Understanding Climate and the Seasons

Posted in About Weather | General Interest.

We learn to live with the weather from almost as soon as we are born. It affects our moods, our health, our work and indeed the relationships we have with others. It is worth understanding a little more about the topic simply because there is little else that has so much of an impact on our everyday lives. We may try and shut it out, create our own micro climates: central heating, air conditioning and double glazing but one way or the other it is likely to affect us. The price we pay for our consumer services is very often in some way dictated, to a greater or lesser extent, by what the weather has been doing in the past month. With the greater emphasis we now place in the 21st century on understanding climatic change and the influence that this also has on our society, there has never been a better time to understand something of the basics of the science of meteorology; the weather.


The definition of the science of the weather is ‘study of the physics, chemistry, and dynamics of the earth’s atmosphere over both the land and the oceans’.

It is by studying these processes that we may hope to better understand -and make more accurate predictions- of all related atmospheric phenomena.

Climate and the seasons

Radiant heat from the sun acts as the principle powerhouse that drives our Earth’s weather and climate. This is not the case throughout the solar system of course; on the gas giants, such as Jupiter and Saturn there are far stronger processes that affect the atmospheric circulation at that distance from the sun partly as a result of the more rapid rotation, and that there are no oceans or landmasses to cause local heating.

On our own Earth, solar radiation is at its most intense nearest the equator where the sun’s rays strike at the most direct angle. The closer on the earth’s crust that any location is to either pole, the smaller the angle and hence the less intense the solar radiation reaching that point will be.

Put simply, our climate is based on the location of these hot and cold air-mass regions. The nature of hot air masses physically is that they tend to rise and expand outwards; cold airmasses tend to sink and contract. This process of expansion and contraction creates a complex atmospheric circulation around the globe, causing a series of winds to blow in different locations.

North of the equator, which is at 0° (degrees) of latitude, a band of winds known as the Trade winds blow from the northeast. These Trade winds were named as they initially helped traders develop routes between the old and new worlds. South of the equator, these trade winds blow from the southeast. Near to the equator there is a region between about 5°N and 5°S where the trade winds of the two hemispheres meet, a hot moist area with very light winds.

WindsAs the two trade winds meet they cause the air to rise. One of the fundamental properties of rising air is that as it does so it cools, becomes denser and moister. Eventually, as rising water vapour scooped up from the oceans with the airmass becomes saturated, clouds and rain develop. It is these resulting bands of cloudy wet weather near the equator that create the very warm humid locations that we call the Tropics.

Early mariners named these areas the Doldrums, because of the low spirits they found themselves in after days of no wind; becoming becalmed in this hot, muggy area could mean death at that time.

Closer to the poles an area of stronger winds circles the globe. In the northern hemisphere we know these as the ‘Westerlies’; they blow generally from the southwest here and in the southern hemisphere from the north west. It is these westerlies that steer the storms we see in winter across the Atlantic towards the western coasts of Europe.

The westerlies and the trade winds both blow away from a belt that lies at around 30°of latitude. In these regions air slowly descends to replace the airmasses that blow away from it north and south. As air descends it generally becomes less dense, gaining heat; any moisture the air contains evaporates in this intense heat. There is little or no rainfall throughout the year. Over the land we see vast regions of tropical deserts as a result, such as the Sahara in Africa and the Sonoran desert in Mexico.

To understand a little more about the processes that create the earth’s climate we need to understand that the Earth rotates about its axis, which is tilted at 23.5°. As mentioned above, the suns ray’s hit the earth’s surface at different angles, so transmitting different radiant energy levels as they do so. This varies the amount of sunlight that certain parts of the earth receive at different times of the year. In the far north and south near the poles there are periods during the year when these places are not facing the sun at all; it therefore remains completely dark; perpetual night time. Similarly, these areas see periods when they constantly face the sun rays, so seeing perpetual sunlight. At the equator more or less the same amount of sunlight is received all year round. As the Earth rotates on it’s tilted axis around the sun, different parts of the Earth receive higher and lower levels of radiant energy each day, each week and each month and it is this process that effectively creates the climatic seasons. We know them as spring, summer, autumn and winter and regulate our lives to them to a greater or lesser degree. 

Classifying the world’s climatic seasons

Up until the latter part of the nineteenth century there was no clear structure that enabled a useful classification of the different types of climate experienced by different parts of the earth’s surface.

However in 1900 a Russian-German climatologist called Wladimir Köppen divided the Earth’s surface into a number of climatic regions that generally coincided with world patterns of vegetation and soils. It was only when routine observations had been available from around the globe for a reasonable period that such a classification became possible. As such observation began around 1860-70 Köppen was able to piece together a reasonable idea of the different climatic regions by the turn of the century; more importantly he was able to look at the similarities that existed in very different parts of the globe.

His system recognises five major climate types, based on the annual and monthly averages of temperature and precipitation and each type is designated by a capital letter. 

A – Moist tropical climates where there are high temperatures all the year round and a large amount of rain all year. 

B – Dry climates where there is little rain all year and a large daily temperature range. (two subgroups, S – semiarid or steppe, and W – arid or desert, are also used with such climatic areas). 

C – Humid middle latitude climates where the differences caused by proximity of the land to the sea or other water plays a large part. Generally these climates have warm, dry summers but cool, wet winters. 

D – Continental climates that can be found in the middle of continents. Here the annual rainfall is not very high and the seasonal temperatures vary quite widely. 

E – Cold climates where there is either permanent ice or tundra (a cold desert) where only a few months of the year have temperatures above freezing, so giving a very short ‘summer’.

Later it become necessary to develop further subgroups; these were assigned a second, lower case letter to distinguish specific seasonal characteristics of temperature and precipitation.

f – These areas are quite moist with reasonable rainfall throughout the year and with no ‘dry’ season. The letter usually accompanies A, C, and D climates.

m – These areas are rainforests with a short, dry season in a monsoon cycle. We use the term ‘Monsoon’ to signify . The letter is only applicable to A type climates.

s – These areas have an almost or completely dry summer season due to the high sun angle

w – These areas have a dry winter season due to the low sun angle

The need to indicate some kind of temperature range for these zones was also catered for with the addition finally of a third letter.

a – signifies a hot summer period where the warmest month is over 22°C (72°F). These areas are found in both C and D climates.

b – signifies a warm summer period with the warmest month below 22°C (72°F). These can also be found in C and D climates.

c – signifies a cool, short summer with less than four months over 10°C (50°F) and can be found in C and D climates.

d – signifies a very cold winter period with the coldest month below -38°C (-36°F) and consequently is found in the D climate region only.

h – signifies that dry and hot weather predominates all year with a mean annual temperature over 18°C (64°F) and is found in B climates only.

k – signifies that dry and rather cold weather predominates all year with a mean annual temperature under 18°C (64°F) and is again in B climates only.

The classification thus allows for seasonal variations where they exist (such as in zone C and also accepts that in some areas there are no variations at all, all the year round, such as in zone A. Those living in the British Isles will probably recognise our climate as being of the ‘Cfb’ type. People living in the upper areas of the Highlands of Scotland or the far Northern Isles might like to suggest that their climate is often of the Cfc category however!