The Australian and English definition of sleet is a combination of snow and rain falling, or snow turning to rain as it falls. That is, if the temperature is not quite at zero degrees celsius (typically between 2C and 3C) and the air is saturated with precipitation, you will usually see a combination of snow and rain. Generally speaking, as the temperature continues to drop below 2C, more snow than rain will appear until only snow will occur at around 1C to 0.5C and below.
In America, sleet is a different phenomenon, being small, clear ice pellets. They are sometimes mistaken for hail but form in a different way and are usually (but not always) smaller - around 5mm or less. This occurs when snow flakes falling through a small layer of warmer air (typically in the mid-atmosphere from 5000 to 10000 feet) begin to melt. They can then re-freeze if they pass back into a layer of colder, sub-freezing air closer to the ground, resulting in little balls of clear ice. In Australia this phenomenon is not called sleet but is usually described by the common name of ice pellets and is sometimes mistaken for hail.
NB: It is possible for snow to occur at temps as high as 5C but the upper temps (at 500hPa) and mid level temps (at 700hPa) need to be cold and the relative humidity needs to be well under 100% at ground level. Such snowfalls are brief and rarely settle.
In the Blue Mountains (and across the Southern/Central and Northern Ranges of NSW) we can also experience graupel which goes by the common names of snow pellets, sago snow or even soft hail. Such icy precipitation is usually less than 10mm in diameter. This phenomenon forms when super cooled droplets in convective clouds (as low as -40C) form over melting snowflakes and produce an outer coating 3m to 5mm thick. The appearance of graupel can vary from a soft, stubby white cone like appearance to something more rounder, brittle and translucent, depending on how cold the air is when it develops in convective clouds.
Snow grains are also observed in the mountains and they are white, opaque grains of ice that are usually very small, typically less than 3mm and are fairly flat or elongated. They tend to fall in weak stratus or fog and the accumulations are very light.
So, just to refresh on the various forms of frozen, winter precipitation in Australia, we have: snow, sleet, graupel (sago snow, snow pellets, wintry hail), ice pellets (american sleet) and snow grains. Remember, sleet is quite easy to identify, it will be observed as genuine snowflakes falling with a bit of rain mixed in.
Amazing Snowflake Image - 1 - magnified, with frozen rime (fibrous material) at each end.
Amazing Snowflake Image - 2 - magnified, with frozen rime (fibrous material) fully covering snowflake.
