11.dewpoint-and-temperature-spread. Dewpoint and Temperature/Dewpoint Spread
The dewpoint is the temperature to which a parcel of air must be cooled, at constant pressure and water vapor content, for saturation to occur. When air is cooled to its dewpoint, the relative humidity reaches 100 percent and water vapor begins to condense out as visible moisture — clouds, fog, dew, or precipitation. The numerical difference between the ambient air temperature and the dewpoint is called the temperature/dewpoint spread (often just "the spread"). It is one of the most useful single indicators of moisture available to pilots, because it tells you how close the air is to saturation right now.
A small spread means the air is nearly saturated and only a slight cooling will produce visible moisture. A large spread means the air is dry and substantial cooling would be required before clouds or fog could form. Standard rules of thumb used in FAA materials and operational forecasting:
- A spread of 5°F (about 3°C) or less, and decreasing, is a strong indicator that fog, low stratus, or mist is likely to form, especially overnight as the surface cools.
- A spread near zero (temperature equals dewpoint) means the air is saturated; expect fog, clouds at the surface, or precipitation.
- A spread of 15°F or more generally indicates dry air and a low probability of low ceilings or fog in the near term.
For convective forecasting, the spread also helps estimate the height of the convective cloud base in unstable, well-mixed air. The standard formula is:
Cloud base AGL (ft) ≈ (Temperature − Dewpoint in °C) × 400
or, in Fahrenheit:
Cloud base AGL (ft) ≈ ((Temperature − Dewpoint in °F) / 4.4) × 1,000
Example: Surface temperature is 25°C, dewpoint is 15°C. Spread = 10°C. Estimated cumulus base ≈ 10 × 400 = 4,000 ft AGL. This works because a rising, unsaturated parcel cools at the dry adiabatic lapse rate of approximately 3°C per 1,000 ft (5.4°F/1,000 ft), while the dewpoint of the rising parcel decreases at only about 0.5°C per 1,000 ft. The two values therefore converge at roughly 2.5°C per 1,000 ft, or about 400 ft per °C of initial spread.
Dewpoint is reported on every routine METAR in the format T/Td (for example, 15/12 means temperature 15°C, dewpoint 12°C, spread 3°C). Watching the trend across successive METARs is more valuable than any single observation. If the temperature is falling toward a steady dewpoint as the sun sets, the spread is closing and the probability of fog or low stratus is increasing.
How the Dewpoint Changes
Dewpoint changes only when the actual water vapor content of the air changes. It is not the same as relative humidity. Relative humidity is a ratio that changes with temperature even if no moisture is added or removed; dewpoint changes only when moisture is added (evaporation from soil, water, vegetation, or advection of moist air) or removed (precipitation falling out, advection of drier air, condensation onto a cold surface).
This distinction matters operationally:
- On a warm afternoon over an irrigated field or after a rain shower, the dewpoint can rise sharply as water evaporates, narrowing the spread and lowering the expected cloud base.
- As a cold front passes and drier air advects in, dewpoint drops abruptly — often the clearest sign of frontal passage on a METAR.
- Overnight radiational cooling drops the temperature toward a relatively stable dewpoint, which is why fog is so common in valleys near dawn.
Hazards Associated with a Small Spread
A small or closing spread should alert the pilot to several specific hazards:
- Radiation fog in low areas after sunset, especially with clear skies and light winds.
- Advection fog when moist air moves over a cooler surface (common along coastlines and over snow).
- Upslope fog and low stratus along rising terrain.
- Carburetor or induction icing, which can occur at outside air temperatures from roughly 20°F to 70°F (-7°C to 21°C) whenever the spread is small and visible moisture is unnecessary — relative humidity above about 50 percent is enough.
- Structural icing in flight when the temperature is at or below 0°C and visible moisture is present, which is far more likely with a small spread.
Preflight Use
During preflight planning under 14 CFR 91.103, the temperature/dewpoint spread should be evaluated at the departure, en route, and destination stations, along with the forecast trend. Compare current METAR spreads with TAF forecasts: a TAF that predicts the spread closing through the period is warning you of deteriorating visibility, even if it does not explicitly forecast IFR conditions until later. Pilots flying VFR should plan alternates and earlier departure or arrival times whenever the spread is forecast to be 5°F (3°C) or less anywhere along the route during the planned flight window.