Hot, cold and dry!

Last time, we covered the extremely humid and wet areas of the planet, the rainforests.  Today we go to the other extreme, the desert, and semi-desert climates.

These are the climates classified in Group B in the Koppen Classification, and are distributed as shown in these images below:

Hot desert regions, BWh in the Koppen Climate Classification. Image from Wikipedia

Cold desert regions BWk classification. Image from Wikipedia

Semi-arid regions BSh and BSk classification. Image from Wikipedia

All these regions are extremely dry, and most are extremely hot too, as you can see, these regions include the Sahara, the Gobi desert, Arizona, Nevada, Central Australia, Iran and large chunks of the Middle East.  Whether an area is classified as arid or not depends on the evapotranspiration that I mentioned last time, which is the sum of evaporation and transpiration from plants.  This image illustrates evapotranspiration:

Evapotranspiration. Image from Wikipedia

All the arid, and semi arid regions have more evapotranspiration than precipitation (fancy word for rain and snow etc, stuff that falls out of the sky).  This means these areas are not able to support much plant life, and consequently, the number of animals which can live in the region are limited.   There are of course, plants which survive, and indeed thrive in these conditions, and they have some amazing adaptations to allow them to do so, the same for the animals which live in these conditions.

It is often assumed that deserts have no rain at all, but, some have over 25cm a year.  The evapotranspiration however, is greater than this.

So, what makes these regions so dry, and mostly hot?

You remember last time I mentioned the ITCZ?  Well, arid climates fall around 30°S and 30°N, which is between the first green lines above and below the equator in this image:

Arid regions are located around the bands with an H to the north and south of the Equator (marked by the ITCZ on this image). Image from Wikipedia

As I mentioned in my first post on climate (HERE), the air circulations from the equator to the edge of the tropics are known as Hadley cells.  To recap, the warm air rises at the equator, and moves north or south (depending on the hemisphere), as it cools, it begins to descend.  This descent occurs around 30°S and 30°N.  This air has deposited much of the moisture it picked up at the equator already (mostly over the rainforest regions), so it is much drier when it reaches these latitudes.

For interested geeky parties like me, these latitudes are also known as the Horse Latitudes (Allegedly due to some old naval tradition for running around on a deck of a ship with a horse…).  They are characterised by high pressure, which leads to clear, cloudless skies (due to the lack of water vapour in the atmosphere) and very high temperatures.  Up here in the North, when we have high pressure it is associated with those lovely (Or not, depending on your perspective) hot summer days, and those crispy cold awesome winter days (Guess which I prefer!).  The lack of clouds in these regions means that whilst the days can be scorchingly hot, the nights can be extremely chilly. The highest recorded temperatures are from desert regions, and they can reach over 40°C in summer on a regular basis.

The deserts which are not hot all year round tend to be the ones at a higher altitude, and these occasionally receive snow in the winter, but their summers are still very hot, and they have very little rainfall.

The semi-arid regions are found bordering desert regions, and the main difference between these, and the arid regions is that they receive a little more rainfall, and so are able to maintain a wider variety of plants and animals.

Now, I should mention that whilst these areas are deserts, Antarctica is also one huge desert, but is not on the list of arid regions.  This is because Antarctica has a polar climate, which we will get to in a later post..

Next time, we will move onto the climate I am most familiar with, the wet, windy temperate climates, where we will bump into the Jet Stream, the Gulf Stream, and probably a few other streams too!  I will try not to make it too long, promise!


Classifying climate

So, as promised last post, today we will look at the different climate zones on the planet, and some of the main features of each.

As mentioned last post, climate refers to the long term patterns in a region, and these are usually classified using the Köppen classification system, which divides the Earth into regions with similar annual variations in climate.  This is how the planet looks under this system:

Köppen climate classification map. Image from Wikipedia

Don’t be put off by all the strange abbreviations at the bottom, the basic thing is that there are 5 main groups, and then lots of sub-groups within each one.

Starting at the beginning of the alphabet:

Group A is the Tropical climate regions, this is broadly the band around the equator, where we find tropical rain forests (group Af), monsoons (Am), and Savannah regions (Aw).

These regions have some of the most productive places on the planet, covering the Amazon Rainforest, the Indonesian rainforests, and the rainforests of central Africa. They also encompass the great plains of Africa, the Serengeti, and places in the US such as Hawaii (Aw) and Florida (Am) and the northern coast of Australia (Aw).

So, what does it mean to have a tropical climate? Well, it means the mean (average) temperature is 18 degrees centigrade (Which is a nice warm day for me here in Denmark!), it also means that the day length is roughly equal throughout the year, and the temperature does not have the wide range of seasonal, or daily variations you find in the mid-latitudes.

Looking a little deeper into each: Af, the Rainforest zones, are the zones closest to the equator, these have minor variations in weather throughout the year, being situated in the Intertropical Convergence Zone that we covered last post.  This image shows the location of the ITCZ in January, and July each year, and you can see that it lines up with the rainforest, and other tropical zones.

Intertropical Convergence Zone location. Image from wikipedia

The rainforest zones have a lot of rain, each month, the average rainfall is around 60mm, which means they get 720mm of rain, on average, every year. The actual rainforests themselves have far more than this, with an average 200cm of rain per year.The reason for this difference is that not all locations within the rainforest zone are actually rainforests, there are cities, and arable lands, and grassland within these areas.

These regions, as well as being very warm, are very humid (As warm air can contain more water than cold air, and there is less wind than in other regions to disperse the moisture heavy air). Much of this humidity is from evaporation from the oceans, but large rainforests such as the Amazon have high levels of evapotranspiration (Evaporation from plants, and transpiration, which is the loss of water from plants when they have their stomata open) that much of the rain which falls over these rainforests actually comes from within it, meaning that they have their own mini water-cycle going on!  This is also why rainforests have a higher rainfall than the zone as a whole.

The tropical monsoon zones (Am) are characterized by, as the name implies, a monsoon, and have a dry month, and little annual variation in temperature.

We usually associate the term monsoon with extremely heavy downpours, but, the term monsoon does not refer to the rain specifically, but to the change in wind direction, which brings this rain.  The change in wind direction has different causes on different continents, but, is (usually) predictable, and arrives at the same time every year, meaning many of these regions have agriculture which relies on the arrival of the monsoon each year at a certain time for the crops, and any delay or shift in timing can have a large impact on the crops, and the food supply.

The monsoon we are most familiar with is the Indian monsoon, so I will briefly explain the mechanism behind this.

In India, in the summer, the air is very warm, the further you go into the continent, the warmer the air becomes, and at the northwest end of the sub-continent is a desert, the Thar desert, which has extremely warm air during the summer months.

This warm air rises, and because the air which has risen needs to be replaced, cooler air moves in, and this air moves from the ocean northwards over the continent.  This air contains a lot of moisture, from evaporation over the ocean.

So, now we have this very wet air moving in to replace warm air which has risen, which would result in normal summer rains if it was not for the rather large blockade at the northern end of India, which we know as the Himalayas.   The warm, moisture filled air cannot move further north into Asia due to the mountains, and it is forced to rise.  As it rises, it cools, and cool air cannot hold as much water as warm air, so, like the bottom of a water balloon bursting, the moisture is deposited out of the atmosphere back to the ground.

Whilst this is vital for agriculture and the plants on the subcontinent, there is also a lot of damage caused every year by flooding and landslides, with houses and roads washed away.

Chittagong in Bangladesh is a city within the tropical monsoon zone, and these next images show the average temperature, and the rainfall for each month.  Note the low variation in maximum temperature, and the very obvious spike in rainfall during the monsoon season.

Average Temperature Chittagong. Image from

Rainfall in Chittagong, Bangladesh. Image from

The cool thing for me about the Indian monsoon is that it only exists because of the Himalaya mountain range blocking the passage of the air further into Asia.  Why do I think this is super cool?   Because, the Himalayas are a relatively new mountain range, geologically speaking, and are only there because India went zooming across the ocean faster than continents usually move (one theory suggests this is due to the plate being thinner than most other continental plates, and so lighter, and able to move faster.  It may also have got a slight speed boost by passing over a hotspot in the Earths mantle) and smacked into Eurasia around 50 million years ago.  To put this in a time perspective, this was after the era of the dinosaurs.  The Himalayas are currently growing at around a centimeter every year, due to the Indian plate moving north-east faster than the Eurasian plate is moving north.

Finally, onto the Savannah zones (Aw).  These regions have very obvious wet and dry seasons.  While these areas can have a lot of rain during their wet months, it is not enough rain to qualify them as monsoons.  This change in weather is due to the movement of the ICTZ, and the rainbelts which lie around it.

This image illustrates the dry season in Brasilia, which is as marked a difference as the monsoon in India:

Rainfall in Brasilia, image from

Eeep….this post got a little longer than I expected, so, next time, we will briefly (I promise!) cover the B and C zones!