Reading the Sky

A cloud is the visible manifestation of liquid water droplets or ice. It forms when humid air cools sufficiently for water vapor to saturate and produce condensation-the dewpoint temperature. On a dry summer day in California, this temperature might be 20 degrees F, and the weather remains clear. On more humid summer days in California, the cloud formation temperature might be 50 degrees, producing morning clouds along mountain peaks. When air is chilled to the dewpoint, the humidity becomes 100 percent and from the texts we expect saturation to occur. But in real life this doesnt always happen. If a given volume of air doesnt contain condensation nuclei-microscopic bits of dust, pollen, etc.-the relative humidity may exceed 100 percent without producing clouds. But for the most part, this relationship between temperature and dewpoint is correct.

Key Takeaways:

• Understanding cloud formation, from humid air cooling to the dewpoint requiring condensation nuclei, and how factors like industrial pollution or African dust contribute to haze, is crucial for pilots to interpret weather.
• Clouds are broadly categorized into cumuliform (heap-like, convective, e.g., cumulonimbus) and stratiform (layer-like, from uniform lift, e.g., nimbostratus), with each group having distinct characteristics and associated flight conditions.
• Specific cloud types across low, middle, and high altitudes provide important insights into potential hazards like icing, turbulence (e.g., altocumulus castellanus, cirrocumulus), or the approach of significant weather systems.
• Pilots can utilize rules of thumb, such as inferring cloud top height relative to the freezing level based on precipitation, or VFR-on-top possibilities for non-precipitating stratiform layers, to supplement and confirm weather forecasts.

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