Japan: Climate and Ecology

Drawing by Steve Lines

LAND OF NINJA (the RuneQuest Japanese pack) has little information on these subjects: even the weather tables were omitted! What follows is culled from various sources such as the ever helpful Encyclopaedia Britannica.

Coast: for each square mile of land there is nine miles of coastline. Coastal sand dunes anchored by pine trees with shallow lagoons are characteristic of the West coast.

Mountains: only 15% is flat enough for agriculture. Deep cut valleys and sharp contours are typical, only the west of Honshu has softer hills. The central ranges rise to 10,000 ft peaks.

Rivers: only three are >200 miles in length. Lake Biwa in central Honshu covers 260 sq miles.

Population: 8th century = 8 million, Tokugawa period (17th-19th century) 25-30 million.


Fog banks off NE coast where cold/warm currents meet.

Jan temps: Tokyo 37°, Osaka 39°, Kyushu 45°.

August temps: Tokyo 77°, Osaka 81°.

Frost free: four months in N, eight months in S.

Rain: maximum early summer, minimum winter, plus in winter cold air from the Asian continent brings rain and snow to the West coast. Early summer rains are required for rice planting. Typhoons in late summer/early fall with torrential rain.

Tokyo has 2.5 times the rainfall of London.



Weather Charts

These were missed out of LAND OF NINJA so I adapted a system devised for my fantasy world of Midgard. It is a continuous rather than a spot check system, requiring the GM to roll up the weather for say a month in advance. All temperatures below in degrees Celcius, wind strengths in RuneQuest units, cloud as percentage of sky covered, rain in millimetres.

Geographical Divisions The main division is as noted above between the East and West coasts with a temperature gradient between North and South. The base areas were NW and NE, with the other four areas differing as follows:

W as NW but temps +2°C
E as NE but temps +2°C
SW as NW but temps as SE, rain as NE
SE as NE but temps +4°C, rain as NE
Southern areas are on Kyushu which does not have additional winter rain.

Wind: Range -10 to +40 except July-Sept -10 to +50.
Roll on D20 for change and on D2 for increase or decrease. If change not possible, reverse.

Cloud Change: 5% cumulative chance of a change from increasing to decreasing & vv in Summer and Winter, 10% in Autumn and Spring. If change not possible, reverse. Actual change is equal to Wind Strength.

Cloud: Range -10 to +100 (winter NW and W 0 to +100, winter NE and E -10 to -90). If not possible, reverse.

Rain: Roll against cloud on D100. If <=, add the digits of the roll and then add the seasonal factor below.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
NW *(2+D10) *(2+D10) *(2+D10) *(2D6) *2 +D6       +D6 *2 *2
NE -D6     +D6 +D6 +D10     +D6 *2   -D6

Typhoon: IF wind strength 41+ THEN cloud becomes 90+D10 and an extra D20 of rain falls. The wind always drops afterwards, and for one week after stays below 30. Cloud always drops to below 50. A typhoon can last for more than one day if winds strengths of 41-50 continue to be rolled.

Temperature Table: The daytime temperature is given. In summer less cloud > hotter, in winter less cloud > colder. The maximum change is 5°.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
NW -10+D10 -10+D10 -10+D10 -5+D10 D10 8+D10 10+D10 10+D10 8+D10 4+D10 D10 -5+D10
NE -3+D10 -3+D10 D6 4+D6 6+D6 8+D10 10+D10 10+D10 8+D10 6+D6 4+D6 D6

Frost Table: gives temperature drop at night at sea level.

Cloud Cover Negative 0-19 20-39 40-59 60-79 80-100
Drop in °C -8 -7 -6 -5 -4 -3

Mountains: -1°C per 1,000 ft on clear nights.

Let's try an example for say NE Nippon in September. The table below shows what happens:

Date Wind Change Cloud % Rain (mm) Temp°C
1 11 5+ 31   14
2 -03 10+ 28   16
3 10 15+ 38   16
4 24 20+ 62 10 14
5 39 25+ 23   18
6 35 30+ 58   14
7 28 5- 30   16
8 41 >> 95 34 18
9 39 5- 56   14
10 19 10- 37   12
11 23 15- 14   11
12 05 20- 09   12
13 -07 25- 14 09 09
14 -05 5+ 09   14

The wind varies between -10 and +50 as this is typhoon season. A 'negative wind' rounds up to zero in reality of course, the intention is to give more calm days than would otherwise occur. On 2 Sept a '-13' was rolled which would have taken the wind out of the range so it was changed to '+13'.

The change column gives the percentage chance of a change in cloud formation. In our case a '24' was rolled on 6 Sept so the chance went back to 5% on 7 Sept and the sign switched to negative indicating cloud decrease. The wind dropped on 5 Sept because 62+39=101, over the maximum, so 62-39=23 instead.

Rain was rolled for on the cloud percentage. On 4 Sept a '32' was rolled which was below the 62% cloud, and the seasonal factor being '+D6' the total rain was 3+2+D6=10.

On 8 Sept the wind reached 41 - typhoon! Cloud cover rose and heavy rain fell (over an inch).