Last updated: 17 November 2008
With grateful thanks to krinnen, aka Gonzalo, who drew the pictures.
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.
The following assumptions have been made:
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.
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 . 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.
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.
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 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.
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 Föhn, 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 . 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
You should now be able to work out, for both January and July, the relative amounts of precipitation on your Map.
You should now be able to work out, for both January and July, the relative levels of temperature on your Map.
The final stage consists of identifying the closest matching climate from the table below; it uses a classification sytem 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 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|
|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:
One final factor to consider is altitude, also known as elevation. In general, temperature decreases with altitude - the higher you are above sea level, the colder it gets - so that a region which would have one type of climate at sea level will have a colder climate at higher elevations. For example, much of south-central Africa around Zambia and Zimbabwe would have a savannah climate at sea level, but because of the elevation has temperate monsoon instead.
The climates appear on the west coast in the following order:
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:
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:
East coasts of continents where there is a monsoon will feature the following climates:
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 taïga, 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.
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.