A capped column snowflake with a collection of rime

If you saw something like this falling from the sky, you might think that the weather outside was indeed frightful. But this dumbbell shaped object is, in fact, a super-magnified snowflake — yes, a snowflake. Not so frightful after all.

This particular snowflake is a capped column, one of many types of snowflakes. The fuzzy texture on its two hexagonal ends is called rime.

Sometimes, when a snowflake, or snow crystal, passes through a cloud on its way down to earth, it collides with super-cold water droplets—clouds, after all, are just a bunch of coalesced drops of water. This collision causes the cloud’s water droplets to freeze and stick to the surface of the snow crystal. When these frozen droplets accumulate on the crystal’s surface, the snowflake becomes bedecked with rime.

If a snow crystal collects too much rime, its original identity can be obscured. Snowflakes that experience this type of identity crisis are called graupel.

This snowflake’s portrait was taken by a low temperature scanning electron microscope.

Photo: Electron and Confocal Microscopy Laboratory, Agricultural Research Service, U. S. Department of Agriculture

The death toll from the May 22, 2011 tornado in Joplin – now 122 — is the latest tragedy of a horrific year for tornadoes. On April 27, twisters in Alabama and nearby states killed 314, the fourth highest in U.S. history. The 480 deaths in 2011 are already the highest number since 1953, and tornado season continues through mid-August.

The Why Files asked Jonathan Martin, an expert on the large atmospheric disturbances that form tornadoes, some questions about tornado prediction. We edited the answers of Martin, a professor of atmospheric and oceanic sciences at the University of Wisconsin-Madison, after the interview.

The Why Files: What must we know to make a good tornado prediction?

The Why Files: Are predictions getting more accurate?

The Why Files: Why so much damage and death this year? Is this a result of climate change?

While the lakes around Madison provide many winter recreation activities, the local lakes do not yield a snow belt.

We refer to agricultural regions in the United States as ‘belts’, such as the cotton belt and wheat belt. This phrase has expanded to cultural (e.g. bible) and climatic regions. The “snow belt” refers to the area downwind of the Great Lakes where the climate includes large amounts of snowfall.

The snow belt exists because snowfall can be enhanced when storms cross large lakes, called the “lake effect.” When cold air moves across a large expanse of warmer lake water, the lower air is warmed and moistened by the water. This enhanced evaporation increases the moisture content of the air mass; this moisture can then precipitate as snow downwind.

The distance the air mass travels over the water, called the fetch, is important in generating snowfall. Generally, larger fetches give more time for the air mass to accumulate moisture, producing more snowfall. A minimum fetch of about 60 miles is needed for lake effect snowfall. Wisconsin, Ohio, Pennsylvania and New York all have major snow belts downwind of Lakes Superior, Erie and Ontario.

Steam Devils in Wisconsin

One of the many advantages of living near large lakes in temperate climates is the steam fog that shrouds them in fall and early winter. Fog is essentially a ground-hugging cloud, composed of tiny liquid water droplets.

Steam fog will develop at this time of year if the air above the lake has very low relative humidity, and a lot of lake water evaporates into invisible water vapor. The more vapor in the air, the closer the air comes to being saturated. When the air finally becomes saturated, some of the vapor begins to condense into liquid water and the steam fog begins to form.

When there is a large difference in temperature between the air and the water at the lake surface, there will also be considerable turbulence in the air over the lake. When a strong cold front passes through, strong winds will carry cold air toward us, and the combination of steam fog production, turbulence over the lake, and strong background winds can create one of nature’s most awesome spectacles – steam devils.

Steam devils are swirling columns of steam fog (analogous to dust devils) that can look like tornadoes of steam fog on a windy, cold winter day. Though it can be physically taxing due to the cold, if you ever have a chance to watch steam devils parade across a cold, watery lake, do not pass it up!

Photo: MODIS Image Gallery by Liam Gumley, SSEC (Space Science and Engineering Center, UW, Madison).

State outline barely visible under snowcover (click image to enlarge), this is Winter in Wisconsin.

The beginning of astronomical winter doesn’t occur until the Winter Solstice, which is usually between December 20-22. On the winter solstice, the sun reaches is most southern point in the sky and our daytime hours reach an annual minimum as the nighttime hours reach an annual maximum.

This is the most recognizable definition of the beginning of winter, but it rarely coincides with an abrupt plunge into the season. In fact, anyone who has lived in the Upper Midwest or the interior of Canada knows that winter usually seems to arrive at least a month before late December.

In Madison, Wisconsin, we usually have had a measurable snow, a day with morning low temperatures in the single digits, and plenty of skating long before the winter solstice. In fact, the all-time record daily snowfall total occurred weeks before the solstice in 1991!

A more meaningful definition of the beginning of winter might be based on when, on average, precipitation has an equal chance of falling as rain or snow. For Madison, that day is November 15. Of course, since this is an average (calculated over many years) any individual year may completely ignore this pattern.

Statistics, after all, apply to groups, not individuals.

A weather watch indicates the possibility of hazardous weather, while a warning means that hazardous weather is occurring, is about to occur or is very likely to occur.

A watch is intended to provide people with enough time to set safety plans in motion for possible hazardous weather. A warning indicates that conditions pose a threat to life or property and people in the warning area should take appropriate protective action.

Watches and warnings outline areas where the weather may occur. Pinpointing the location of hazardous weather in advance is extremely difficult. For this reason, watches are usually issued for large regions, sometimes covering several states. Warnings are issued for much smaller areas, often only a county or two, because they are based on actual observations of hazardous weather.

The National Weather Service issues weather watches and warnings under specific weather conditions. A severe thunderstorm watch means that conditions are favorable for the development of severe thunderstorms in and near the watch area. A warning means that a severe thunderstorm has been sighted visually or indicated by radar and it is producing hail 0.75 inch or larger in diameter and/or winds of at least 58 mph.

Finally, a weather advisory may be issued when actual or expected weather conditions are not hazardous, but may cause inconvenience or concern.

Photograph of Cumulus clouds in fair weather taken by Michael Jastremsk

Clouds are made of water and clean water is clear. So why are clouds white? Because clouds are made of billions of small water droplets and ice crystals.

When light beams interact with particles suspended in air, some of the energy is scattered, which means the light beam changes direction, and usually color as well. The amount of light scattered is a function of the size of the particle relative to the wavelength of light falling on it. Cloud particles are large enough to scatter any color of light that falls on them. The repeated scattering of light, called multiple scattering, causes whitish light because enough light of all colors is scattered to your eye, and those colors combine to make white light.

You can demonstrate this with a glass bottle. Without the labels, the glass will look clear. Wrap the bottle in a rag, smash it, and pour the small pieces into a pile. The pile will be whitish, even though each tiny piece is clear.

The bottoms of even the whitest clouds appear gray, sometimes ominously so. Why? Because multiple scattering above the cloud base redirects the incoming sunlight out the top and the sides of the cloud, leaving very little light to emerge from the cloud base, which is dark.

The probability of precipitation (fondly known as PoP) has been part of weather forecasts since the late 1960s, and is the only forecast element that includes a probability. Unfortunately, there is confusion about the exact meaning of a “60 percent chance of precipitation.” Part of that confusion, we are sad to say, reflects discrepancies in interpretation and application among meteorologists.

First, let’s understand what a PoP does not mean: It is not the percent of time precipitation will be observed over the area. Nor is it the percentage of meteorologists who believe precipitation will fall!

The definition most commonly among meteorologists is this the confidence probability that at least one one-hundredth inch of liquid-equivalent precipitation will fall in a single spot. One common way to grasp a PoP of 60 percent is this: If we had ten tomorrows with identical weather conditions, rain would fall on six of them at a given point, or 60 percent of the days. Just as importantly, rain would not fall on four of those days.

But this PoP is a forecast for ten potential tomorrows, not a forecast for the next ten days. Although this may be confusing, weather forecasters have no problem thinking of tomorrow’s weather as a group of potential tomorrows. One thing you can take to the bank: as the PoP increases, precipitation becomes more likely.

Operate your own rainbow, then find out!