The Australian and English definition of sleet is a combination of snow and rain. 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 0.5C. It is possible for snow to occur at temps even up to 5C but the upper temps (at 500hPa) need to be cold and the relative humidity needs to be quite low. Such snowfalls are brief and rarely settle. At temperatures near or below freezing, you can also see what are sometimes called snow grains. These are small shards of ice (usually less than 5mm in diameter) that typically fall from weak or de-activated stratus clouds. They are the frozen form of very light drizzle.
In America, sleet is defined as tiny frozen raindrops, or ice pellets. They are sometimes mistaken for hail but form in a different way and are usually (but not always) smaller. This occurs when snow flakes falling through a small layer of warmer air in the atmosphere will begin to melt. They can then refreeze if they pass back into a layer of colder, sub-freezing air closer to the ground, resulting in little balls of clearish ice. In Australia this phenomenon is not called sleet but is sometimes described by the common name of ice pellets. A similar looking phenomenon in winter is small or wintry hail. This forms in a different way, circulating in congestive clouds that usually have weaker updrafts than storm clouds in spring and summer. Hail in this context is usually small, less than 1cm across and quite brittle.
In the Blue Mountains and across the Central Ranges of NSW we can also experience 'snow pellets' or 'sago snow' that is usually less than 10mm in diameter. This phenomenon is also called graupel which forms when super cooled droplets in convective clouds form over melting snowflakes and produce an outer coating 3m to 5mm thick.
So, just to refresh on the various forms of frozen precipitation, we have: snow, sleet, sago snow (graupel, snow pellets), ice pellets (american sleet) snow grains and small/wintry hail. That should confuse you!
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.
