Wednesday, December 18, 2013

Snow to Water Ratios, or How Much Water is in that Snow

If you are a CoCoRaHS observer in a part of the country that receives snow, then you no doubt have reviewed the snow measurement training  that's available on the CoCoRaHS web site, as well as the snow measurement animations on YouTube. You have reviewed these, right?

Taking a snow core
for a water equivalent measurement.
Snow measurement involves several more steps than just measuring what's in your gauge. One of the measurements we ask observers to make is the liquid water equivalent of the new snow measured from a snow core taken from the snow board or other flat surface. Some may ask "Why bother?  Doesn't one inch of water equal ten inches of snow?"   No, not really.

The adage "one inch of rain equals ten inches of snow" is a myth. It happens, of course, but only under certain circumstances. Observers should never "measure" snowfall by using the 10:1 ratio to convert the water in the gauge to snow depth. That's why we ask observers to measure the depth of new snow, and to measure the liquid water equivalent of that snow.

A stellar dendrite snow crystal
So what's with the 10:1 rule? It was first provided to weather observers by the U.S. Weather Bureau in 1875, although it later was qualified to be a rough approximation. It tends to occur or be close to 10:1 when the surface temperature is around 30°F during the snow.  However, the snow to water ratio, or snow density, is dependent on many complex factors. Among these are are the temperature and water vapor within the dendritic growth zone in the clouds (where the temperature is -12°C to -18°C), the depth of the dendritic zone. Dendrites are beautiful, ornate snow crystals that most people are familiar with. Additional factors affecting snow density include the shape of the snow crystals, vertical motion in the clouds, the amount of liquid water in the cloud, and the thermal profile of the layers from cloud level to the surface. Once the snow starts accumulating on the ground, compaction due to the weight of the snow begins to take effect. Wind will also affect the density of the snow. Strong winds break up the snow flakes/crystals into smaller pieces that allow the snow to compact further. Climatologically, the 10:1 ratio tends to occur only over a relatively limited area. A climatology of snow to water ratios was published in 2005 and showed that on average, a ratio of 13:1 was more typical.

30-year climatology of snow to water ratios.
Credit:  A Climatology of Snow-to-Liquid Ratio for the Contiguous United States Martin A. Baxter, Charles E. Graves, and James T. Moore, St. Louis University, October 2005. AMS Journal of Weather and Forecasting

Snow falling on Saturday
Snow to water ratios can change during a storm, and that can be a challenge for forecasters when trying to determine how much snow will result from the amount of liquid water expected. A case in point is the storm that hit my area this past weekend.  The snow started here on Friday afternoon. By Saturday morning at observation time (it was still snowing) there were 6.8 inches of snow that melted down to 0.67 inch of water. The snow to water ratio was 10.1:1. There was little wind during the storm and the snow came straight down, frosting trees, fences, and any horizontal surface, turning everything into a winter wonderland. It continued snowing until about noon Saturday at which point there was another 1.7 inches of snow. However, the lower levels of the atmosphere had warmed some so the snow was denser. That snow melted down to 0.28 inch of water for a snow to water ratio of 6.1:1. That's quite a difference.

Another 0.8 inch of snow on Sunday night melted to 0.06 inch water for a snow to water ratio of 13.3:1.  So over a period of a little over 48 hours I measured 9.3 inches and snow to water ratios ranging from 6:1 to 13:1.

In general, the colder it is, the less dense the snow will be. Snow falling at 20°F will tend to be dry and "fluffy" while snow at 32°F will tend to be wetter and denser, the good-packing snowball-making, snowman-making kind of snow.

Tuesday, December 3, 2013

Brrr-ace Yourself!!

We've had some cold weather in the last two weeks, but nothing like what will occur over the next several days. In the upper atmosphere a broad trough low pressure extends across the continental U.S.  and over the next several days that trough will deepen as a sharp ridge develops just off the west coast.

500 millibar map (~18,000 ft.) for 6:00 a.m.Tuesday, December 3

That will clear the way for cold air from the Arctic to plunge southward well into the U.S.  Snow cover is fairly extensive across Canada, but the southern extent of the snow is a little north of normal in the central and northern Plains. That will change over the next few days.

Snow on the ground for December 3, 2013.
While the upper air system gets its act together, a complex low pressure system will be organizing over the central Rockies, producing precipitation eastward along and behind the leading edge of the cold air. Winter Storm Warnings are in effect from western Nevada through the central Colorado Rockies, and from the eastern two thirds of Montana across the Dakotas and Minnesota to northwestern Wisconsin.

Watches, warnings and advisories in effect as of 7:40 p.m. CST.
Click this link for the latest version.

Four to eight inches of snow is likely from southern Utah to western Lake Superior, with amounts possibly reaching 12 inches or more in the Colorado Rockies and along the west side of Lake Superior.  On Thursday and Friday a mix of wintry precipitation including freezing rain is possible from southern Oklahoma through Arkansas into western Kentucky and Tennessee.

By Saturday the leading edge of this cold air mass will push off the east and Gulf coasts, but another reinforcing surge of cold air will push through the the central U.S. early next week as  the closed upper low settles in over the Great Lakes.

Forecast 500 millibar map for Tuesday, December 10.
Maximum temperatures are expected to be below zero in the northern Plains northern Great Lakes by Saturday. It will warm slightly and then the reinforcing surge of cold air will bring maximum temperatures back down by Tuesday.

Forecast maximu, temperatures for Saturday, December 7 (left) and Tuesday, December 10 (right).
On Saturday minimum temperatures are expected to be well below zero in the northern Plains and Rockies, and below freezing from the Appalachians to the Sierra Nevadas

Forecast minimum temperatures for Saturday, December 7.

The outbreaks of very cold air over this much of the country in early December do happen, but not very often. Severe cold air outbreaks have occurred in December 1977, 1983, 1989, and 2000. December 1983 still ranks as the coldest December on record for the Midwest, with December 2000 the second coldest and December 1989 the third. The unusually cold weather the first half of December 1977 was followed by a very mild period, and December 1977 ranks as the 27th coldest. However, just after Christmas that year it turned cold again, and the winter of 1977-1978 ended up going into the record books as one of the most severe winters on record for the eastern two-thirds of the country.

So, button up and batten down. Be sure to keep abreast of the latest weather developments at your local NWS office web site. It looks like winter weather will be holding on for much of the country, especially the central U.S., through the middle of the month.

Temperture outlook for the period December 11-17, 2013.
Credit: NOAA/NWS Climate Prediction Center

CoCoRaHS observers, it's time to review snow measurement techniques!  See the training animations at the CoCoRaHS YouTube channel. They are a great refresher and it will take only around 20 minutes to view them all.

Monday, December 2, 2013

Need Some CoCoRaHS Holiday Gift Ideas?

Snow stick
The holiday season is upon us, and if your searching for gift ideas for the CoCoRaHS observer in your life (or something to put on your own list) here are some suggestions. Some of these are of more interest to those in CoCoRaHS, but others will appeal to any weather enthusiast.

Now that winter is here, any CoCoRaHS observer that has to deal with snow will appreciate an extra outer cylinder. When it's snowing they just have to swap out cylinders and bring in the full cylinder to melt the contents. No waiting for the snow to stop if it's snowing at observation time. Even those who never see a snowflake during the year will appreciate the convenience of an extra outer cylinder when it's raining at observation time.

Staying with the snow theme, an aluminum snow stick will make the job of measuring snow depth a little easier. The snow stick is a heavy duty 30-inch aluminum ruler that is graduated in tenths of an inch.

A fan of snow or hail will appreciate a CoCoRaHS precipitation series t-shirt. They feature the symbol for the precipitation, a description, and the CoCoRaHS logo. They are available in adult sizes from small to XXL.

CoCoRaHS t-shirt, caps, and sweatshirts are also available.  There is also an "observer" polo shirt with the CoCoRaHS logo with the word "Observer" underneath. It's available in royal blue only.

The gauge parts and accessories and CoCoRaHS clothing can be found at

Fans of snow or photography will appreciate two books about snowflakes by Kenneth Libbrecht.  The Snowflake - Winter's Secret Beauty and The Art of the Snowflake: A Photographic Album feature breathtaking microphotography of snowflakes. The first book describes the many different types of snowflakes and other information about snow. The second book is more of a chronicle of snowflake photography from 1890 to the innovations in digital photography.

I'm a big fan of non-fiction books about significant weather events. Here are a few of my favorites. If you have any books to add to this list please feel free to mention them in the comments.

Isaac's Storm by Erik Larson.  This is the story of Isaac Cline, who was the chief meteorologist at the Galveston, TX office of the U.S. Weather Bureau during the Galveston Hurricane of 1900.

The Worst Hard Time: The Untold Story of Those Who Survived the Great American Dust Bowl by Timothy Egan. This book tells the story of those who suffered through the Dust Bowl in the Great Plains. It was the inspiration behind Ken Burns' dopcumentary The Dust Bowl.

The Children's Blizzard by David Laskin. This is a story of a Plains blizzard on January 12, 1888 that left 500 people dead on the prairie, many of them children,

White Hurricane - A Great Lakes November Gale and America's Deadliest maritime Disaster by David G. Brown.  This book is an account of the fierce Great Lakes storm in November 1913.  Here's my post about this storm.

 The Long Winter by Laura Ingalls Wilder. The Ingalls family deals with the brutal winter of 1880-1881 in the Dakota Territory.

Last but certainly not least, a great gift is a SAME encoded weather radio. Every home is required to have smoke detectors - every home should have a weather radio!  There are a number of models with a variety of features available.  You can learn more about NOAA Weather Radio and what's available in this post from last April.