Recently, I read an article about temperature and climate charts for D&D. This is one of those times when you have to ask yourself if the benefit outweighs the effort. On a day to day basis, I really don’t see the usefulness of detailed climate information. But as a generality, I think it is important to at least have a rough idea of what to expect in a region.

There is nothing wrong with static weather. So what if every day is sunny and clear, with moderately warm days, comfortably cool nights, and amazingly, all the plants are green and thriving despite the lack of rain. It’s a fantasy game where you are busy fighting monsters and saving fair damsels. Is it really necessary to worry about the weather?

On a day to day basis, I would say no. But you should be familiar enough with climatology to know what is reasonable weather and what isn’t. In northern Alaska, you wouldn’t see a month long 120° heat wave. In an equatorial rain forest, you wouldn’t see a blizzard. Recognizing such extreme anomalies may seem like common sense, but where is the cutoff where heat waves/blizzards are reasonable?

My Old Charts

I have some very detailed charts, based on those that Gary included in his World of Greyhawk folio, that predict: temperature, humidity, and wind speed. They take into account terrain type, elevation, latitude, and wind chill. Although not perfectly accurate, they offer a very workable model. After a great deal of work fine-tuning them, they got stuck in a folder and never used. I just never needed that level of detail and the amount of work required to use them was prohibitive.

An Overview

A quick search for climate shows up a wiki page on Climate Charts. A climate chart shows the average range of temperatures and precipitation, by month, for a given region. Here are sample charts of three regions, as well as a description, taken from that site:

As we can see from the chart, Maribor has a temperate climate with hot summers and freezing winters. It lies in the northern hemisphere, so the temperatures peak in July and August. The temperature in Labuan, which lies in the heart of the tropics, hardly changes through the year. Instead of summers and winters, there is a dry season in the beginning of the year, followed by a wet season with high rainfall. Cuzco also lies near the equator, but at a much higher altitude in the Andean highlands, and also much drier. Like in Labuan, the daily high temperature barely changes through the year, but they are significantly cooler due to the altitude. Nights in Cuzco are much colder than during the day, especially in the dry months from May to August.

Most of the US more or less resembles the first chart. The range of temperatures remains fairly constant each month, while the average temperature raises significantly in the summer and drops in the winter. This climate type is all I am familiar with so, subconsciously, I assume that it is representative of a global condition. As you can see from the other charts though, this isn’t true. Near the equator, where the angle to the sun never becomes extreme, there is little variation between summer and winter. A little research will show that there are actually a number of factors that make it difficult to accurately describe planet-wide climate in a single model.

A Compromise

The single most useful climatological piece of information we could generate would be the temperature. From that, we can extrapolate whatever else we need. In fact, I wouldn’t suggest actually using such charts to generate temperature so much as use them for a guide as to what ranges of temperatures are reasonable and then assign them, on the fly, as befits the situation.

MonthAverage Temp
Jan33°
Feb43°
Mar55°
Apr68°
May77°
Jun80°
Jul77°
Aug68°
Sep55°
Oct43°
Nov33°
Dec30°

Okay, enough talk. Let’s get started. I want to base my model at 45° latitude with an average daily high of 80°F in the summer and 30°F in the winter. I’m going to use a sine curve to vary the temperature between those two extremes, which gives the Average Temp on the chart to the left. From this average temperature, I will subtract 10°F to create a Base Temp. Then, to find the high and low for the day you will add 1d20 for the high or subtract 1d10 for the low. This will generate the average daily highs I desire, with an average daily low about 15°F cooler.

There is no method of accounting for different latitudes that is both easy and accurate. So instead, I’m going with what’s easy. Between 30° and 60° latitude, 1° of change in latitude equates to 1°F of change in temperature (cooler as you move toward the pole, warmer as you move toward the equator). Each degree of latitude above 60° (toward the pole) drops the temperature by 2°F, while every 2° of latitude below 30° (toward the equator) raises the temperature by 1°F.

For being such a crude method, this technique isn’t too bad. It generates average daily temperatures between -45° (winter) and 5° (summer) at the pole, and between 60° (winter) and 110° (summer) at the equator. In real life, the north pole ranges from -15° to -45° in the winter, and stays around 0° in the summer, while the equator maintains a temperature of roughly 90° all year round.

Using This Info

With the chart above, you can simply look up the month and see what the average daily high should be. The average daily low will be about 15°F cooler. With that information, I’d suggest simply “winging it” whenever the subject comes up. As I said earlier on, I don’t see much use in worrying about the weather. However, it can be fun to mix things up once in a while. Throw in a snow day now and then. From the chart above, you can see that snow is reasonable all winter long and in late fall or early spring. That’s pretty much common sense for the latitude given. But if you change the latitude significantly, this will provide guidelines for how far that snow zone can reasonably be shifted.

As far as actual climate (blizzards, fog, wind storms, etc), I think those sorts of things are best placed as needed, instead of being left to the luck of the dice. When travelling high in the mountains, constant snow (occasionally turning to blizzard) makes a good story. Perpetual rain when travelling through murky swamps or heavy fog on the moors just adds atmosphere.

Keep in mind that blizzards, heavy rain, and heavy fog significantly reduce visibility. Finding that temple in the marsh may seem easy when looking at a map in town, but sloshing through the mud in a heavy rain, with visibility dropping down to almost nil, is an entirely different story.