This time of year, meteorologists across the country are being asked “Will we have a White Christmas this year?” And I bet even those of you who are known as Earth/Environmental Science teachers to your friends and family are being asked the same question.
Such is the life of those us “in the know,” who understand, at least to a small extent, how to interpret weather data and can understand what that data tells us about Earth’s complex systems. Everyone expects us to know everything about the weather. So here’s some inside information to help you out when you’re asked.
How white will the winter be?
Unfortunately, for those of us who enjoy waking up Christmas morning to find snowflakes gently falling towards the Earth and all the evergreen trees covered in a bright blanket of freshly fallen snow, this year’s long-range outlook doesn’t look too promising for most of the country. Indeed, for most of us it’s been several degrees warmer than normal. On the other hand, this is probably good news for those who are traveling this holiday to see friends and family.
Either way, the question of how “probable” a White Christmas is for any given city from year to year is still a fun data exercise to explore, and it doesn’t have to take a lot of time either.
A) Looking Back: The Probability of Snow
When meteorologists develop their predictions on the weather, they always start with climatology, that is, the average weather condition expected based on many years of observations. They will then alter their forecast based on short term processes like front locations and cloud formations, which affect the weather in the near-term from that expected based on climatology alone.
Unfortunately, it is hard to make short term predictions more than a week or two out, since fronts and clouds are highly variable. So, when predicting whether or not it will snow on any given day, the best guess one can make far out in advance it to look at climatic predictions and make a “best guess” of what to expect. Of course, the end result is that it either snows or it doesn’t, and a probability map only tells us, say, how many years out of 30 one can expect to see snow on the ground. But such is the nature of weather forecasting. Closer to an actual date, when one knows the local temperature trends and locations of nearby weather systems, a more precise prediction be made.
Map of the historical probability of snowfall on December 25 (NOAA)
But back to our White Christmas question, a few years ago the National Weather Service created several maps depicting the probability of snow on the ground (in increments of 1 inch, 5 inches and 10 inches) on Christmas morning. You can find these maps and a nice summary in NOAA Magazine.
At the very least, these maps would probably be fun to share with your students as a quick activity, in which you can ask them to find their hometown and determine how often they can expect a White Christmas in their future. You can also ask where they need to go in order to ensure they wake up to one, or on the other hand, where you need to go to ensure you don’t.
Suggested Research Questions
- Based on your memory (or ask your parents), can you remember how many times you woke up on Christmas morning to find snow outside? Compare your memory to the probability found for your hometown on the map.
- Find the probability of snowfall for your hometown and the hometowns of any family friends or relatives that live out of state? Are they different?
- Compare the snow probability maps to a national map of elevation/topography. Do you notice any correlations?
- Can you explain the patterns of higher/lower snow probabilities? Is there a constant north/south relationship? Does proximity to the ocean play a role? What about elevation?
- Define what you think the probability numbers shown on these maps actually mean.
B) Looking Forward: Forecasting Snowfall
Thanks to many major advances in technology, we are able to observe and forecast the weather better than ever before. Today’s weather models are fairly accurate at predicting conditions up to a week out, though forecasters still have trouble with large storms like blizzards and hurricanes after a couple days.
When it comes to predicting snowfall, the National Operational Hydrologic Remote Sensing Center, has some wonderful real-time datasets to play with.
National Snow Analyses http://www.nohrsc.noaa.gov/nsa/
This page contains neat images on recent snowfall and ground snow cover. The animations are particularly interesting, because you can observe recent snow storms as they cross the US. At the bottom of the page is a written snowfall forecast which is updated daily.
3D Snow Analysis http://www.nohrsc.noaa.gov/earth/
For those of you who like to use Google Earth in your classroom, this page features national real-time snow-fall images you can import into Google Earth to play around with. Also available are snowfall station reports which link back to dynamic time-series plots for each station.
Satellite Snow Cover Observations http://www.nohrsc.noaa.gov/nh_snow-cover/
On this page, you can access recent maps of snow cover across the US and even the northern hemisphere.
Snow Pictures http://www.nohrsc.noaa.gov/snowsurvey/photos/index.html
You can find cool images from around the country of snow and its impact on rivers and streams.
Finally, on this page, students can access forecast maps of weather fronts (up to 6 days out) and snowfall probabilities (up to 3 days out). In a few days, forecasts for 12/25 will become available.
Suggested Research Questions
- Describe the current pattern of snow cover in North America. What factors might explain the patterns you see?
- Has the snow cover pattern changed at all in the last week? Why may have caused these changes?
- Is there a relationship between this week’s snow cover and the climatic probability map of a White Christmas? Describe why you think there might be differences between the two.
- How does snow impact rivers and lakes?
- After analyzing all of the snowfall forecast maps, can you make a prediction on whether it will snow soon in your state and if so, how much?
- Is there a relationship between weather fronts and snow fall?
C) Relevant References
A quick web search revealed the following additional White Christmas activity. It involves mapping out the probability of snowfall for several cities and then drawing contour lines. It is well suited for those of you who would like to work further on graphing and mapping skills. http://www.educationworld.com/a_lesson/02/lp290-05.shtml
If you live in the Northeast, and especially in the Mid-Atlantic, you might be interested in the 3-day weather forecast animations we produce for research purposes in the Rutgers COOLroom. http://marine.rutgers.edu/cool/weather/WRF/
And, the NWS Graphical Forecasts page has some great images of future weather conditions. Be sure to check out “Snow Amount” which shows detailed snowfall maps for 3-days, and also “Weather” which shows potential areas of rain & snow up to a week out. http://www.weather.gov/forecasts/graphical/sectors/conusLoop.php
(Originally written December 18th, 2006)