How does the rain catcher from Tomb of Annihilation work mechanically?

The rain catcher item states:

"[The rain catcher] can catch 2 gallons of drinking water per inch of rainfall and hold up to 8 gallons"

My question is: How do I calculate how much rain has fallen in a given period? The adventure states that it is almost constantly raining in Chult, but I don't see any system to generate exactly how much rain falls. For example, if my party wants to set up a rain catcher in their camp each night for 10 hours, how do I calculate the water collected?

• I'm afb, so can you check for me: does p 14 have a "weather" section with some d4 rolls indicated for rainfall and storms? If so I'll be able to describe the system our table derived from those specs. – nitsua60 Sep 30 '17 at 17:13
• @nitsua60 The only probability listed is "On days that receive rain, there is a 25% chance of a full blown tropical storm..". It goes on to say that travel is limited in these cases. Nothing about rainfall in inches. – Destruktor Sep 30 '17 at 17:37
• I found this ironic when I came across it as I am working on an API for weather generation for fantasy games, as it always seems to be an after thought if brought up at all in any game I have been in. – Slagmoth Oct 2 '17 at 2:59

To calculate the water gathered by a rain catcher each night in gallons, multiply the number of inches that fall during the night by 2. This is because the rain catcher gathers 2 gallons per inch of rainfall. It also holds up to 8 gallons.

To figure out how much rainfall there is, there's no answer given in Tomb of Annihilation (ToA). The ToA DM Screen published by Gale Force Nine includes the same table found in the Dungeon Master's Guide (DMG) for rolling for precipitation, but the table isn't tailored for Chult's climate.

It rains in a rainforest about 250 days each year, or about 70% of the days each year. Rainforests average 80 inches of rainfall a year, and the wettest place on Earth receives 467 inches of rainfall a year. One other useful piece of information is that Heavy Rain is considered three or more times as much rainfall as Light Rain.

The chart in the DMG only has it raining 40% of the days. To match a rainforest, roll on the table for every 8 hour period, so once for the 8-hour day and twice during the 16-hour night. This makes it rain 78% of the days each year, but doesn't increase how often the players will encounter daytime Tropical Storms.

Roll Weather 3 Times per Day

Roll on the chart 3 times every day, once during the daytime traveling, and twice during the resting hours. Note that the collection is for an 8 hour period, so players should get to collect twice during the long rest. This chart tries to keep things as close to the DMG and DM Screen as possible.

d20       Intensity   Rainfall   8-hr Collection
1-12        None         0              0
13-17       Light       0.031"      0.5 gallons
18-20       Heavy       0.094"      1.5 gallons
^- 25%  Tropical Storm  0.19"        3  gallons


Using this table results in annual rainfall of 222 inches, but also tries to give the players a good chance of getting some rainwater by not dumping it all on them during Tropical Storms. Rainfall in the chart is per hour. By rolling 3 times, some days it might be worth hunkering down to catch some rainwater since there's no guarantee the rain will still be there come nightfall!

Roll Weather 2 Times per Day

An alternative chart is here which requires only two rolls, once for the day and once for the night. This version results in 227 inches of annual rainfall, and rain on 75% of the days.

d20       Intensity   Rainfall   Nightly Collection (16 hr)
1-10        None         0                 0
11-14       Light       0.016"         0.5 gallons
15-17       Medium      0.047"         1.5 gallons
18-20       Heavy       0.078"         2.5 gallons
^- 25%  Tropical Storm  0.19"           6 gallons


Roll Weather Once a Day

And finally a once per day chart. Roll once for the whole day! One change is that to determine Tropical Storms, only check on a roll of 18+. This chart results in 228 inches of annual rainfall, and rain on 75% of the days.

d20         Intensity   Rainfall   Nightly Collection (16 hr)
1-5           None         0                 0
6-10          Light       0.016"         0.5 gallons
11-14        Medium       0.03"          1   gallon
15-20         Heavy       0.047"         1.5 gallons
18+,25%  Tropical Storm   0.094"         3   gallons


1d4 Rainfall

Here's an extra simple method for generating rain each night. Just roll 1d4 and lookup how many gallons were gathered.

d4    Intensity   Nightly Collection (16 hr)
1       None                0
2       Light           0.5 gallons
3       Medium           1  gallon
4       Heavy           1.5 gallons


This method results in annual rainfall of 205 inches, and rain on 75% of the days.

To get more rainwater each night, parties should bring more rain catchers. Characters may also be able to forage for water (see rules in the DMG).

As far as I know there aren't any actual guidelines for how much it rains in Chult except for "A day without rain is rare." Legendarypants had a blog post about this and they ended up using a combination of Wikipedia and the DMG:

I looked up a rain forest on Wikipedia, found out it rains something like 275 days of the year, did a quick calculation, and figured out that rolling twice per day on the precipitation table from the DMG is about right.

http://www.legendarypants.net/tomb-of-annihilation/

The weather table on page 109 of the DMG just lists "light rain" or "heavy rain." Google tells me that light rain is up to 0.10in/0.25cm per hour and heavy rain is in excess of 0.30in/0.76cm per hour.

That would mean in a light rain it would take at least 40 hours to fill up your raincatcher. In a heavy rain it would be 27 hours or less.

One more thing to note if you end up using the chart from the DMG. Page 11 of ToA tells us that on a heavy rain day, there's a 25% chance of a tropical storm.

• hmm... seems like making a new table might be easier than rolling allthose dice. there is a 25% chance for no rain, then use the existing proportion from DMG for light/ heavy: 5/8 for light and 3/8 for heavy. Then 25% of the time heavy rain is a tropical storm. This leaves the 24% chance for no rain, 46.9% chance for light rain, 22.1% chance for heavy rain and 7% chance for tropical storm. – Destruktor Sep 30 '17 at 18:40
• Do note that the 'Heavy Rain' value cited in this post is the low end of what is considered heavy rain. The upper end for 'Heavy Rain' is 2 inches per hour...above which it is called 'Violent Rain.' The record for a Tropical Storm in the US is 6 inches of water in one hour. – guildsbounty Oct 12 '17 at 16:09

Using those numbers, here is what I generally came up with:
On any given day, roll 1d4.
1 = No Rain
2 = Light Rain (0.1 inch per hour)
3 = Moderate Rain (0.2 inches per hour)
4 = Heavy Rain (0.5 inches per hour)
On a heavy rain day, roll 1d4: on a 4, it's a Tropical Storm (1 inch per hour)

Travel is assumed to be 8 hours travel and 16 hours camping. Rain catchers need to be stationary to set up, and collect 2 gallons per inch of rain. I'm also rounding down to the nearest whole number to accommodate for set-up and rain intensity shifting throughout the day. So in one day (16 hours of camping), a rain catcher collects:
Light Rain: 3 gallons
Moderate Rain: 6 gallons
Heavy Rain: 16 gallons*
Tropical Storm: 32 gallons if traveling, 48 gallons if camping for 24 hours*
*Heavy Rain and Tropical Storms require someone to empty the rain catcher one or more times to get full amount, or else they hit the 8 gallon max.

Obviously this isn't EXACT to the real world, but is a much simpler version that will speed up the process of figuring it all out.

• Welcome to the site! Please take the tour when you get a chance. Note that we prefer that answers offering homebrew or house rules be based on experience. Have you tried this system yourself? How did it work out? – Oblivious Sage Oct 12 '17 at 14:31
• Welcome to the site, Matthew! This is a great progression from the other answer, however, we expect each answer to stand on its own. In order to do that, you could just copy the needed numbers from Orvir's answer into your own, citing where they came from. – Derek Stucki Oct 12 '17 at 14:35
• This method results in 1,752 inches of rainfall a year. Earth's rainforests average 80 inches a year, while the wettest place on the planet gets 467 inches due to monsoons. – Lexaire Feb 6 '18 at 5:35