Dew Point
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Unit: Degrees Celsius (°C)
The dew point is the temperature at which air becomes fully saturated and water vapour begins to condense. Unlike relative humidity, the dew point is an absolute measure of moisture in the air — it doesn’t change as temperature rises and falls throughout the day.
Why it matters for fire weather
Section titled “Why it matters for fire weather”The dew point tells you how much moisture is actually in the air mass, independent of temperature. This makes it more stable and reliable than RH for assessing the background atmospheric moisture level:
- Low dew point (below 5°C) — the air mass itself is very dry. Even if morning RH is high (because temperatures are also low), the afternoon will bring significant drying as temperatures rise and RH plummets.
- High dew point (above 15°C) — significant moisture in the air mass. Afternoon RH will remain higher than in a dry air mass, and fuels can recover more moisture overnight.
The gap between air temperature and dew point — the dew point depression — relates directly to VPD. A large gap means dry air with high evaporative demand.
How it works
Section titled “How it works”Dew point is derived from temperature and relative humidity measurements. It represents the actual water vapour content of the air in a way that doesn’t shift with daily temperature changes.
This stability is what makes dew point valuable for fire weather assessment:
- A sudden drop in dew point signals the arrival of a dry air mass — potentially several days of elevated drying conditions
- Two days with identical afternoon RH can have very different fire weather implications if their dew points differ significantly
Key thresholds
Section titled “Key thresholds”Dew point describes the moisture state of the air mass. Like other individual variables, it gains fire weather significance in combination with temperature, wind, and drought conditions.
| Dew point | Physical significance |
|---|---|
| < 0°C | Very dry air mass. The atmosphere contains very little moisture. Afternoon drying will be intense regardless of temperature. |
| 0–5°C | Dry air mass. When combined with high temperatures, produces very high VPD and rapid fuel drying. |
| 5–10°C | Moderate moisture. Typical of continental air masses in transitional seasons. |
| 10–15°C | Moist air mass. Afternoon RH stays relatively higher, and overnight moisture recovery for fuels is more effective. |
| > 15°C | Humid air mass. Common in maritime and tropical air masses. Limits how low afternoon RH can drop, even on hot days. |
How to read it in Wildflyer
Section titled “How to read it in Wildflyer”Dew point appears in the expert view alongside other atmospheric variables. Because it stays relatively constant through the day (unless a new air mass moves in), it’s useful for:
- Multi-day outlook — a dry air mass arriving means sustained drying conditions, regardless of daily temperature swings
- Overnight recovery — low dew point means fuels won’t recover moisture overnight as effectively
- Comparing days — two days with the same afternoon RH can have very different drying conditions if their dew points differ
Sources
Section titled “Sources”- World Meteorological Organization (2018). Guide to Meteorological Instruments and Methods of Observation. WMO-No. 8.
- Van Wagner, C.E. (1987). Development and structure of the Canadian Forest Fire Weather Index System. Forestry Technical Report 35, Canadian Forest Service.