Winter Hydrometeor Classification Ground Truth Program

Project Status: Active 5 Mar 2008 -- A potentially major winter storm is in the offing, perfectly situated in the project observing area. The event will likely start with rain, then change to sleet, ending finally as snow. There may be a brief period of freezing rain, but only a little. Much more important will be snow amounts. Capturing the change form rian to snow is important, so as soon as the change occurs, please log your observations.

NEW! Snowflake Identification Manual!

If you live within 90 miles of Norman, OK, we invite you to be a part of NSSL’s science and research.  We are looking for volunteers of any age and education level (including teachers, families and classes) to observe and describe what kind of precipitation is falling at your location during winter weather events.  The project is called the Winter Hydrometeor Classification Ground Truth Field Program, and is the first of its kind.

The project supports NSSL scientists involved in developing new radar technologies and techniques to determine what kind of precipitation — such as snow, ice, or rain — is falling at any given location.  As a volunteer, you will provide valuable information that will allow us to compare what you have observed with what the radar has detected.

You can spend as little (or as much) time as you wish on making observations, because the basic idea is simple: NSSL will collect radar data during winter storm events within about 90 mi of Norman, OK, but we also need to know what's happening on the ground, where you live. Is it snowing, raining, sleeting or doing nothing at all? While we have recruited some volunteer observers to help us (mainly from public schools within 90 mi of Norman), we need more observations. In fact, we need as many observations from as many different locations as we can get.

Ideally, observations are taken every hour throughout the event, but we know that this is not usually possible for anyone, so any observations that you provide are helpful. A complete observation consists of the date, time, precipitation type and location. Locations can be either a complete address or — even better! — an accurate latitude/longitude .

Some notes on precipitation types:

On the observation reporting form, you will see the following categories for precipitation type:

• None
• Rain
• Hail
• Freezing Rain
• Drizzle
• Freezing Drizzle
• Snow
• Ice Pellets/Sleet
• Graupel/Snow Grains

Except for None, each category has very technical definitions, but for our purposes, we can keep this simple.

Liquid Precipitation:

Rain differs from drizzle only in the size of the droplets. If you can see individual drops falling (or hear them at night), it's rain. If it's very fine and the individual droplets can't be seen easily (or heard at night) it's drizzle.

Freezing Precipitation:

Freezing precipitation (freezing rain and freezing drizzle) may not always be easy to detect. Obviously, the air temperature must be below freezing for freezing precipitation and that's almost always sufficient. To be sure, look for ice forming on shrubs or tree limbs. Looking for ice on the ground isn't a good strategy because the ground is almost always warmer than the air and is one of the last places to accumulate ice.

Frozen precipitation:

Snow is, well, snow. That's an easy one. You can read more about identifying the different types of snowflakes in the Snowflake Identification Manual.

Hail comes exclusively from thunderstorms. Hail in winter is rare, but especially in winter when thunderstorms occur, it's always a possibility. In winter, the temperature within the thunderstorm is cooler overall, allowing hail to be formed at lower altitudes. Because the temperature below the thunderstorm is also cooler than in spring or summer, even small hail is likely to survive to reach the ground.

Ice pellets/sleet looks like little ice BBs. A few are nearly perfect spheres, but most are hemispheres. They have the clear/translucent character of ice.

Graupel/snow grains are like bits of frost you might scrape off of a freezer or, alternatively, tiny bits of Styrofoam. They are white and sometimes shaped like tiny Apollo capsules. Graupel particles tend to be very light, fall more slowly than ice pellets, and can be easily crushed under finger pressure. They're literally *grains* of snow. We don't see graupel/snow grains here often so if you do see it, you're getting a real treat.

If you have any questions, feel free to contact Dr. Kim Elmore by email at kim.elmore@noaa.gov or telephone at (405) 325-6295.