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.
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.
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.
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.
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.
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 [2][7]. The relative amounts of
precipitation due to various factors are shown in the following
table.
Ingredients
You will need the following items before you can proceed any
further.
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.
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.
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.
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.
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:
