Snow readings

[Mtn.Marty]

Yesterday, we received 2.75 inches of snow. I measured this with a ruler on a table in the backyard. Later in the day, the rain gauge read 0.04 inches, I assume because snow melted into the cup on the station. Now I know that it might not equate to the water equivalent of the snow, but I wonder how close it might be. Also, if you keep track of yearly precipitation, then that should also include the water equivalent of the snow. For example, last year I had 34 inches of rain, but that does not include snow melt. Just thinking...

[Blicj11]

Marty:

Good post. There are some old rules of thumb floating around about water equivalency of melted snow, none of which are accurate.

I kicked this around for years and last summer I decided to install a heater in the rain collector cup of my Davis VantagePro Plus ISS. I bought the heater from Davis years ago when I purchased my first weather station but had never gotten around to installing it as it requires power be run to my ISS. I finally did that last summer. This winter I turn the heater on when it snows and turn it off at the end of the storm. This has given me same-day precipitation during snow storms and over the course of the winter I get much more recorded precipitation than before when I was just waiting for whatever was still in the cup to melt on its own.

There is another method that requires a little more manual work but is much simpler and way less expensive. There is an organization called CoCoRaHS that collects precipitation information from private residents in the US and Canada. Many of us WeatherCatters are members. We take a manual measurement of melted snow and use that as a water equivalency. CoCoRaHS uses a standard 4" diameter rain gauge with which to collect and measure precipitation, including melted snow. Check out their website at http://www.cocorahs.org

[Mtn.Marty]

Interesting site Blick. Right now I don't know whether I'm interested in purchasing a separate rain gauge ($30) and manually reading it every day at 7am. I'm also wondering what use this data actually is. I currently report my date to WG and CWOP in any case.

What's your thoughts about developing a method of estimating the water content of snow and manually using the rainfall editor to reflect that amount of precipitation?

[Blicj11]

Right now I don't know whether I'm interested in purchasing a separate rain gauge ($30) and manually reading it every day at 7am.

Amen! I report in by 9:00 am on most mornings, but it is a hassle to melt snow and measure the water. I do it because their method of calculating water equivalency is accurate. Also, the regional coordinator in Utah is a professor at Utah State and they sold me the rain gauge for $20.

I'm also wondering what use this data actually is.

A partial list of who uses the CoCoRaHS data: The National Weather Service, other meteorologists, hydrologists, emergency managers, city utilities (water supply, water conservation, storm water), insurance adjusters, USDA, engineers, mosquito control, ranchers and farmers, outdoor & recreation interests, teachers, students, and neighbors in the community are just some examples of those who visit the web site and use the data.

What's your thoughts about developing a method of estimating the water content of snow and manually using the rainfall editor to reflect that amount of precipitation?

Absolutely you should do it! Find a cheaper gauge or some other method of capturing the snowfall, melt it and enter the precipitation amount in the rain editor. If you just wait for what's in the cup to melt, you will lose a lot to evaporation and what you have left over is usually recorded on the wrong date.

[Mtn.Marty]

OK, how's this for a plan. I only get maybe a half dozen days of snow per winter, here, so I don't want to burden myself too much? I looked up on a NASA website that there are approximate water conversions for snow depending on temperature. So, 30 degrees equal 10:1; 25 deg. = 15:1; 18 deg = 20:1, etc. So, if I measure the snow depth on my outdoor table in inches and then convert to water equivalent using the ratios above, I can get at least close to the water content. Then I would add that to my daily rain editor. I might even go so far as to cut a standard snow board and paint it white to use to collect the snow.

Using this method, my snowfall yesterday of 2.75 inches falling at around 25 degrees would compute as 2.75/15 = .18 inches. Now I would subtract the .04 inches from melting in the gauge and thus add 0.14 inches to the rainfall for that day. Does this make sense?

[Blicj11]

Yes. It's a pretty good approach. Just bear in mind that snow to water conversion tables are not always reliable due to the variable nature of snow storms. Some snow storms are wet and some are dry. But it is true that the colder the air, the less moisture it typically contains, so I like the NASA approach based on temperature.

I think using this you will end up with a more accurate annual rainfall than you had before.

[LesCimes]

An added benefit of have the separate rain gauge is having a backup for the electronic one.  My electronic gauge on the VP2 was malfunctioning intermittently for a few months last spring. Had I been comparing the old style gauge with the VP2 readings I would have caught the problem a lot sooner and would have been able to enter in the missed data into WeatherCat. Lesson learned. I have't joined the CoCoRaHS system because I'd have a hard time keeping up with daily data entry (feel a bit guilty about that) but I sure am glad to have their rain gauge. And who knows, one day I may start entering the data into CoCoRaHS.