Tecnobosque Wildfire Knowledge Graph · Fuels & Weather
Reference Concept · v1.0Definition → relationship → calculation → evidence context
The difference between saturation vapour pressure and actual vapour pressure, expressing atmospheric drying demand.
VPD
What is vapor pressure deficit?
The difference between saturation vapour pressure and actual vapour pressure, expressing atmospheric drying demand.
For Tecnobosque, vapor pressure deficit is useful only when its source, spatial context and observation or modelling window are stated. The same label can otherwise hide important differences between field observations, remote sensing, forecasts and reconstructed incident evidence.
Why it matters
Vapor Pressure Deficit is useful because it separates one specific part of wildfire behaviour, exposure, operations or evidence from nearby concepts that can otherwise be mixed together.
The difference between saturation vapour pressure and actual vapour pressure, expressing atmospheric drying demand.
VPD = e_s − e_aUse consistent vapour-pressure units and state the temperature convention.
What changes vapor pressure deficit?
Field observation, remote sensing, model output and retrospective reconstruction can produce values with different assumptions and uncertainty.
A point, parcel, fire-front segment, incident perimeter and regional product can represent very different physical scales.
Wildfire conditions evolve rapidly, so timestamps and averaging periods are part of the meaning of the value.
How to interpret vapor pressure deficit
- Identify exactly what object or process the value describes.
- Check units, spatial scale, timestamp and source before comparing values.
- Separate direct observations from modelled or reconstructed estimates.
- Preserve uncertainty and later revisions instead of overwriting earlier evidence.
What vapor pressure deficit cannot tell you by itself
- Vapor Pressure Deficit is not a complete description of wildfire hazard or risk by itself.
- Values from different methods or scales may not be directly comparable.
- A technically precise number can still be misleading when context or uncertainty is omitted.
Vapor Pressure Deficit FAQ
Is vapor pressure deficit one fixed value for an entire wildfire?
Usually not. Wildfire conditions vary in space and time, so the value should be attached to a defined place, object and observation period.
Can vapor pressure deficit be used alone for an emergency decision?
No. Operational decisions require current local authority information and a wider set of conditions than a single technical concept can provide.
Vapor pressure deficit: atmospheric drying demand, not fuel moisture
Vapor pressure deficit (VPD) is the difference between saturation vapour pressure at a given air temperature and the actual water-vapour pressure. It is commonly expressed in kilopascals. For the same air temperature, lower relative humidity generally means higher VPD.
VPD describes an atmospheric moisture gradient that can influence plant water stress and evaporation, but it is not a direct measurement of fine dead-fuel moisture or live vegetation water content. Plants and fuel particles respond over their own timescales, modified by exposure and physiology.
Example: at a fixed temperature, moving from 60% to 30% relative humidity increases the fractional gap from saturation from 0.40 to 0.70, a factor of 1.75. VPD itself is therefore 1.75 times higher under these simplified constant-temperature conditions, not simply 30 percentage points higher. Include temperature when calculating real values.
Fire-weather interpretation should consider wind, fuel receptivity, antecedent weather and local topography. VPD can be a valuable explanatory variable but does not predict a specific wildfire ignition or growth rate on its own.
See Relative Humidity, Dead Fuel Moisture and Fire Weather.