Geoff’s Climate Cookbook

Written by Geoff Eddy

All text in green added by The Astrographer

Last updated: 27 February 2006

With grateful thanks to krinnen, aka Gonzalo, who drew the pictures.

This should provide more reliable hosting for the page, but the formatting suffers a bit in adaptation to wordpress. I’ll do what I can, but I’m not really a wp guru…

Contents

Introduction

This page is part of my essay about creating an

Earthlike planet; it is intended to guide the creator of such a

planet, after he or she has drawn a Map, through the process of

working out the climates which characterise a particular area. As far

as learning about the physical causes of climates goes, there’s no

substitute for a good textbook; however, textbooks tend to work

backwards from observed phenomena to inducing the causes, whereas the

typical conworlder needs to know the causes before he or she can

deduce the observed phenomena – which is what this page is for.

Please note that predicting climates is notoriously complicated and

full of approximations, which is why there are no equations on this

page and very little quantification. Ideally, I would be able to offer

a program which would convert a Map of a planet and its physical data

– such as axial inclination and distance from the sun – into a diagram

showing the climate at every point of interest on the planet’s

surface; when I’ve written this program I will be able to retire for

good on the money. In the meantime, the best I can do is talk in

generalities without going into too much specific detail.

If you find this page useful, please let me know! As ever, I welcome

corrections and suggestions for improvements.

Basic principles

Virtually everything important about climates can be deduced from the

following physical principles, which are referred to in [square

brackets]:

All heating comes from the sun. Water heats and cools much more slowly than land; water thus

acts as a stabilising effect on temperature. Hot air rises, cold air sinks; this is because air

expands as it heats up and thus becomes less dense. Cold air gives rise to areas of high pressure, and

hot air gives rise to areas of low pressure. Wind flows from areas of high pressure to areas of low

pressure. Due to the Coriolis effect – the effect of the rotation of the

earth on the flow of air – winds are deflected to the right in

the northern hemisphere, and to the left in the southern. Rising air is conducive to the fall of precipitation, sinking

air is not. Warm air carries more moisture than cold air.

Ingredients

You will need the following items before you can proceed any

further.

The axial inclination of your planet, which is 23.5

degrees for the Earth. The lines of latitude at this distance

from the equator are known as the tropics, and those at

the same distance from the poles are called the polar

circles. Two identical copies of your Map, which should show the

locations of as much land as you know about, the

locations of the mountains, the lines of

latitude in increments of no greater than fifteen

degrees, and the tropics and polar circles. Label one copy

“January” and the other “July”. A transparent drawing medium which can be marked and erased

without damaging the Map. In the physical world, this means

several sheets of tracing paper or something made of clear

plastic; on a computer, the equivalent is a drawing program

which can handle layers, such as the GIMP. Something erasable with which to draw on the transparent

medium; for tracing paper, coloured pencils (not pens)

are suitable. You will need several colours. Something with which to erase the above, because you

will make mistakes, and lots of them.

The following assumptions have been made:

Your planet rotates from west to east, like the Earth.

Your planet has a similar diameter and rotation period to the

Earth. These quantities are respectively 12750 km and 24

hours.

For ease of reference, “January” and “July” refer respectively to the

periods shortly after the sun reaches its furthest south and north

respectively, and “April” and “October” to those just after it passes

directly above the equator northwards and southwards respectively. The

“just after” is necessary because the atmosphere acts as a drag on the

heating and cooling processes; thus the hottest time of the year in

the northern hemisphere is typically around mid-to-late July, some

weeks after the summer solstice on 21 June.

This image lets you know when you

should think about drawing something.

Pressure-cooking

The first stage consists of locating the large-scale areas of high and

low pressure.

The default

The most important is the low-pressure belt called the

inter-tropical convergence zone, or ITCZ, about which the

temperature and pressure characteristics are theoretically

symmetrical; this zone is caused by the rising of hot tropical air

[3][4]. In April and October, the ITCZ lies more or less along the

equator. In the northern summer, it moves northwards, reaching its

farthest north in July; its most southerly position is attained in

January. The range of movement on Earth is about 5 degrees of latitude

over the oceans, and up to 40 degrees over land.

About one-third of the way from the ITCZ to the poles is the

high-pressure belt known as the subtropical high-pressure zone,

or STHZ, which is caused by air from the ITCZ cooling and sinking back

to the ground [3][4]. Between the STHZ and the poles is the polar

front or PF, a band of low pressure where cold air from the poles

meets warm air from the STHZ. The interaction between these air masses

at the polar front is responsible for the rain-bearing low-pressure

areas familiar from weather forecasts.

If the surface of the planet was uniformly water, the distribution of

these pressure belts and the prevailing winds

would be as shown below, allowing for seasonal movements, which would

be slight.

Adding land

The presence of land has two effects on the pressure distribution,

both results of principles [2][3][4]: the pressure belts front bend

northwards over land in July and southwards in January, and they are

broken up by seasonal pressure-areas over the land. In general, the

larger the area of land, the more noticeable the effect.

In winter, the cooling of the land creates a high-pressure area over

the interior, which merges with the high pressure area around the

STHZ and leaves low-pressure systems over the oceans:

while in summer the land warms to create a low-pressure area, which

joins up with the ITCZ and the PF, leaving high-pressure areas over

the oceans:

In general, these pressure areas are located east of the longitudinal

(east-west) middle of the continent, and are more intense when the

surrounding land mass is larger. This is particularly noticeable with

Asia; if the Eurasian landmass was reversed laterally, the pressure

areas would be considerably less intense. Correspondingly, the

pressure gradient is greater on east coasts than on west coasts; the

precise difference depends on the shape of the continent.

Figures 7p-4 and 7p-5 on this

page show how this works out for the Earth; the animation, one of

many from here,

is also here. Note

particularly the considerable northward movement of the ITCZ in July

over Africa and Asia, the continuous low-pressure zone over the

Antarctic Ocean where there is no land to disrupt the southern PF, and

the change in the air pressure over the interior of eastern Asia.

You need to draw similar diagrams

showing the pressure for January and July. Start by drawing with the

ITCZ, STHZ, and PF, then locate the continental pressure-areas, and

finally join them up as in the diagrams. Different colours for each

stage are a good idea.

Ventilate…

Wind, in meteorological terms, is a flow of air from an area of

high pressure to an area of low pressure [5]; the strength (speed) of

the wind increases with the difference in pressure. Winds have two

important effects on climate: they transport moisture, and –

for our purposes – they are the principle cause of the ocean

currents. Winds pick up moisture as they blow over the oceans and

deposit it as rain or snow over land. Obviously, a wind can only carry

a finite amount of moisture, so it wil become dry after blowing across

a large area of land.

Winds

The winds we are interested in here are those which blow at the

surface. Because of the Coriolis effect [6], the winds do not blow

directly from high pressure to low pressure, but are deflected to

blow, in the northern hemisphere, clockwise around high-pressure areas

and anticlockwise around low-pressure areas. In the southern

hemisphere the deflection is in the opposite direction. This

deflection gives rise to the trade winds over the oceans; in

the northern hemisphere they are south-westerlies in mid-latitudes and

north-easterlies otherwise, and in the southern hemisphere

north-westerlies and south-easterlies respectively.

The monsoon

On the east and south-east coasts of sufficently large land masses,

pressure gradient will be sufficiently extreme that the resulting

winds will override the prevailing trade winds; they will blow

offshore into the ocean in winter, while the summer low-pressure area

will pull in moisture-laden air from the ocean. This important

seasonal reversal of the winds is, of course, the monsoon; it

is prototypically observable in south-east Asia. The two pictures

below show the general directions of the prevailing winds in winter

(above) and summer (below). Note particularly the monsoon effect on

the east coast.

A good question is: How large is “sufficiently large”? North America

has no monsoon as such, so somewhere between the size of it and of

Asia is probably as good an answer as any.

In winter, the continental high-pressure areas are responsible for

cold waves, which are flows of very cold air eastwards to the

offshore oceanic low. These cold winds pick up moisture as they pass

over the sea, which will be deposited as snow on any mountains they

encounter; western Japan is a terrestrial example.

Ocean currents

The formation and movement of the ocean currents is a complicated

subject, much of which is not of interest here; for our purposes we

are only concerned with currents on the surface of the oceans, which

are caused wholly or mainly by the winds. The Coriolis effect comes

into play again here, deflecting the currents from the path of the

wind; the deflection is greatest (up to 45 degrees) at high latitudes

and least (about 5 degrees) at the equator.

Ocean currents come in two flavours, depending on the direction in

which they flow: poleward currents, which carry water from hotter

areas to colder areas, are classified as warm, while

equatorward currents are similarly classified as cold. Note

that these are relative terms, thus a particular warm current flowing

to a cold region may actually be colder than a cold current which

flows to a warm region.

The oceanic high-pressure areas of the STHZ give rise in low latitudes

to warm currents along the east coasts of continents and cold currents

along the west coasts. The reverse distinction obtains in

mid-latitudes, because the wind blows around the oceanic low-pressure

areas in the opposite direction. The currents affecting the sample

continent shown above would thus be as follows, with warm currents

shown in red and cold currents in blue:

The Gulf Stream, which keeps western Europe much warmer in winter than

the north-eastern USA and south-eastern Canada, is a classic warm

current.

Now is a good time to add the

prevailing winds and ocean currents to your Maps for both January and

July. The currents are easy; don’t forget that the winds will blow

more or less in S-shaped double spirals.

… add water…

The annual distribution of the fall of precipitation in the form of

rain and snow is one of the factors which characterise a

particular climate. Rain and snow result from four processes:

Moist winds blowing onto land, as previously mentioned.

blowing onto land, as previously mentioned. Orographic lifting of moisture-carrying winds as they

blow over mountains and are forced to rise; the air cools as

it rises, depositing its moisture on the windward side of

the mountains [8].

of moisture-carrying winds as they blow over mountains and are forced to rise; the air cools as it rises, depositing its moisture on the windward side of the mountains [8]. Convection due to the heating of the air. Again, the air

cools as it rises and loses its moisture [8]. (Although the

ITCZ passes over the Sahara Desert, it does not cause rainfall

because of the dryness of the air; there is very little

moisture for the rising air to pick up.)

due to the heating of the air. Again, the air cools as it rises and loses its moisture [8]. (Although the ITCZ passes over the Sahara Desert, it does not cause rainfall because of the dryness of the air; there is very little moisture for the rising air to pick up.) Frontal lifting along the polar front. Here the warm air

from the STHZ is lifted up by the colder air from the

poles, causing the low-pressure areas which weather forecasts

warn about; further details are beyond the scope of this page.

An important detail about orographic lifting should be observed: after

the wind crosses the mountains it sinks, expands, and warms back up

again. These winds on the leeward sides of mountains (the

rain-shadows) are thus characteristically warm and dry, and are

known as chinook or Fˆhn, or colloquially as

“snow-eaters” after their ability to melt snow in otherwise cold

climates.

Finally, cold currents cool and stabilise the air, inhibiting the

formation of precipitation, while warm currents heat and destabilise

it, encouraging precipitation [2][7]. The relative amounts of

precipitation due to various factors are shown in the following

table.

Factor High precipitation Low precipitation Pressure ITCZ, on or near the equator STHZ Mountains Windward sides Leeward sides, in rain-shadow Prevailing winds Onshore Offshore or parallel Coastal currents Warm Cold, especially in low latitudes Location West coasts subject to the PF,and some way inland Interiors

You should now be able to work out,

for both January and July, the relative amounts of precipitation on

your Map.

… and place in the oven.

The annual variation in temperature is the other characteristic

feature of a climate. As a first approximation, the temperature is

highest at the equator and decreases steadily towards the poles [1],

subject to the following modifications.

Effect of the oceans

Variations in temperature are lowest along the coasts and highest in

areas remote from maritime influence [2]. The variation increases with

the distance from the oceans, and less so with distance from the west

coast; the eastern regions of continental interiors thus experience

the greatest variations in temperature. Incidentally, another

consequence of [2] is that the hottest and coldest times of the year

occur two to three weeks earlier in these regions than at the coasts.

Effect of moisture

Heat is more readily transmitted through clear skies than cloudy

skies; consequently, the less cloud an area receives, the greater will

be its temperature variation during a single day. The higher the

temperature, and the clearer the skies, the more moisture will be lost

during the day through evaporation, which is the opposite of

precipitation. The greatest amounts of evaporation are found in land

areas influenced by the STHZ, where the high-pressure belt is not

conducive to precipitation and thus cloud-formation [7]. These areas

are thus the hottest of all during the day, and cold at night.

You should now be able to work out,

for both January and July, the relative levels of temperature on your

Map.

Checklist

On both of your Maps you should now have indications of the

following:

The main pressure-belts (ITCZ, STHZ, and PF);

The oceanic and continental areas of high and low pressure;

The prevailing winds;

The main ocean currents;

Temperature and precipitation, on land at least.

The final stage consists of identifying the closest matching climate

from the table below; it uses a classification system similar to the

widely-used system developed by Wladimir Köppen.

Temperature Precipitation Location, for

checking Name Köppen Summer Winter Summer Winter latitude in degrees Tropical rainforest Af Hot Hot Wet Wet 0-10 Tropical monsoon Am Hot Warm Very wet Short and dry 5-15; east and south-east coasts only Savannah Aw Hot Warm Wet Long and dry 5-15 Hot desert BWh Very hot Warm Dry Dry 10-30, especially on west coasts with cold currents Hot steppe BSh Hot Warm Low to dry Low to dry 10-35; typically next to deserts Cold desert BWk Hot Cold Dry Dry Interiors, rain shadow Cold steppe BSk Warm Cold Low to dry Low to dry Interiors, rain shadow Maritime east coast Cfa Hot Warm to mild Wet Moderate 20-40; east coasts only Maritime west coast Cfb, Cfc Warm to mild Cool to cold Wet Wet 40-60; west coasts only Mediterranean Csa, Csb Hot Mild Dry Moderate 30-45, west coasts only Temperate monsoon Cwa, Cwb Hot Mild to cold Wet Dry 20-40; east coasts only Laurentian Dfa, Dfb Warm to mild Cold Moderate Low 40-60; not on west coasts Subarctic Dfc, Dfd Mild to cold Very cold Moderate Very low 60-80; not on west coasts Manchurian Dwa, Dwb Warm to mild Cold Moderate Dry 40-50; east coasts only Subarctic east Dwc, Dwd Mild to cold Very cold Moderate Dry 45-70; east coasts only Tundra ET Cold Very cold Low Dry 60-80 Icecap EF Very cold Very cold Low Dry 75+

The climates given in italics are those which, generally

speaking, are subject to the same influences throughout the year. The

other climates may be regarded as transitions between these; for

example, the mediterranean climate is a combination of hot desert in

the summer and maritime west coast in the winter.

Note the following:

Steppe and desert climates experience large

diurnal variations in temperature, which means cold nights.

and climates experience large diurnal variations in temperature, which means cold nights. In the subarctic and tundra climates, winters are

long, dark, and cold, and the other seasons are short.

and climates, winters are long, dark, and cold, and the other seasons are short. Some sources mention Köppen climate types As and Ds, which are

like Aw and Dw but with the dry season in summer rather than

winter. I don’t know what causes these particular climates;

they are very rare anyway and can probably be safely ignored.

The progression of climates

Moving from the equator to the poles, the climates appear in

the well-defined sequences described below. It is instructive to

compare these found on the Earth.

The climates appear on the west coast in the following

order:

Tropical rainforest .

. Savannah .

. Hot steppe , with dry winters. The boundary between this

and the savannah is the line where evaporation equals

precipitation.

, with dry winters. The boundary between this and the savannah is the line where evaporation equals precipitation. Hot desert , due to the influence of the cold

current, which is also responsible for coastal fog on the

west coasts of desert climates.

, due to the influence of the cold current, which is also responsible for coastal fog on the west coasts of desert climates. Hot steppe again, this time with dry summers.

again, this time with dry summers. Mediterranean . The boundary between this and the steppe

is, again, the line where evaporation equals

precipitation. Coastal fog is often experienced in summer.

. The boundary between this and the steppe is, again, the line where evaporation equals precipitation. Coastal fog is often experienced in summer. Maritime west coast , cooling steadily poleward. These

climates are warmed by the ocean currents.

, cooling steadily poleward. These climates are warmed by the ocean currents. Tundra .

. Icecap.

Continental interiors, and areas in the rain-shadows of

north-south mountain ranges, will experience dry versions of the

climates to the west. The equivalent order of climates would be:

Tropical rainforest or savannah .

or . Hot steppe , with dry winters.

, with dry winters. Hot desert .

. Hot steppe .

. Cold desert in areas far from the west coast.

in areas far from the west coast. Cold steppe .

. Laurentian in its colder incarnations. Round about here,

the colder temperatures reduce evaporation to the point that it

no longer exceeds precipitation.

in its colder incarnations. Round about here, the colder temperatures reduce evaporation to the point that it no longer exceeds precipitation. Subarctic .

. Tundra .

. Icecap.

On the east coast, there are two cases to consider, depending

on whether the land mass is large enough to generate monsoons. East

coasts not subject to the monsoon will feature the following

climates:

Tropical rainforest , or savannah if the land is

high enough, as in east Africa.

, or if the land is high enough, as in east Africa. Maritime east coast . The difference between this and

the preceding is largely one of winter temperatures.

. The difference between this and the preceding is largely one of winter temperatures. Laurentian , becoming steadily colder polewards.

, becoming steadily colder polewards. Subarctic .

. Tundra .

. Icecap.

East coasts of continents where there is a monsoon will feature the

following climates:

Tropical rainforest , which may be absent.

, which may be absent. Tropical monsoon , prototypically.

, prototypically. Temperate monsoon . This is the same as tropical monsoon,

but with colder winters; equivalently, it is equivalent to

maritime east coast with dry winters.

. This is the same as tropical monsoon, but with colder winters; equivalently, it is equivalent to maritime east coast with dry winters. Manchurian . Effectively a laurentian climate with dry

winters.

. Effectively a laurentian climate with dry winters. Subarctic east . Similarly, this is the subarctic climate

with dry winters.

. Similarly, this is the subarctic climate with dry winters. Tundra .

. Icecap.

Vegetables

One of the reasons for being interested in climate is to discover the

types of vegetation which grow in a particular region. This section

describes, in general terms, the vegetation types asociated with the

climate types. More detail, with information about the fauna, can be

found with a Google for “biomes”; for example

Introduction

to biomes, Habitats

and biomes, Blue Planet

Biomes, World

Biomes, and – the most detailed – Kˆppen biomes.

The vegetation of the icecap climate is the simplest to

describe: there is none at all, because the temperature is below

freezing for most or all of the year. Tundra climates similarly

discourage growth for most of the year, but some vegetation grows in

the short summer, typically small mosses, lichens, and alpine

plants. Equatorward, where the climate borders subarctic, stunted

trees may grow.

The characteristic vegetation of the subarctic, subarctic

east, and manchurian climates is extensive coniferous

forest known as taÔga, typically made up of spruce, fir, scots

pine, and larch; larch is commonest in the coldest and driest

climates, and the deciduous birch, aspen, and alder are also found in

the lower altitudes. Despite the low amounts of precipitation, even

lower evaporation means that enough moisture is retained to allow the

growth of vegetation. Conifers have needle-like leaves to preserve

water and strong branches to endure the snow which lies on them for

much of the winter.

A mixture of coniferous forests and broadleaved forests characterises

the maritime and laurientian climates; the dominant type

of forest depends on the proportion of the year in which the

temperature is less than 5.5 degrees centigrade (this is 42 degrees

Fahrenheit, interestingly). The progression is from evergreen

broadleaved through deciduous broadleaved to coniferous as the winters

become colder; thus if the temperature is always above 5.5 degrees

(i.e. the proportion is zero), the forest wil be mainly or entirely

evergreen broadleafed. The dominant type of tree will be coniferous if

the proportion is greater than 50%, and deciduous broadleafed if it is

between 0% and 50%.

Mediterranean vegetation needs to guard against losing water in

the dry summers, and tends towards scrub made of small plants with

hard leaves, similar to the chaparral familar from many Western

movies. The trees are either coniferous or evergreens with small waxy

leaves and thick bark; evergreen oak, pine, cedar, and above all olive

are typical mediterranean trees.

Too little moisture is retained in the steppes to allow trees

to grow; the principal vegetation is thus extensive grassland,

including many cereals. Grassland is also characteristic of the

savannah, in which the vegetation dies back in the dry winter

but grows vigorously in the summer, reaching heights of up to six

feet. Trees in the savannah tend to be isolated and adapted to retain

water for the long dry season, such as the baobab. The vegetation of

the deserts is scanty, patchy, and specially adapted to the

conditions; plants tend to be fleshy and leafless, such as the

cactus.

The characteristic vegetation of the tropical rainforest

climate is, of course, tropical rainforest: lush, abundant forests

with massive trees and an enormous variety of other plants which grow

all year round in the ever-present moisture, The large amounts of

precipitation leach nutrients from the soil, and as a result the trees

have shallow roots and large buttresses at the bases of their

trunks. Monsoon vegetation is intermediate between rainforest

and savannah: the forests are less dense, many varieties of tree

become deciduous to cope with the dry winters, roots are longer, and

the plant types are less diverse.

What if?

The principles described up to now should work well enough for an

Earthlike planet. This section is intended as a catch-all for

questions not otherwise answered.

… my planet rotates in the opposite direction?

Easy – just interchange “east” and “west”.

… my planet rotates very fast?

The three bands of prevailing winds in each hemisphere are due to the

speed of the planet’s rotation. Above a certain speed of rotation, for

which I am unable to provide figures, the three will become five (they

cannot become four), and in between the STHZ and PF there will appear

another belt each of of low pressure and high pressure. These will

still move north and south with the sun, and the principles can be

applied as before.

Bear in mind that above a certain speed of rotation the planet will

disintegrate; I have no idea what limit this fixes on the maximum

number of bands of prevailing winds. A faster rotation will also lead

to shorter days and nights, which will doubtless have other

consequences.

… my planet has a small axial inclination?

The north-south movement of the pressure belts will be correspondingly

less, and smaller areas will be subject to the climates which undergo

seasonal changes; annual temperature ranges will also be less. The

tropical rainforest, maritime, hot desert, and icecap climates will be

favoured.

… my planet has a large axial inclination?

The reverse of the preceding; season effects will be increased, and

the areas subject to the tropical rainforest, maritime, hot desert,

and icecap climates will be less. A large enough inclination – about

40 degrees – will eliminate these climates altogether.

Paper references