The short answer to this is, it depends who you ask.
According to one early-reader guide to snow and the Inupiaq language (what we generally mean when we say "Eskimo"), you can talk about snow in terms of
- how packed-down its gotten
- whether it's good to use for building things
- if it's already been made into something
- if it's still falling or if it's on the ground
- whether or not it has been moved by wind or something else.
But another crucial way for the Inupiaqs to describe snow is in terms of how good it is to turn into drinking water. I'm not going to get the diacritic marks right at all, but here are those terms in general:
- nutagaq -- freshly fallen, light snow, easy to blow away
- silliq -- next layer down in a snowdrift, packed more tightly
- pukak -- at the very bottom of a snowdrift, just above the ground, grainy snow that feels like pebbles or salt. This is the best kind of snow to melt into water because the compression has worn the points off the snowflakes. This allows the flakes to be packed closer together, which means there's less air between the flakes. It's also the easiest to scoop into a container. When you melt pukak, you'll get the most water from it.
There are a lot more than three layers of snow here. But I think the same principle that the bottom part is the best for melting still applies.
(Photo from the Snow Hydrology Gallery at UCSB)
A snowboarder, however, will tell you about various forms of snow in terms of its surface:
- Powder -- Freshly fallen, untouched, uncompacted, airy, soft snow.
Powder is new-fallen, soft, and ideally pristine snow like this, near Salzburg
(Photo from the Hotel Gasthof zum Kirchenwirt)
- Crud -- Powder that's been packed down, tracked, footprinted, or otherwise mucked up.
- Corn snow -- After several cycles of nightly freezing and daily thawing, the snow gets wet and grainy and heavy.
- Crust -- Hard crust on top of powder beneath. Your feet tend to punch through this. The crust forms when sun melts the top layer of snow, but the colder temperature freezes it again.
- Loose granular -- Wet or icy snow that's been groomed into smaller, loose pellets.
- Wet granular -- Very wet snow, usually occurring in the spring, easy to form into snowballs.
- Slush -- When the air temperature rises above freezing, snow crystals change to larger pieces of ice. Heavier and wetter than snow.
Corn snow forms when the snow melts just enough to create kernels of snow surrounded by slick patches of melted snow-water. This kind of snow will support your weight if it's still cold enough, but if the day gets any warmer, the water between the kernels will increase and your feet will sink through to slushy snow beneath.
(Diagram from Avalanche.org)
- Ice -- Most heavy-snow areas will never entirely turn to ice. But the top layers can melt and freeze several times until they become ice -- solid, hard, and slick.
Meteorologists will tell you about these categories of falling snow:
- Snow -- ice crystals that have ganged together to form flakes at temperatures below freezing.
Snow falling in Cleveland during a snow storm in December 2007
(Photo by Chris Bennis, sourced from WYKC in Cleveland)
- Snow pellets -- As ice crystals or snow flakes fall, supercooled water gathers on the crystals. This can happen as a snowflake melts about halfway and then re-freezes. THey have small air pockets locked within them. Pellets will break apart or be crushed when pressed.
- Sleet or Ice pellets -- Similar to snow pellets in appearance, but these are frozen raindrops that do not have air pockets. Usually this starts as snow way up in the atmosphere but melts on the way down to the earth and then passes into a subfreezing layer where it freezes again and turns into ice.
- Snow grains -- Very small grains of ice, solid version of a drizzle, little accumulation.
- Ice crystals -- Crystals of ice that are so small, they float in the wind.
- Hail -- Falling, dense ice at least 5 mm in diameter. Forms first as ice crystals and supercooled water attaches to it and freezes there.
Because hail forms as ice with water that freezes onto it, it often takes on a layered or clumpy shape, as in this piece of hail measuring in at an astonishing 6 inches in diameter -- that's about the size of a grapefruit.
(NOAA photo posted at tornadochaser.net)
- Graupel -- Same thing as hail, except less than 5 mm in diameter.
- Freezing rain -- Liquid precipitation that turns to ice after it hits the ground.
If you talk to a chemist or a physicist, they'll probably classify snow according to the individual flakes or crystals. And there are all kinds of ways in which the crystals have been categorized.
- In 1951, the International Commission on Snow and Ice produced a simpler system of grouping the flakes into 7 general categories.
- spatial dendrites
- capped columns
- irregular crystals
Process by which a snow crystal grows. Depending on which classification system you use, the different steps of the process might fall into a different category of crystal.
(Image from Mystery in the Air by Pete Dunkelberg)
- Another snow researcher thought the 7 categories were way too simplistic, so he devised his own system using 35 types of snowflakes.
Kenneth Libbrecht of Caltech's abbreviated guide to snowflakes
- Still other snow researchers have devised their other systems using as many as 80 categories. Then the International Commission on Snow and Ice met again and revamped their whole system. They said that each snowfall will differ in terms of the snow's
- grain shape
- grain size
- liquid water content
- snow temperature
- and the snowfall in general will also have its own characteristics of
- thickness (amount of snowfall)
- surface roughness
- load-bearing capacity
- water equivalent
- aspect (slope)
- If I remember my math right, that's 13! or 6,227,020,800 possible different kinds of snowfall.
(You can have this image, along with several others, as your screen saver, from e-motional.com)
Alaska Native Education Program, Immiugniq CH3 - How Many Kinds of Snow Are There?
LINGUIST mailing list, Eskimo Words for "Snow"
ABC of Snowboarding, Snow Types
Mike Doyle, Your Guide to Skiing, About.com, Types of Snow
Jeff Haby, theweatherprediction.com, Precipitation Types
Kenneth G. Libbrecht, Caltech, A Guide to Snowflakes
Argonne National Laboratory, Ask A Scientist Weather Archive, Types of Snow Crystals, February 26, 2004
Working Group on Snow Classification, The International Classification for Seasonal Snow on the Ground, 1985(?)