Atmospheric Conditions And Weather

Torrential downpours. Dome Mountains. Groves and arroyos. In order to construct a setting that is both fantastic and believable it is necessary to conceive the setting itself. Whether your heroes tread through sylvan glades or live upon a mountain’s rocky slopes the tone is set by the terrain you describe and the language you use to describe it. Geography is the root of your setting, the building block of all that comes after.
What follows is an examination of clouds and storms, terrain and other useful items that play into the creation of the physical setting for your fantasy adventure or soon to be written novel.
Source: Gary Gygax's World Builder


Black: These dark clouds often form as a result of upper layers blocking the sun and making lower clouds appear black. This is often a sign of rain.
Brown: This cloud color is seen mostly in windy areas, where dust is coloring the clouds after being blown up from the earth.
Cirro-cumulus: This cloud formation is a high altitude formation usually in the shape of ripples or grains. Also called mackerel sky.
Cirro-stratus: High altitude covering of clouds that is thin, hazy and often produces a halo effect.
Cirrus: This is the most elevated of all the cloud formations. It often appears like thin bands and has been called both mare tails and cat-tails.
Cloud bank: Name given to a large layer of clouds viewed from a distance.
Cumulus: These are the large clouds, often appearing hemispherical on top and level on the bottom. These clouds often layer over each other and carry rain.
Cumulo-nimbus: Large cloud formations often consisting of great towers or pillars formed in the bank.
Golden: This cloud color often occurs in early to late morning, as the low sun shines through clouds, giving them a golden appearance.
Gray: This cloud color forms when the top layer of the cloud is blocking sunlight from the lower layers, often giving the clouds a gray color on bottom and a white color on top. Often called leaden clouds.
Leaden clouds: Clouds of a gray color, often covering the entire sky and close to the ground.
Lowering clouds: Dark clouds seen as being pushed closer to the earth, often a sign of storm.
Mares tails: Name given to Cirrus clouds by farmers and landsmen. Also named cat-tails by sailors.
Nimbus: This form of cloud is generally gray in tint and found in areas with prolonged rain. This is also used to describe a raining cumulus or cumulo-stratus clouds.
Pink: This cloud color is often observed in early evening as the sun is just beginning to set.
Puffy: Name often given to cumulus clouds for their large, cotton look.
Purple: This cloud color is seen in evening often resulting from gray or black clouds reflecting the setting sun’s rays.
Red: This cloud color is seen in late evening, usually when the sun has already set and the last rays are striking the clouds at an angle.
Rolling: Clouds being pushed along by the wind, usually rapidly. These clouds appear to be rolling over each other.
Strato-cumulus: A form of clouds between cumulus and stratus, turning a black or bluish tint at the horizon.
Stratus: Clouds formed in horizontal layers or bands.
Thundercloud: A cloud charged with electricity, often producing lightning and thunder. These are most often cumulo-nimbus clouds.
Thunderhead: This is the swollen upper part of a thundercloud; often this is accompanied by a thunderstorm.
White: This color of cloud is most common, often seen when there is only a single layer of clouds which will most likely not produce rain.

Fog, etc.

Dust: Clouds of dirt and other particles blown into the air by a strong wind.
Fog: A mass of clouds lying close to the ground and often limiting vision.
Fogbank: A large mass of fog seen from a distance, often on the sea.
Haze: Moisture or dust in the atmosphere that diminishes visibility.
Miasma: Unhealthy or poisonous particles in the atmosphere, often in the form of vapor.
Mist: A mass of water vapor lying low in the air or in contact with the ground.

Table 2:1a Type of Rain & Visibility*

Roll d10 for variable affect.
Roll Rainfall Vis/max. miles
1 Intermittent 5
2 Sprinkle 3
3 Drizzle 3
4 Mist 2
5 Cloudburst 1 1/2
6 Shower 1
7 Downpour 7/8
8 Rainstorm 1/2
9 Thunderstorm 1/4
10 Torrential 1/16

Rainfall by hour 2:1b

Type mm/in Description and Effects
None 0 / 0 No rain; doesn’t preclude suspended moisture such as mist, fog, or low cloud
Trace 0.1-1 / 0.004-0.040 Usually spitting, small droplets; dampens rather than wets things, like heavy fog or light drizzle.
Light 1.1-2 / 0.044-0.080 Typical of a summer sun-shower; wets in half an hour. Puts out candles, campfires burn with much smoke.
Moderate 2.1-4 / 0.084-0.160 ‘Normal’ rain, medium droplets; wets in a quarter hour, soaks in a half hour. Puts out campfires.
Heavy 4.1-8 / 0.164-0.320 Causes roaring noise on roofs, misty spray on roads; wets in minutes. Puts out bonfires.
Downpour 8.1-16 / 0.324-0.640 Large droplets; drowns out speech; wets in seconds, soaks in minutes. Puts out scrub fires.
Torrential 16.1+ / 0.644+ Flattens vegetation; drowns out shouting; soaks to the skin in seconds. Puts out forest fires.
Wetting applies to the rained-on landscape in general as well as a normally = dressed man. The notes on fires assume an hour of average rain (e.g. 6 mm/h = for heavy rain). The intensities of rain by day and by hour (trace, light, = and so on) are different things. Moderate rain for a day might fall either as continuous trace rain or as a single torrential storm.
Type mm/in
None 0 / 0
Trace 1-3 / 0.04-0.12
Light 4-12 / 0.16-0.48
Moderate 13-20 / 0.52-0.80
Heavy 21-40 / 0.82-1.60
Downpour 41-100 / 1.62-3.98
Torrential 101-304 / 4.00-11.98
Cataclysmic 305+ / 12.00+

Rain with an intensity above 60 mm/hr (2.4 in/hr) for at least 5 minutes is = known as a cloudburst: this is like standing under a small waterfall. Earth = heaviest recorded rainfall in a day is 1880 mm (74 in). The heaviest rainfall = in an hour is 285 mm (11.2 in).


Cyclone: A storm resulting from the rapid movement of air around a lowpressure center. Often accompanied by destructive weather of other sorts. Cyclones move clockwise in the northern hemisphere, and counter clockwise in the southern.
Gale: A strong wind with speeds from around thirty to sixty miles an hour.
Hailstorm: Any storm which produces hail, a particle of ice, which can range in size from microscopic to inches in diameter.
Hurricane: A severe tropical storm often in the form of a cyclone traveling over the ocean. This classification is also used for winds greater than seventy five miles an hour.
Ice storm: Storms in which the rain or snow falling will freeze on contact forming layers of ice wherever it touches.
Lightning storm: An electrical storm, which produces much lightning and may or may not produce other effects such as rain.
Monsoon: A seasonal wind, which blows one direction part of the year, and the opposite direction the latter part of the year. Often one direction of wind will bring nearly constant rain.
Rainstorm: Any storm producing rain, which may be accompanied by other effects as well.
Sleet storm: Any storm producing sleet, a form of partially frozen rain or rain mixed with snow.
Snow storm: A storm which produces snow, ice crystals which collect wherever they land and remain frozen.
Tempest: A violent windstorm, which may be accompanied by other effects such as rain, snow, or hail.
Thunder & lightning storm: A storm which produces lightning and thunder regularly, often associated with rainstorms as well.
Thunderstorm: A storm that produces thunder but may not necessarily produce visible lightning.
Tornado: A violent windstorm that can be seen as a downward spike of cloud that may or may not touch the ground.
Twister: An informal name for a cyclone or tornado.
Typhoon: A violent whirlwind or cyclone often found in the sea, characterized by uprising winds.
Windstorm: Any storm that produces unusually strong wind, may be accompanied by other storms. For the effects of wind and wind speed consult Tables 2:4 and 2:5.

Table 2:2 Ice Strength in lbs.

Ice of 2” thickness supports per square foot 200#
Ice of 3” thickness supports per square foot 300#
Ice of 4” thickness supports per square foot 400#
Ice of 6” thickness supports per square foot 600#
Ice of 8” thickness supports per square foot 800#
Ice of 10” thickness supports per square foot 1,000#

Table 2:3 Type of Snow & Visibility*

Roll d6 for variable affect.
Roll Snowfall Vis/max. miles
1 Flurry/Flurries 4
2 Showers 2 3/4
3 Sleet 1 1/4
4 Storm 1
5 Blizzard 5/8
6 Whiteout 1/20
Note: 4” inches of melted snow produce 1” water.
* Visibility assumes flat terrain.

Table 2:5a Wind: Beaufort Wind Force Scale

In order to determine the variable windspeed roll a percentile dice and consult the following table.

% Description mph Observations
1-10 Calm 0-1 Smoke rises vertically
11-25 Light Air 1-3 Smoke drifts
26-39 Slight Breeze 4-7 Wind felt on face; leaves rust
40-50 Gentle Breeze 8-12 Leaves and twigs in motion
50-56 Moderate Breeze 13-18 Loose paper raised; flags flap
57-62 Strong Breeze 19-24 Small trees sway
63-69 Strong Breeze 25-31 Large tree branches bend
70-78 High Wind 32-38 Whole trees bend; walking into wind difficult
79-84 Gale 39-46 Twigs break off trees
85-89 Strong Gale 47-54 Signs blown down
90-94 Whole Gale 55-63 Trees uprooted; structural damage
95-98 Storm 64-74 Widespread damage
99-100 Hurricane 75+ Severe and extensive damage

Table 2:5b Wind: Beaufort Wind Force on Sea

Beaufort scale Description Effects at sea knots mph
0 Calm Sea like a mirror 0-1 0-1
1 Light air Ripples on sea 2-3 2-3
2 Light breeze 1' wavelets, glassy crests 4-5 4-7
3 Gentle breeze 3' breaking wavelets, few whitecaps 6-9 8-12
4 Moderate breeze 5' waves, whitecaps 10-14 13-18
5 Fresh breeze 8' waves, many whitecaps 15-18 19-24
6 Strong breeze 13' waves, white foam, and spray 19-24 25-31
7 Near gale Heaped sea, some blown foam 25-30 32-38
8 Gale 25' waves, streaks of blown foam 31-37 39-46
9 Strong gale 30' waves, visibility gets worse 38-44 47-54
10 Storm Heavy rolling sea, white with foam 45-51 55-63
11 Violent storm 50' waves, crests blown to froth 52-59 64-73
12 Hurricane Sea white, air full of foam and spray 60+ 74+
Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License