Skip to main content

The mystery of snowfall

By Ian Roulstone
February 13, 2014 -- Updated 1225 GMT (2025 HKT)
Vehicles are piled up in an wreck Friday, February 14, in Bensalem, Pennsylvania. Traffic accidents involving multiple tractor-trailers and dozens of cars completely blocked one side of the Pennsylvania Turnpike outside Philadelphia. Vehicles are piled up in an wreck Friday, February 14, in Bensalem, Pennsylvania. Traffic accidents involving multiple tractor-trailers and dozens of cars completely blocked one side of the Pennsylvania Turnpike outside Philadelphia.
HIDE CAPTION
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
Southeast storm moves north
<<
<
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
>
>>
STORY HIGHLIGHTS
  • Ice and snow storms are crippling some states in the Southeast
  • Ian Roulstone: Predicting how much snow will actually accumulate is difficult
  • He says the physics of snowfall is complex, involving factors like temperature, moisture
  • Roulstone: Making the final call comes down to "seat of the pants" knowledge in the end

Editor's note: Ian Roulstone is professor of mathematics at the University of Surrey in England. He is co-author of "Invisible in the Storm: The Role of Mathematics in Understanding Weather" (Princeton University Press).

(CNN) -- A snow-covered landscape is one of the classic images showcasing the beauty of weather on Earth. We are awed by the grandeur of white-capped mountains and the almost magical quality of snow-covered trees. We are also frustrated when the tempests of winter reach far and wide, striking as they have done this year in America's southern states.

When it comes to forecasting the likelihood of a blizzard, the weather anchors know what to say. But when asked to predict how much snow will actually accumulate, they will give estimates. Why?

The computer models used in weather forecasting do not actually predict snow. A single variable is used to predict water in its various forms -- liquid, vapor or ice -- so we need other information, such as air temperature, to decide whether snowfall is likely.

Ian Roulstone
Ian Roulstone

Models forecast the amount of liquid water produced when air rises above the height at which water vapor begins to condense. This is commonly known as the Quantitative Precipitation Forecast (QPF). While QPFs are an important ingredient when it comes to predicting snow, there are many other very subtle factors that can easily tip the balance in favor of rain, or make it very difficult to distinguish between different types of snow.

If temperatures are low enough to allow precipitation to fall as snow, then the forecasters need some way to convert the QPF to an equivalent snowfall. The ratio that we use to calculate the liquid water to snow equivalent is around 1 to 10. That is, if the QPF predicts 1 inch of rain, we can anticipate the amount of snow produced would be 10 inches.

Unfortunately, life's not always that simple.

The liquid water to snow ratio can vary depending on whether the snow is "wet" or "dry." Dry snow is the term used to describe small powdery flakes, and it forms when there is very little moisture available. Under these circumstances, the rain to snow ratio can be considerably higher, with values of 1 to 20 not uncommon.

On the other hand, if there is abundant moisture and the snowflakes are larger and wetter, a ratio of 1 to 5 may be typical. Therefore we need to have a very accurate forecast of the levels of moisture in the atmosphere, together with the variation of temperature with altitude, to even get off on the right foot when it comes to predicting snowfall.

Our forecast models represent a snapshot of the weather at any moment in time by using huge arrays of numbers to describe states of the atmosphere. These numbers represent basic variables such as moisture and temperature.

Calculating how these many millions of "weather pixels" will change requires superfast computation and large amounts of memory. The inevitable limitations on available computer power and data storage force a trade-off between the geographical coverage of models and the detail that we can expect from them. This trade-off can be critical when it comes to calculating reliable QPFs.

Forecasters often have to resort to methods they learned at college, involving dew points, temperature soundings from meteorological balloon ascents and real-time reports from weather stations to assess the impact of a snow storm.

The bottom line is that snow -- one of our most loved, and occasionally loathed, features of weather -- is real tough to forecast well.

We may be able to capture the beauty of Mother Nature with high resolution digital images, but capturing the physics behind the snowfall in our sophisticated weather prediction models is much more challenging.

Making the final call comes down to "seat of the pants" knowledge in the end.

Follow us on Twitter @CNNOpinion.

Join us on Facebook/CNNOpinion.

The opinions expressed in this commentary are solely those of Ian Roulstone.

ADVERTISEMENT
Part of complete coverage on
December 20, 2014 -- Updated 0242 GMT (1042 HKT)
Conservatives know easing the trade embargo with Cuba is good for America. They should just admit it, says Fareed Zakaria.
December 20, 2014 -- Updated 0112 GMT (0912 HKT)
We're a world away from Pakistan in geography, but not in sentiment, writes Donna Brazile.
December 19, 2014 -- Updated 1709 GMT (0109 HKT)
How about a world where we have murderers but no murders? The police still chase down criminals who commit murder, we have trials and justice is handed out...but no one dies.
December 18, 2014 -- Updated 2345 GMT (0745 HKT)
The U.S. must respond to North Korea's alleged hacking of Sony, says Christian Whiton. Failing to do so will only embolden it.
December 19, 2014 -- Updated 2134 GMT (0534 HKT)
President Obama has been flexing his executive muscles lately despite Democrat's losses, writes Gloria Borger
December 18, 2014 -- Updated 1951 GMT (0351 HKT)
Jeff Yang says the film industry's surrender will have lasting implications.
December 18, 2014 -- Updated 2113 GMT (0513 HKT)
Newt Gingrich: No one should underestimate the historic importance of the collapse of American defenses in the Sony Pictures attack.
December 10, 2014 -- Updated 1255 GMT (2055 HKT)
Dean Obeidallah asks how the genuine Stephen Colbert will do, compared to "Stephen Colbert"
December 18, 2014 -- Updated 1734 GMT (0134 HKT)
Some GOP politicians want drug tests for welfare recipients; Eric Liu says bailed-out execs should get equal treatment
December 18, 2014 -- Updated 1342 GMT (2142 HKT)
Louis Perez: Obama introduced a long-absent element of lucidity into U.S. policy on Cuba.
December 16, 2014 -- Updated 1740 GMT (0140 HKT)
The slaughter of more than 130 children by the Pakistani Taliban may prove as pivotal to Pakistan's security policy as the 9/11 attacks were for the U.S., says Peter Bergen.
December 17, 2014 -- Updated 1600 GMT (0000 HKT)
The Internet is an online extension of our own neighborhoods. It's time for us to take their protection just as seriously, says Arun Vishwanath.
December 16, 2014 -- Updated 2154 GMT (0554 HKT)
Gayle Lemmon says we must speak out for the right of children to education -- and peace
December 17, 2014 -- Updated 1023 GMT (1823 HKT)
Russia's economic woes just seem to be getting worse. How will President Vladimir Putin respond? Frida Ghitis gives her take.
December 17, 2014 -- Updated 0639 GMT (1439 HKT)
Australia has generally seen itself as detached from the threat of terrorism. The hostage incident this week may change that, writes Max Barry.
December 12, 2014 -- Updated 2020 GMT (0420 HKT)
Thomas Maier says the trove of letters the Kennedy family has tried to guard from public view gives insight into the Kennedy legacy and the history of era.
December 15, 2014 -- Updated 1456 GMT (2256 HKT)
Will Congress reform the CIA? It's probably best not to expect much from Washington. This is not the 1970s, and the chances for substantive reform are not good.
December 15, 2014 -- Updated 2101 GMT (0501 HKT)
From superstorms to droughts, not a week goes by without a major disruption somewhere in the U.S. But with the right planning, natural disasters don't have to be devastating.
December 15, 2014 -- Updated 1453 GMT (2253 HKT)
Would you rather be sexy or smart? Carol Costello says she hates this dumb question.
December 14, 2014 -- Updated 2253 GMT (0653 HKT)
A story about Pope Francis allegedly saying animals can go to heaven went viral late last week. The problem is that it wasn't true. Heidi Schlumpf looks at the discussion.
December 14, 2014 -- Updated 1550 GMT (2350 HKT)
Democratic leaders should wake up to the reality that the party's path to electoral power runs through the streets, where part of the party's base has been marching for months, says Errol Louis
December 13, 2014 -- Updated 2123 GMT (0523 HKT)
David Gergen: John Brennan deserves a national salute for his efforts to put the report about the CIA in perspective
December 12, 2014 -- Updated 1426 GMT (2226 HKT)
Anwar Sanders says that in some ways, cops and protesters are on the same side
December 11, 2014 -- Updated 1439 GMT (2239 HKT)
A view by Samir Naji, a Yemeni who was accused of serving in Osama bin Laden's security detail and imprisoned for nearly 13 years without charge in Guantanamo Bay
December 14, 2014 -- Updated 1738 GMT (0138 HKT)
S.E. Cupp asks: How much reality do you really want in your escapist TV fare?
ADVERTISEMENT