Icing is the formation of ice on the balloon. There's much more information available about heaver-than-air craft (airplanes) than lighter-than-air craft (balloons). Most information presented here is relevant to heavier-than-air craft, and it's applicability to balloons will be specifically stated, if known.
The remarkably short lifetime of many pressurized balloon flights at 300 mb had been tentatively explained by overloading with ice. This phenomenon was definitely observed and well documented in one recent flight. When the balloon entered a dense cirrus cloud the observed ice layer growth rate was very fast, 2 μm min−1. This transport of ice particles cannot be accounted for by eddy diffusion alone in the vicinity of the craft. The proposed explanation of the observed accretion rate of ice is the fall of the large ice particles on the top of the balloon.
Icing is usually restricted to the lower 30,000 feet (9,140 m) of the troposphere1).
This is caused by average size freezing rain or drizzle drops hitting an aircraft surface and spreading out before freezing on the sub-zero aircraft surface2). It will most likely occur in the troposphere, below the jet stream. It is very heavy, and hard to get rid of.
This occurs when tiny super-cooled liquid water droplets in a cloud freeze instantly on contact with a sub-zero aircraft surface. It forms a rough, crystalline, brittle deposit that has many airspaces inside. Temperatures possible: 0C to -40C, most likely from -10C to -20C3).
Frost occurs when moist air comes in contact with a sub-zero surface. The water vapor forms ice deposits directly on the surface without going through the liquid phase. It forms a fluffy rough coating that can be brushed off easily. Typical conditions required for frost require a clear night, calm air, and high humidity. Frost can occur when an aircraft descends from sub-zero air into warmer, moist air.4)
Our research has show that cirrus clouds are not uncommon along the jet stream, and are likely caused by the low pressure in the jet stream. A cirrus cloud that forms at 300mb to 200mb altitudes should form ice crystals, not liquid water droplets5).
Cumulus clouds that extend upward through these altitudes are uncommon, however they do occur. Cumulonimbus cloud formations from storm cell lifting can carry supercooled liquid water droplets to and beyond the level of 200mb. Liquid droplets in cumulonimbus clouds are likely to be present in temperatures from 0C to -20C, and uncommon from -20C to -40C, and very unlikely below -40C6)7).
Risk of airframe icing in certain types of clouds8):
|Risk||Cumuliform Clouds||Stratiform Clouds||Freezing Rain/Drizzle|
|High||0 to -20C||0 to -15C||0C and below|
|Med||-20C to -40C||-15C to -30C|
|Low||-40C and below||-30C and below|