the unique dry summer /wet winter pattern of the Mediterranean
climate zones.
This latter, together with its westerly ocean facing sibling, the Oceanic, demonstrate mild
temperature ranges from winter to summer.
By contrast the Continental climate zones that follow show much more extreme temperature
ranges, with subdivisions of these being the cool deserts, with absence of rain, Humid
Continental, where we see snow and rain defining seasonal precipitation and Continental Monsoon,
where this pattern is varied by an absence of winter precipitation.
The final continental climate of the Subarctic is then seen with extreme temperature ranges
featuring very cold winters yet warm summers.
As we reach the poles, freezing temperatures dominate with only a brief cool summer in
the tundra bringing any respite to these forbidding lands.
And so there we have the seasons of Planet Earth.
I hope you found it enlightening in answering your questions about the changes throughout
the year that affect us so much.
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Thanks again for watching and I’ll see you soon in the next chapter of my Climate Casebook.