Cooling due to evaporation

Question

I'm reading "The water wizard" by Callum Coats ed. Gill Books. At page 9 I read:

Terracotta exhibits a porosity particularly well-suited to purposes of water storage. This is because it enables a very small percentage of the contained water to evaporate via the vessel walls. Evaporation is always associated with cooling (vaporization, however, with heat) and, according to Walter Schauberger (Viktor's physicist son), if the porosity is correct, then for every 600th part of the contents evaporated, the contents will be cooled by 1 °C (1.8 °F), thus approaching a temperature of +4 °C (+39.2 °F).

(emphasis mine)

I'm aware that evaporation reduces the temperature of a liquid in an open system. What I don't understand is:

1. is it correct that just 1/600 of water that evaporates reduces the temperature of 1 °C (no matter the current temperature) ?

2. the common sense and the daily experiences tell the water does not reach 4 °C even if it evaporates completely. Or in other words: how can the water's temperature drop below the ambient temperature?

Is the above sentence simply wrong or are there conditions (not made explicit in the text) that make these statements valid?

I read this, this and this questions but they didn't answer to my doubts.

Answer 1

The figure 1/600 comes from the specific heat of water divided by its latent heat of vaporization. The specific heat of water tells you the amount of energy it takes to raise its temperature, and is approximately 4.18 kJ /(kg$\cdot$ K). The latent heat is the energy required to evaporate the water, and is approximately 2460 kJ / kg. [1]. The ratio of these two numbers is approximately 1/600 which means that the energy required to raise the temperature an amount of water by 1 K is approximately the same as the energy required to evaporate 1/600 of the water.

The temperature of water can indeed drop below the ambient temperature due to this cooling effect. This is the same reason that people can survive in temperatures above body temperature - by sweating, you take advantage of evaporative cooling to maintain a stable body temperature even if the environment is hotter.

The equilibrium temperature of a body of water will be determined by the heat balance between the water and the surroundings. Water stored in a container from which it can evaporate will generally be cooler than the environment due to evaporation, but how much cooler depends on several factors, e.g. the temperature and humidity of the surroundings, the heat conductance and permeability (porosity) of the container, etc. Determining the equilibrium temperature is therefore not trivial - but it does seem plausible that terracotta amphorae would be good containers for storing water coolly.

[1] Numbers obtained from https://www.engineeringtoolbox.com