The essential answer
Humidity describes water vapor in air; fog contains suspended liquid droplets. A humidity sensor therefore cannot by itself establish how much liquid water a mesh could intercept. Relative humidity also depends on temperature, so equal readings need not mean equal vapor concentrations. For a fog collector, the useful distinction is between water already condensed into droplets and vapor that would require a phase change. Treat humidity as environmental context, and measure the droplet-bearing flow when estimating collection potential.
How to reason about it
Imagine drawing a boundary around a collector. Droplets can cross that boundary and collide with a surface without first being condensed there. Vapor crosses too, but it does not automatically enter the collection bottle. A surface that cools enough for condensation introduces another mechanism, with a heat balance that a simple fog interception calculation omits. This separation prevents a common bookkeeping error: counting every gram of atmospheric water as immediately recoverable. Temperature and relative humidity help explain whether conditions favor fog formation, while liquid water content characterizes the suspended liquid reservoir. A useful diagnostic record keeps these quantities in different columns, with their units and sensor locations. If collection continues when no droplets are detected, investigate dew, rain contamination, residual drainage, and measurement limits before assigning the result to vapor harvesting.
ILLUSTRATIVE · NOT A REPORTED RESULT
An example to work through
Assume two hypothetical air samples both read 95% relative humidity. A droplet instrument reports 0.20 grams of liquid water per cubic meter in sample A and no detectable droplets in sample B. If 100 cubic meters pass an ideal interception surface, A contains 20 grams of suspended liquid. The same liquid estimate cannot be assigned to B from its humidity reading. This example assumes representative sampling, constant conditions, and no condensation on the collector. It does not specify vapor mass because air temperature and a vapor-density calculation would also be needed.
Where interpretation fails
Do not call a mesh a dehumidifier merely because its bottle gains water on a humid morning. Fog interception, dew condensation, rain capture, and delayed drainage can overlap. Their dependence on airflow, surface temperature, and time differs, so the mechanism must be supported by measurements.
The next decision
Before calculating yield, decide which water phase your model includes. Use humidity and temperature for context, and a liquid-sensitive measurement for interception estimates. If the mechanism remains ambiguous, report observed bottle mass and the ambiguity rather than converting humidity into a collection efficiency.
Sources & context
These sources support the concepts noted below. Our synthetic examples and research framing are educational interpretation; they are not findings from these organizations or validation of our ventures.
- NWS: Discussion on Humidity ↗
Definitions of vapor humidity, saturation, and suspended droplets.
Source check: Official page opened; definitions reviewed
Educational research context, not a validated device specification or clinical protocol. Read our editorial and correction policy.