Weather -- when does fog appear?

[aboyd]

I've been using a weather generator to plot out the weather for a few days in advance of my player's activities. It makes for a nice log, like this:

Day 1 - Cloudy
They made it to town.

Day 2 - Partly Cloudy
Shopping & picking up rumors.

Day 3 - Heavy Rain (2 hours)
Explored the haunted house.

Day 4 - Heavy Rain (7 hours)
Recuperating at the inn.

Anyway, here's the problem. They are going to encounter something that only appears on foggy nights. I would like to be educated enough to know when fog appears. So far, all I know about the conditions is "warm & wet air meets a cold surface," such as a lake.

That makes me think that fog precedes a rain, since the air would be thick with moisture. So on any day slotted as "rain" I could say the night before was foggy. The problem is that rainy days are usually cold.

What is a good generality that I can fall back upon? Warm day followed by rainy day? Vice versa? I just want something simple that is at least grounded in 5th-grade level knowledge of how weather works. It doesn't need to be smarter than that -- just smart enough for some believability (mostly for myself, since the players will likely never care).

Any experts (or even just armchair warriors) out there want to give me something plausible?

 

[Jeff Wilder]

In the Bay Area, fog usually occurs on a warm day followed by a rapid cool-off at night and in the early morning. That happens quite a lot here. And yes, fog can produce rain and other forms of precipitation.

From HERE:

"Advection fogs occur when warm moist air moves over a cold surface. Advection is the horizontal transport of heat by winds and slow drifting movements of the air. The stronger the wind, the deeper the fog layer. In the San Francisco Bay Area, the movement of the air, the sea-breeze, from the Pacific Ocean inland to radiation-cooled land, produces its spectacular fog. Along the coast, during the clear summer days, the land warms more than the adjacent water. As the air warms over the land, it expands. Land air flows up and out over the ocean. The air pressure is decreased over the land and increases over the ocean. Air circulation begins, and the movement of the air is now from the ocean to the land, a sea breeze. Although little of original air returns to the land, most if not all of the lower air along the surface of the ocean flows inland. It usually starts around 10:00 a.m. far offshore, and ends up extending seaward and inland about 10 to 30 miles each way. At evening it subsides. Then the land breeze starts up because the land cools more than the water. The air over the land becomes denser as it cools. On top of this cooled land air mass, the air from the ocean begins to flow inward. The air pressure increases over the land. The land breeze removes the cooled air. Land breezes usually only extend about 5-6 miles over the sea, and inland. That is because the temperature differences between land and sea are less during the night. The fogs seen further offshore are also advection as they are the result of adjacent bodies of contrasting temperature water.

Evaporative fogs occur when cold air moves over warm water. The fog looks like steam rising as it begins at the surface of the water as moisture is evaporated from the air, and builds upward. An evaporative fog also develops when rain falls from a warm layer of air to a colder layer of air below it and the air is stable or there is only a light wind. The boundary between the two layers of air is known as a front.

Ground fogs and high-inversion fogs are radiation fogs as their air radiates/loses heat. Ground fogs occur primarily in the clear-weather mornings as they form from the cooling of the surface of the earth, and the lower air at night producing an inversion of temperature. The result is convection prevention and reduction of turbulence. Convection is the vertical movement of the air brought about by the transfer by internal mass movements of the air containing the heat. It can only occur in liquids and gases. Ground fogs rarely go above 100 feet above the surface of the earth. High-inversion fogs are prevalent and dense in winter in the San Joaquin Valley and in valleys near the southern and central California coast. They are really low stratus clouds. Cool quiet moist air lies on the earth. Over it at about 300 feet to 2,000 feet above the surface of the earth is warmer drier air. The result is fog.

Upslope fogs are formed due to adiabatic cooling of air moving upslope against a mountainside or a gradually up-slope plain. When air ascends, the pressure on it decreases. Therefore, the air expands losing energy/heat in its expansion. The change in temperature is an internal change because of pressure. Adiabatic is the term for it meaning “without transfer of heat.” When dry air rises above the ground, its cooling is at a fairly uniform rate of 5.5 F per 1,000 feet. When dry air sinks to the ground, its heating is at the same rate."

Interesting, that site has more complete info than the Wikipedia entry.

(BTW, the fog is one of the reasons I moved to San Francisco, and one of the reasons I now live overlooking the Pacific, only a half-mile inland. The fog here gets crazy, and even though I ride a motorcycle, I love it.)

 

[Kid Charlemagne]

To give a couple of examples - around me, it gets foggiest when there's been several days of rain soaking the ground and then the temperature increases markedly - the moisture evaporates and is visible in the form of fog. It can get very dense sometimes.

The second foggy situation I encounter is when I visit my parent's place in Arkansas - there's a fast moving, very cold river there, and when the warmer air hits the surface, it causes evaporation and creates a foggy area right around the river.

The fog in London (historically speaking) is largely smog - smoke and fog, partly from pollution, partly fog. My recollection is that the smoke and foggy conditions feed on each other, resulting in even foggier conditions.

 

[Mercutio01]

There is a lot of fog in the Persian Gulf, but not all that much rain. This happens in the late night/early morning when the air temperature cools off and approaches the water temp. The humidity increases almost exponentially as this happens, and it creates thick dense fogs which burn off as the sun rises and begins to heat the air again.

 

[Scott DeWar]

I have lived in every thing from coastal to desert. I have witnessed fog in all climets, yes even the mojavi desert and the egyptian sahara.

typically i have noticed fog occuring when there is warm water meating a somewhat cooler air mass. warm ocian current, warm heavy rain followed by a coldfront moving in, 40 degree over night temp with a rain the previous day on any land mass. You get the picture?

 

[ragboy]

In the context of an adventure, fog appears when the story calls for it. i.e. when you decide. Unless your players are more into accurate simulation...

No disrespect intended, but it's easy to get mired in details at the expense of the game. YMMV.

 

[Armadillo]

In the context of an adventure, fog appears when the story calls for it. i.e. when you decide. Unless your players are more into accurate simulation...

No disrespect intended, but it's easy to get mired in details at the expense of the game. YMMV.

Truly. This narrative approach is one of the best ways to handle weather in a game. Set the weather according to theme, drama, or what will provide the most interesting terrain for the fight.

Alternatively, you could take a simulationist approach. Find a real world analogue for your campaign location, and then copy down a few weeks worth of actual weather history for use in your campaign. I had a GM do this for a Twilight 2000 game, which was great for atmosphere and easy to do, if you assume the weather in post-apocalyptic Poland is comparable to last year's weather in Poland.

 

[Kaisoku]

Fog happens when there's any form of moisture (airborn or water sources) and a rapid temperature change (either warming up or cooling down).

So you can pretty much call up a mist or fog or whatever whenever you feel like by simply saying "the temperature raised <lowered> quickly overnight".

You could even add a spook factor by making the temperature change seem unnatural (if that's what you are going for in the encounter).
The PCs are going about their daily shopping and whatnot, when suddenly, without any wind, the temperature drops a few degrees and people get the shivers. A fog/mist rolls in and voila, spooky encounter has been set up.

 

[aboyd]

Thanks. Depending upon the fog, the group may get a chance to meet the Midnight Peddler from the Tome of Horrors. Since it's essentially a free Divination spell, I don't want to concoct a time when the narrative is right for it -- according to narrative, the time will never be right, because I would never require or inject such a thing in my narrative. My intention is for the Midnight Peddler to be a disruptive force that could conceivably trash my entire campaign, or not. I certainly don't want to trash all my work, but the idea is that I'm deliberately allowing a wild card to enter the sandbox, and if it shoots the party off into a direction that I never planned for, so be it. If it gives them unexpected advantage over the game, so be it. If it does nothing, so be it.

So in order to feel that I am not ramming this down their throats, I made it contingent upon the weather. I generated 90 days of weather. I wanted to flag which days were potentially foggy, as that is a requirement for the Midnight Peddler to appear. Then, if they are looking for him on the night that fog is scheduled, lucky them. If they look for weeks but only on clear nights, unlucky them.

How I ended up deciding if a night was foggy was simply to look for rain. A rainy day followed by a sunny day = foggy night between the two days. That gives the Midnight Peddler a potential of 8 days he might appear.

Thanks everyone for the help.