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FAA Aircraft Dispatcher (ADX)MeteorologyMedium

A dispatcher is estimating the height of convective cloud bases at an airport reporting a surface temperature of 82°F and a dewpoint of 62°F. Using the standard rule of thumb, approximately what height AGL should the dispatcher expect the cloud bases to form?

  1. A2,000 feet AGL
  2. B12,000 feet AGL
  3. C8,000 feet AGL
  4. D5,000 feet AGL
Show answer & explanation

Correct answer: C. 8,000 feet AGL

The temperature-dewpoint spread is 82°F − 62°F = 20°F. Using the rule of thumb that cloud base height (ft AGL) equals the spread multiplied by 400, 20 × 400 = 8,000 feet AGL.

Why the other options are wrong

  • A. This would correspond to a spread of only about 5°F, far less than the actual 20°F spread.
  • B. This would require a spread of 30°F, more than double the actual computed spread.
  • D. This corresponds to a 12–13°F spread, not the 20°F spread calculated.

Convective Cloud Base Estimation

A rule of thumb used to estimate the height of cumuliform cloud bases using the surface temperature-dewpoint spread.

  • Formula: cloud base (ft AGL) = spread (°F) × 400
  • Metric equivalent: spread (°C) × 1,000 / 2.5
  • Applies best to convective (cumulus) cloud formation, not stratiform layers

Memory trick: 'Spread times 400 gets you off the ground' (in feet AGL).

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