FAA-H-8083-28B, ch. 6 and 18

Temperature-dewpoint spread

The spread is one number you can read straight from a METAR, and it tells you how close the air is to saturation and fog.

Part 107, explained › Weather · Last reviewed October 25, 2026 against the eCFR and faa.gov. UAV Campus is not affiliated with the FAA.

What is the temperature and dew point spread, and why does it matter?

The spread is the difference between air temperature and dewpoint. As it shrinks, relative humidity rises, and at zero the air is saturated (100 percent). Surface spread helps you anticipate fog, which seldom forms when the spread is greater than 2°C (4°F). It has little bearing on precipitation.

Dewpoint is the temperature to which air must be cooled for the water vapor in it to condense. The amount of water vapor air can hold depends on its temperature, so a warm parcel can hold more. Cooling air without adding or removing water vapor raises its relative humidity until the temperature reaches the dewpoint.

Surface aviation weather reports such as METARs give both temperature and dewpoint (18/16 in the sample METAR). A small spread on a calm, clear night with moist air is a warning that fog may form. Fog forms when the temperature and dewpoint become identical or nearly so.

Heat is a separate issue for performance: how temperature changes density altitude is explained on our other factors affecting performance page.

Sources: FAA-H-8083-28B, p. 83 · FAA-H-8083-28B, p. 84 · FAA-H-8083-28B, p. 222 · FAA-H-8083-28B, p. 302

What the FAA says

FAA-H-8083-28B, p. 84

As the spread decreases, relative humidity increases. When the spread decreases to zero, relative humidity is 100 percent, and the air parcel is saturated.

FAA-H-8083-28B, p. 84

Surface temperature-dewpoint spread is important in anticipating fog but has little bearing on precipitation.

FAA-H-8083-28B, p. 222

Fog seldom forms when the temperature-dewpoint spread is greater than 2°C (4°F).

FAA-H-8083-28B, p. 302

Dewpoint is the temperature to which a given parcel of air must be cooled at constant pressure and constant water vapor content for the air to become fully saturated.

Sources: FAA-H-8083-28B, p. 84 · FAA-H-8083-28B, p. 84 · FAA-H-8083-28B, p. 222 · FAA-H-8083-28B, p. 302

Exam traps

  • Dewpoint is the temperature to which air must be cooled to become saturated, not a measure of how much water the air can hold.
  • The amount of water vapor air can hold depends on its air temperature: warmer air can hold more.
  • When the spread reaches zero, relative humidity is 100 percent and the air is saturated.
  • Surface spread is a fog clue, not a rain clue: it has little bearing on precipitation, which needs air saturated through thick layers aloft.

Quick quiz: 3 questions

Similar to the FAA test, not the actual test questions. Each answer shows its FAA source.

Question 1 of 3

What is meant by the term "Dew Point"?

  1. A The temperature at which condensation and evaporation are equal.
  2. B The temperature at which dew will always form.
  3. C The temperature to which air must be cooled to become saturated.
Show the answer
Answer: C. The temperature to which air must be cooled to become saturated.

The Dew Point is the temperature at which the air becomes fully saturated.

FAA source: FAA-H-8083-28B ch. 6, 6.5 Dewpoint

Question 2 of 3

The amount of water vapor which air can hold depends on the

  1. A air temperature.
  2. B Dew Point.
  3. C stability of the air.
Show the answer
Answer: A. air temperature.

The amount of water vapor that an air mass can hold is dependent on its temperature. A warmer air mass can hold more water.

FAA source: FAA-H-8083-28B ch. 6, 6.4 Relative Humidity

Question 3 of 3

What situation is most conducive to the formation of radiation fog?

  1. A Warm, moist air over low, flatland areas on clear, calm nights.
  2. B Moist, tropical air moving over cold, offshore water.
  3. C The movement of cold air over much warmer water.
Show the answer
Answer: A. Warm, moist air over low, flatland areas on clear, calm nights.

Radiation fog occurs when the ground cools rapidly and the surrounding air reaches the dew point. This happens with little to no winds.

FAA source: FAA-H-8083-25C ch. 12, Fog

What spread usually means fog is possible?

Fog forms when temperature and dewpoint become identical or nearly so, and it seldom forms when the spread is greater than 2°C (4°F).

Source: FAA-H-8083-28B, p. 222

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More on this topic: Part 107 weather, task by task.