Sleet is translucent balls of ice that are frozen raindrops. The most common forms of precipitation are rain, snow, freezing rain, and sleet. In Wisconsin, precipitation usually begins as ice particles in a cloud. The temperature conditions below the cloud base determine if the precipitation ends up as rain, snow, freezing rain, and sleet. The altitude of the melting line, or the height where the air temperatures are at freezing, determines the type of precipitation. If the freezing line is above the cloud base, than the ice particles will melt as they fall and we will have rain. If the ground is at, or very near freezing, then the precipitation will be snow, as the ice crystals won’t have enough time to melt. In freezing rain and sleet, the melting line is between the ground and the cloud base, and the ground temperature is below freezing. So, the ice crystals melt and become liquid drops with a temperature near freezing. The main difference between getting freezing rain or sleet is the height of the melting layer and the temperature of the surface. Freezing rain occurs when the liquid drops hit the ground and freeze on contact. Sleet occurs when there is enough time for the liquid drops to refreeze before they hit the ground.

A mixture of rain and snow is not sleet, even though that term is often used in common parlance. A mixture of rain or snow is a term used to describe either wet snow or a mixture of snow and rain.

Last week we were visited for the second time this winter by a sizable snowfall. A reasonable question to ask is where does the water come from to make the snow? A general answer is not available as the variety of snowstorms that we get in Wisconsin result from different sources of water. In fact, even for this last storm in the southern part of the state, the answer depends on where you live. In Madison, the bulk of that water that fell as snow in that last storm most likely originated in or near the Gulf of Mexico, transported northward on southerly winds in advance of the storm center itself. As the air containing the water vapor rose, it condensed or was deposited onto ice particles in the clouds and eventually grew to a sufficient size to fall from the cloud as a snowflake. In Milwaukee, however, an additional source of water vapor to form the snow was Lake Michigan. Air carried over the lake was able to gather more water vapor from the lake surface due to evaporation, thus enhancing the amount of water vapor in the air near Milwaukee. Upon rising, this increased water vapor content led to increased snow amounts there. Finally, the process of transforming water from invisible vapor to ice releases a huge amount of energy to the surrounding atmosphere. In fact, the amount of energy released by this phase change during a 5″ of snowstorm over Dane County alone is enough to power the Madison metro area for 400 days!

No. These storms are individual weather events, which cannot be used to support or refute climate trends. Which also means that the warm weather in Vancouver is not evidence that global warming is occurring. Weather relates to events that will happen over the next few days; climate describes what happens over decades. To make such claims about these storms and climate would be similar to saying “Looks like car accidents rates for WI are going down this year.” after returning from an incident free car ride on a Sunday afternoon. Accident trends are not about individual experiences, and climate and climate trends are not about individual weather events.

It is reasonable to ask, will we see more or less of winter storms in the coming decades with a warmer climate? There is some evidence that a warmer climate will result in more frequent large snowstorms. While the average global temperature 25 years from now will be warmer than current conditions, we will still have winters and summers. A warmer atmosphere means that there will be more water vapor in the atmosphere which could lead to greater snowfall amounts from individual storms during winter. In addition, a warmer world leads to a warmer ocean. So, as winter storms move along the eastern seaboard there could be more evaporation from the warm water. This provides more water for precipitation and more fuel to strengthen the storm.

When the snow builds up on my patio, it starts to evaporate after a few days, even though the temperature is still below freezing. On average, what percentage of our snowfall each year evaporates back to the air?

The transition of water from the ice phase (snow) to the gas phase (water vapor) is called sublimation. Sublimation is a common way for snow to disappear in cold, dry winters.

On days when temperatures are above freezing, we can see the melting process as snow turns to liquid water, which evaporates, gets absorbed into the ground, or runs off. We do not see sublimation because the snow turns directly into water vapor without first melting, but we do notice that the snow is decreasing and may even disappear on cold winter days.

The rate of sublimation is a function of the weather. It takes a lot of energy to turn ice into water vapor: about seven times as much energy as would be needed to boil that water. This energy comes primarily from the sun, so sunny weather is best for sublimating snow. Wind also helps, as it removes the water molecules that have left the snow. Low humidity also helps to increase the rate of snow loss.

So the amount of snow that sublimates depends on the winter weather at your particular location.

Snow carried by wind can reduce visibility and cover roads. We cannot “switch off” the wind; but we can slow it with obstacles. Obstacles like trees and fences break wind into a swirl of eddies, reducing its overall speed. This is why a wooded, fenced-in suburb is generally much less windy than a lake shoreline.

Snow fences use this concept to keep snow from blowing across roadways. Snow flying on high winds past a snow fence will get caught in the turbulent eddies created by the fence. As the air slows, it will drop some of the snow just beyond. Eventually a large pile of snow can accumulate downwind of a snow fence.

If the fence is located upwind of a road, the snow collects around the fence and doesn’t drift onto the road. If the fence were too close to the road, some of that snow would drop onto the road and cause traffic problems. Optimally, the fence should be set back about 35 times its height from the road, so a two foot fence should be placed about 70 feet upwind from the road.

Snow fences work best when they are placed on a long open expanse upwind of the road, and the wind blows mostly from one direction.

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.

The term ‘black ice’ refers to either a new layer of transparent ice on water, which allows us to see the deep water below, or a layer of clear ice on a roadway, which makes for hazardous driving. In both cases, the ice is transparent, not black, and so it shows the color of the underlying surface.

Photo: AdamKR

Black ice in Halifax

The ice is clear because no air bubbles are trapped in it. Lots of trapped air turns ice white; snow is white because of air trapped between crystals.

Driving on a road covered with black ice is particularly dangerous because the roadway can appear to be merely wet, so drivers may not recognize the slippery conditions until too late. Similarly, a sidewalk covered with clear ice will look dark gray – like a wet sidewalk. This ‘grey ice’ can be hazardous to walking!

Because bridges span the open air, they cool faster than other roadways, so black ice often occurs first on bridges. That’s why we see those warnings that “Bridge May Freeze Before Road Surface.”

Yes. A storm that includes any occurrence of thunder with snow is called a thundersnow event or thunder snowstorm. Lightning and thunder go together; you can’t have one without the other. Thunder is generated when the lightning heats the air five times as hot as the sun, which causes the air to rapidly expand and generate a sound wave.

There is much we don’t know about lightning but we do know that it requires both ice crystals and liquid water in a cloud with strong updrafts. Since winter storms usually have mostly ice crystals, thunder is uncommon in winter storms. And not only is thundersnow rare, it is also hard to hear, as the accompanying snowfall acts as an acoustic suppressor, or sound absorber.

Between 1961 and 1990, there were only 191 reports of thunder snowstorms, and less than six official observations of thundersnow within southern Wisconsin. Madison’s most memorable thundersnow event occurred at 5:09 PM January 26, 1996, when lightning produced an eerie purple sky. States with the best chance of thundersnow are eastern Nevada and Utah, followed by the Central Plains and Great Lakes states.

Photo of snow in west Sierra Nevada by Itrovert

Sometimes on a sunny day, freshly fallen snow may appear to sparkle or glitter. This happens because when light hits an object light, it can be absorbed, in which case the object is heated; transmitted, in which case light passes through the object; or reflected, in which case it bounces back.

Clear water can transmit or reflect light (think of how a calm lake can reflect an image of the trees on the shoreline). Flat snowflakes resting on top of a blanket of snow also can act like a mirror, reflecting a portion of the sun’s image toward your eye. Each ‘sparkle’ is a reflection of the sun’s image from a single crystal. Whether we see the sparkle depends on the angle between the sun, the snowflake’s position on the snow and the location of our eye. When the angle is right, when we walk by a field of snow, we’ll see glitter as the sun is reflected by different snowflakes.

You may notice some faint colors in the sparkles. As light travels between air and water, the colors in the light separate, much as they do in a rainbow. The sun’s ray can reflect off different surfaces of the crystal. If the sunlight reflects off the back of the ice crystal, the colors may disperse as the light enters and exits the crystal, making those faint colors.

62 degrees below zero! From Wikimedia

Snow can make both ‘squeaky’ and ‘crunchy’ sounds. Snow is a mixture of ice crystals, liquid water and air, and the sound it makes when you walk on it depends on the proportions of this mixture.

When you walk on snow, your boots apply pressure that can break its ice crystals. When the snow is warmer than about minus 35 degrees F, its ice crystals are surrounded by a film of liquid water that lubricates them so they can slide past one another without breaking.

The more water around the crystals, the less likely the breakage, so if the snow is colder than 14 degrees F, your boot will crush the ice crystals, making that squeaking, or creaking, sound. Snow above approximately 14 degrees F contains enough liquid water for the crystals to “flow” silently under your boot. You’ll never get a squeak striding through slush!

The crunchy sound, on the other hand, is made by cold, packed snow. Snowpack is made of ice grains. Where they touch, they bond or weld together like a matrix. These bonds are weak in fluffy snow and strongest in cold, dense snow. The crunchy sound occurs when the bonds between the ice grains in the snowpack break apart when you step on them.

You’ll never hear the crunch walking in fluffy snow!