All right, as promised, here goes.
I do not apologize for the roughness of these tables; I conceived of this method only about a week and a half ago, and all but one of these tables was created last night. You can consider that it means those tables are probably junk…but no worse than the weather tables I’ve seen in 3e editions. I think that this group works pretty well together, and I’ll try to demonstrate that after completing the explanation.
If you’ve read yesterday’s post, I made the proposal that the appearance of cold fronts could be used as a singular method for determining the weather. I’d like to state again that this is a simplification. That is the point. Obviously the science of meteorology cannot be simplified to the consideration of cold fronts alone. But I don’t hope to explain meteorology…I just want a working system that will enable me to easily incorporate weather into my campaign.
The first requirement would be to determine the likelihood of a front occurring:
This table would be consulted on the day after a cold front occurred, and each day following, until a successful roll indicates the arrival of another front. You will note that the likelihood increases as the days pass; this likelihood could easily be adjusted for specific areas of your world or earth…more often for mountain regions, less often for areas nearer to the equator. That would depend on how turbulent you believed were the conditions on your world. For myself, I wanted a reasonable possibility of a front every 4 to 6 days, with a chance (however unlikely) that two weeks of steady weather might occur.
The modifier to wind strength is there so that fronts that follow immediately on top of one another are unlikely to be violent ballbusters. Again, this modifier could be dropped if you thought appropriate, or rearranged to reflect a specific region.
In any case, this brings us to the wind strength table:
The effects indicated are from the Beaufort Scale, which you can research to your heart’s content; various online descriptions of the scale give slightly different wind speeds—I’m using the numbers from my encyclopedia. The effects are merely there to give an impression of what the conditions might be like during travel or during an encounter. It’s interesting to imagine a battle during a strong gale, with bits and pieces of roof tile bouncing off the combatants causing damage.
Please note the front strength given on the left, as it is needful on the storms table.
First however, the level of humidity of the entering cold front and of the pre-existing warm front must be determined:
I’ve arranged this table to show the most common weather provinces. I admit that the numbers are almost entirely ad hoc—I give them here only to get an idea of what they might be. At some point, when I have a week or so, I might dig up some data on humidity of fronts moving into given regions and do the table more accurately; for the moment, however, this is good enough to show how the table ought to work.
It’s quite simple. Roll a % die to see if the cold front is composed of moist air…a failure indicates that it’s dry. Sorry about the cut off headings. They should read,
“% chance that arriving cold front is moist” and “% chance of moist air mass at point of arrival of front.” The latter refers to the pre-existing warm front.
It is also necessary that I post the following table, describing weather grades. This is based in part on the system first proposed in the Wilderness Survival Guide from AD&D, but the numbers are my own:
This is pretty straightforward and easy to understand also. I constructed it a few years ago as I was struggling with cold and heat effects and how they’re mitigated by clothing; but that’s another story.
Okay, there’s only one table left:
This table is suitable only for the turbulent zone between 30 and 60 degrees latitude…and it quite general in its descriptive effects to allow plenty of latitude on the part of the DM in describing just what the conditions are. This table should be used as a guideline. Again, at some point I may sit down and devise a much more complicated and detailed descriptive table of the specific results, for each season and for each weather province, but that’s not necessary for this demonstration.
The temperature drop refers to the number of grades that the temperature falls below the average temperature for a given locale. Since in the days following the previous cold front the weather has been steadily improving, the actual drop will be greater…unless fronts have followed two days in a row, in which case it is possible that the second, more likely weaker front would actually be warmer than the first front. If you follow me.
Regarding the rainfall modifier given: if we presume sixty cold fronts in the space of a year, which would be five per month. To determine the given rainfall of a specific locale, divide the monthly average by five and then multiply against the modifier given to determine the number of inches of rain.
Yeah, I know. This is all as clear as mud. Let’s work out an example using all the tables.
Let’s say the party is in Helsinki, Finland, or in a region very much like it. It is October the 16th. A cold front arrived last Friday, and since that time—up until yesterday—the weather has been steadily improving. But for today we’ve rolled a 3 on a d20, indicating a cold front.
There is no modifier to the wind strength, so we roll a % die, obtaining a 64, which we compare against the “maritime” column, as Helsinki is within 100 miles of the sea (its right on the Baltic, for those who don’t know). The wind strength table indicates a fresh wind, with small trees swaying and moderate waves on the sea—a “weak” front.
Helsinki is in a humid cool continental zone; in the autumn there is a 15% chance that the cold front is moist, and a 73% chance that the existing warm front is moist. We roll a 35 and a 22, indicating that a dry cold front is moving against a moist warm front.
As it is a dry cold front, the wind would originate in the north to northeast, blowing from the east ice cap/north Russian land mass (a moist front would originate in the Barents Sea, still open at this time of the year). Remember that all cold fronts originate from a easterly direction and are turned counter-clockwise by the motion of the earth.
We compare the relationship of the fronts to the Weak Front Effects table and we find that, in the autumn, it begins to rain. This rain will increase in intensity through the day and overnight, while tomorrow will be cool and the skies relatively clear. By tomorrow afternoon the cold front will have passed entirely, whereupon we can roll to see if the warm front that flows into the air space behind the front is moist or not; we roll a 51 and find that it is. We get no cold front for Friday or Saturday, so the warm front raises the temperature a grade or two, bringing with it the cloudy skies (and gentle rain) associated with the Baltic Sea basin. For each day that a cold front does not occur, there is a 50% chance per day that the temperature will rise one grade, until a maximum of 4 grades above average is reached. At some point, the next cold front will interrupt this increase, and the pattern begins again.
All there is left to determine is the actual temperature and the actual rainfall. For that, we go to the climate table for Helsinki, where we discover the average temperature for October is 5.5 degrees Celsius, or 41.9 degrees Fahrenheit. That makes the average temperature grade “M”; the temperature drop from the front that occurred today is to drop this 1-2 grades; we roll and find the temperature today is “K”.
I haven’t mentioned it yet, but it should be obvious to some that the temperature K would refer to the mean temperature for the day…the daily high would be one grade better, and the daily low would be one grade worse. Thus, during the afternoon, the temperature would be “L”, meaning rain during the day, turning to “K” in the evening (whereupon the rain would turn to sleet), then “J” overnight (meaning snow) and “K” the following morning. At that point there is a 50% chance that the mean daily temperature would rise to “L” with a high of “M” tomorrow.
Greater increases are possible from the onset of warm fronts…you might want to try an initial increase of 1-4 grades in the summer, 1-3 grades in the spring, 1-2 grades in autumn and 1 grade in winter. Play with it, see what results work for you.
We can now discover the average rainfall for Helsinki for the month of October: total, 68.5 mm or 2.7 inches. Divided by 5, and multiplied by a modifier of 1x according to the table, indicates that during the 24-hour period of this cold front, 13.7 mm or 0.54 inches of rain falls.
Now, how simple is this system? Very simple. In no time at all it’s easy to see that Helsinki is nowhere that a party wants to be in October. Time to get on a boat and head south.
Oh, incidentally, the temperature in Helsinki at the moment (I believe it is 5 P.M.) is grade “O” and the wind from the SE…one of those Baltic Sea moist warm fronts that bring occasional rain.
I need to fix the flaw in my reasoning above. IF the warm front that follows a cold front is a moist front, then rainy weather follows sporadically, with cloud cover, until the next occurrence of a cold front. The skies clear following a cold front only if the warm front that follows is a dry front.