Moisture, Clouds and Precipitation
Moisture, Clouds and Precipitation covers humidity measures, the dewpoint, cloud formation and classification, and the precipitation types the exam expects you to link to stability.
Humidity
- Dewpoint (Td): the temperature to which air must be cooled at constant pressure to become saturated.
- T − Td spread: the smaller it is, the closer the air is to saturation and the higher the relative humidity.
- Mixing ratio is conserved when air rises; relative humidity rises as temperature falls.
Cloud base and formation
Convective cloud base ≈ 400 ft per °C of surface T − Td spread (derived from DALR 3 °C/1000 ft and a dewpoint fall of about 0.5 °C/1000 ft). Cooling mechanisms: lifting (orographic, frontal, convergence), radiation, and advection over a cold surface.
Classification
- High: cirrus, cirrostratus, cirrocumulus.
- Medium: altostratus, altocumulus.
- Low: stratus, stratocumulus, nimbostratus.
- Vertical: cumulus, cumulonimbus.
Precipitation
Continuous rain or drizzle comes from stratiform cloud; showers from cumuliform cloud. Growth occurs by the Bergeron (ice crystal) process and by coalescence. Hail needs strong updraughts and repeated freezing cycles. Virga is precipitation that evaporates before reaching the ground.
Worked example
Surface T 20 °C, Td 14 °C gives a spread of 6 °C, so the convective base is 6 × 400 = 2400 ft AGL. At an aerodrome 500 ft AMSL the base is 2900 ft AMSL, consistent with a TAF of BKN030 - a usable but low ceiling.
Traps
- Forgetting AGL versus AMSL when converting the cloud base.
- Thinking air must cool to 0 °C to condense; it must cool to the dewpoint.
- Assuming relative humidity cannot reach 100% below freezing.
- Confusing drizzle from stratiform cloud with showers from cumuliform cloud.