Practice test
Unit 9 · Weather Theory · Lesson 2 of 4

Moisture, stability and clouds

9 min readLesson 39 of 59

By the end of this lesson you'll be able to:
  • Explain dewpoint and the temperature-dewpoint spread
  • Compare stable and unstable air
  • Identify the main cloud types and what they mean

Dewpoint

The dewpoint is the temperature to which air must be cooled to become saturated. When the temperature and dewpoint get close (a small spread), expect clouds, fog or low visibility. If the spread is 2°C or less and closing, fog is a real possibility.

A rule of thumb for estimating the base of cumulus clouds: divide the temperature-dewpoint spread in °C by 2.5 and multiply by 1,000 (that's in feet AGL). A spread of 10°C gives a cloud base around 4,000 feet AGL.

Stable and unstable air

Stable airUnstable air
Stratiform (layered) cloudsCumuliform (piled-up) clouds
Smooth airTurbulence
Fair to poor visibility (haze, smoke)Good visibility, except in showers
Steady precipitationShowery precipitation

Air is unstable when a rising parcel stays warmer than the air around it, so it keeps rising. That's how cumulus clouds grow into thunderstorms.

Cloud families

  • Low clouds (surface to about 6,500 ft AGL): stratus, stratocumulus, nimbostratus. Low ceilings and drizzle or steady rain.
  • Middle clouds (about 6,500 to 20,000 ft): altostratus and altocumulus.
  • High clouds (above about 20,000 ft): cirrus, cirrostratus, cirrocumulus. Made of ice crystals.
  • Clouds with vertical development: towering cumulus and cumulonimbus, the thunderstorm cloud. Stay away.

"Nimbo" or "nimbus" in a cloud's name means it's producing precipitation.

Key takeaways

  • Small temperature-dewpoint spread = clouds, fog, low visibility.
  • Cloud base ≈ spread (°C) ÷ 2.5 × 1,000 ft AGL.
  • Stable: stratiform, smooth, hazy, steady rain. Unstable: cumuliform, bumpy, clear, showers.

What the examiner may ask

Practice answering out loud before you open each answer.

The temperature-dewpoint spread is 1°C and closing at sunset. What do you expect?
Fog or low clouds are likely, especially with light winds and clear skies.
What clouds signal instability?
Cumuliform clouds, especially towering cumulus and cumulonimbus.
What's a temperature inversion and why does it matter?
Temperature increasing with altitude. It traps haze and smoke (poor visibility) and can create wind shear at its boundary.

Common mistakes

  • Departing at dusk with a small, closing temperature-dewpoint spread.
  • Assuming smooth air means good visibility.
  • Flying under building cumulus on a hot afternoon.

Read it in the FAA sources

PHAK · Chapter 12Weather TheoryOpens the PHAK at this chapter ↗

Lesson check

Get every question right to complete the lesson. Take your time; wrong answers explain why.

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