Vapour Pressure Deficit (VPD)

Unit: Kilopascals (kPa)

VPD is the difference between the amount of water vapour the air can hold at a given temperature and the amount it actually holds. Think of it as the atmosphere’s “thirst” — the higher the VPD, the more aggressively the air pulls moisture from vegetation and fuels.

Why it matters for fire weather

VPD is a more direct measure of atmospheric drying than either temperature or relative humidity alone. Here’s why:

At 30°C and 40% RH, VPD is approximately 1.7 kPa. At 40°C and 40% RH, VPD is approximately 2.8 kPa.

Same relative humidity, but the hotter air dries fuels 65% faster. VPD captures this because it accounts for both temperature and humidity in a single number.

A landmark global analysis by Jolly et al. (2015) found that fire weather seasons have lengthened across 25% of Earth’s vegetated surface since 1979, with increasing VPD identified as a key driver of this trend.

VPD is a core component of the HDWI (Hot-Dry-Windy Index), which multiplies VPD by wind speed to produce a measure of atmospheric fire environment.

How it works

VPD is calculated from temperature and humidity:

1. Saturation vapour pressure (e_s) — the maximum water vapour the air can hold at a given temperature. This increases exponentially with temperature (roughly doubling every 10°C).
2. Actual vapour pressure (e_a) — how much water vapour is actually present (derived from RH or dew point).
3. VPD = e_s − e_a

The exponential relationship with temperature is what makes VPD so responsive to heat. Each additional degree of warming at low humidity produces a disproportionately larger increase in drying power.

Key thresholds

VPD thresholds describe the physical stress the atmosphere places on vegetation and fuels. These values are drawn from fire weather and plant physiology research.

VPD	Physical significance
< 1.0 kPa	Moderate evaporative demand. The atmosphere is relatively moist and pulls water slowly from fuels and vegetation.
1.0–2.0 kPa	Increasing evaporative stress. Many plant species begin to close stomata (pores) to conserve water, reducing their ability to regulate moisture.
2.0–3.0 kPa	High evaporative demand. Fine fuels dry rapidly. Vegetation under prolonged stress at this level becomes increasingly flammable as it can no longer replace lost moisture.
3.0–4.0 kPa	Intense atmospheric drying. Even live vegetation loses moisture faster than it can replace through root uptake. Conditions are conducive to significant fire behaviour when combined with wind.
> 4.0 kPa	Extreme evaporative demand. Conditions associated with the most destructive wildfire events globally. During the 2023 Mediterranean fires, sustained VPD above 4 kPa accompanied temperatures of 40–45°C with 10–20% RH.

How to read it in Wildflyer

VPD appears on the weather timeline and in the expert view. It peaks in the early-to-mid afternoon, matching the most fire-weather-relevant window of the day.

VPD is particularly useful during heat waves — temperature and RH may look familiar individually, but VPD reveals the compounding effect. During prolonged heat events, VPD can remain elevated overnight, meaning fuels don’t recover moisture as they normally would.

Sources

• Jolly, W.M., Cochrane, M.A., Freeborn, P.H., Holden, Z.A., Brown, T.J., Williamson, G.J., & Bowman, D.M.J.S. (2015). Climate-induced variations in global wildfire danger from 1979 to 2013. Nature Communications, 6: 7537.
• Sack, L., John, G.P., & Buckley, T.N. (2018). ABA accumulation in dehydrating leaves is associated with decline in cell volume, not turgor pressure. Plant Physiology, 176(1): 489–495.