Herman lives in a house on the Caribbean island of Kinja, where the weather is warm all year. Let’s assume all Kinjan houses are of similar construction, and no houses have a water heating system. Herman takes a shower every day at 5 pm, and at the end of his shower, he measures the water temperature coming out of the shower head. Below are the results for a 150-day period.
What is the likeliest explanation for the drops (blue) and spikes (red) in water temperature?
These fluctuations happen to all houses on Kinja, sometimes on the same days, and sometimes not.
Updates: As mentioned in the comments:
Herman’s shower takes the same amount of time every day.
Seeing "exactly" the same temperature numbers on various days is not important, and is just to simplify things. The numbers just convey drops (blue), spikes (red), and normal (black).
The following groundbreaking info was found by @oleslaw:
The houses on Kinja have cisterns.
As further info:
Cisterns are built below ground level, often where a house’s basement would be. Cisterns are common in the Caribbean islands where well water is brackish, and there is no municipal water system to pipe water to houses.
There are two main ways for water to get into a cistern. In the more desirable way, the water is cooler than normal, and it’s free. In the other way, the water is sometimes warmer than normal, and it’s not free.