RogerBW's Blog

Twilight for Harpoon 28 October 2014

Sighting conditions in Harpoon are typically set by the scenario author. This means there's little guidance to the designer. Particularly if the scenario spans a large area, it might take several game hours to resolve, and it would be good to know how conditions change over time.

The only real game material in the 4.1 rules is on pages 4-16 and 4-17 under 4.5.5. Effects of Weather and Light. One could take this to imply that lighting thuds from "day" to "night" conditions, since there's certainly no guidance for anything in between.

On the other hand, the 4.1 quickstart and scenario book includes the scenario The Gibraltar Question, which has "Dusk is at 1730, with visibility dropping to 40% and to 25% by 1830". That seems a bit more reasonable.

Given that we have computers now, can we do this a bit more accurately when writing scenarios? I will define "twilight" for game purposes as nautical twilight, the time when the sun is between the horizon and twelve degrees below. If the sun is above the horizon, it's "day", if it's more than twelve degrees below, it's "night", and in either case the standard rules still apply.

One could pick a spot in the middle of the battlefield and use an on-line calculator (or custom software, easy enough to write) to generate the transition times. (Even at the equator, a mere fifteen nautical miles east or west can make a minute's difference in the time of sunset. Ultimately one really ought to generate lighting conditions per unit, which could be especially relevant if they're at high altitude, far from the rest of the battle, or moving fast enough to beat the sun across the sky, but that's more trouble than I want to go to.)

In twilight conditions, combine three columns on the sighting conditions table: "day precip" (in case of rain), "clear day" (for haze and fog), and "clear night" shifted up one row, so a twilight with full moon gives you 75% surface range, and a new or below-horizon moon gives 10%. (And yes, this means you need to know moonrise and moonset times too.)

(Should you multiply the percentages together or take the worst? The rules aren't clear. My inclination is to take the worst.)

Spotting modifiers for "night" are applied in twilight, but reduced: gun flashes or a ship on fire gives ×2 sighting range, running lights give ×1.5, and missile launches give ×3.

One could get more sophisticated and say that if you're trying to sight into the sunrise or sunset your effective range is reduced, while if you're sighting into the reflection of the sun or moon a silhouette is easier to spot, but I think this is well below reasonable resolution.

I have written software for this. Here for example are the next few lighting change events for the game I'm running at the time of writing:

               Time     Event     State  Moon
1997-03-29T18:36:30       now       Day   -
1997-03-29T18:41:00    Sunset  Twilight   3/4
1997-03-29T19:26:00      EENT     Night   3/4
1997-03-30T05:03:02   Moonset     Night   -
1997-03-30T05:41:00      BMNT  Twilight   -
1997-03-30T06:27:00   Sunrise       Day   -

Of course, what would be really nice would be a random offshore weather generator, which given lat/long and time of year would generate wind/sea state, precipitation and clouds/fog, as well as how they change over time. But writing that would be a distinctly challenging task.

  1. Posted by John Dallman at 08:43pm on 28 October 2014

    A random weather generator isn't that hard, if you have enough data for the area. One that makes weather change in plausible ways over a few days is much harder.

  2. Posted by RogerBW at 09:09am on 29 October 2014

    Getting historical data (a) at sea rather than on land and (b) for anywhere in the world is proving quite difficult. The NOAA seems to have various data sets, but they don't say what most of them are and they tend to be in obscure binary formats. The simulation element is relatively easy with enough data, even over a few days' timescale: Markov chains.

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