wouldn't a cumulus cloud have bigger droplets than a stratus cloud... and therefore be more susceptible to clear ice?

If the stratus cloud was described as 'nimbo stratus' it would have bigger droplets than a small cumulus. Any cloud with 'nimbo' or 'nimbus'' in its name such as nimbo stratus or cumulonimbus will have large droplets and therefore be likely to produce clear ice if flight is above the freezing level. Large droplets do not reflect light as efficiently as small droplets, so clouds with large droplets always look darker than clouds with small droplets.

Bob

Thanks Bob!

Page 78 of the BOM Manual of Meteorology has a great diagram in figure 11.2.

It shows the three TEMPERATURE thresholds as:

0 deg to minus 10 deg (Clear Ice)
minus 10 deg to minus 15 deg (Mixed - clear and rime)
minus 15 to minus 20 deg (rime ice)

Problem was I got a mock exam question wrong on this, asking ice type on minus 5 deg, light winds in Altocumulus cloud! I had assumed clear ice due to the above temp range from the BOM book.

If you then go to the chart on page 45 of the same book, you see a potentially conflicting bit of information which shows the icing for this cloud type as usually light rime. Go figure...(light rime was the answer in the mock exam).

The BOM Aviation Meteorology on pages 20-21 gives a summary of cloud types that includes most likely icing to be encountered in various cloud types. Also in my CPL bet book I have provided a similar summary on pages 4.7 to 4.16.

The main difference between clear ice and rime ice is the time it takes for the supercooled droplets to freeze. Large droplets have to lose more latent heat and therefore take longer to freeze. This gives them time to splash and spread across the surface before they freeze, giving rise to clear ice. Small droplets [drizzle sized] freeze almost instantaneously giving rise to the granular structure of rime ice. Very small droplets can 'snap' freeze at any temperature below zero.

Although the temperature ranges given are a guide, the droplet size is the most important factor in deciding which type of ice forms. Cloud types that have weak updrafts are unable to support large droplets so are likely to contain only small droplets. These clouds have less vertical development and are often referred to as 'thin'. CASA base their questions on the BOM Manual of Met, so if you go for the ice type listed in those summaries, you will be more likely to get the mark.

Thanks Bob - so Droplet size for cloud type is the key, rather than temp. Thanks for the great pics in the Irex book!