Can you explain the difference between black ice and gray ice?

The term ‘black ice’ refers to two conditions: a new layer of ice on water, which appears dark in color because the ice is transparent and so we see the deep water below, or as a layer of clear ice on a roadway, which makes for hazardous driving conditions.

In both cases the ice is not black but transparent, and therefore shows the color of the underlying surface.

The ice is clear because no air bubbles are trapped in the ice.

Lots of trapped air makes an object look white. Snow looks white because of air trapped between crystals.

The danger of driving on a road covered with black ice is that the roadway can appear to be merely wet.

Drivers may not recognize the slippery conditions until it is too late.

Crashes also occur when a roadway freezes over quickly because of freezing rain.

A freezing rain situation occurred in the northeast on Jan. 18, resulting in several multiple vehicle pile-ups and some deaths.

The temperature of the road was below freezing, and when rain fell it quickly froze, resulting in extremely hazardous conditions.

Even walking can be seriously affected on certain days.

Keep in mind that your thermometer placed several feet above the ground may register an above-freezing temperature while the temperature at the surface, where ice can form, may be below freezing.

If a sidewalk is covered with clear ice, it will look dark gray — like a wet sidewalk.

This “gray ice” can be hazardous for walking and undoubtedly contributes to thousands of personal injuries during winter.

Category: Meteorology, Seasons, Weather Dangers

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How unusual is our roller coaster winter?

It doesn’t take an exceptional attention span to realize that this year’s cold season (starting in November) has been very changeable.

November was 6.1 degrees colder than normal, then December was surprisingly mild (5.8 degrees above normal). As of Thursday — mid-month — January has been 8.5 degrees below normal.

Thus far we have had only 14.9 inches of snow and every month has been substantially drier than normal as well.

This is shaping up to be a very unusual winter in many respects. After a relative warm up, forecast models are suggesting a return to very cold temperatures as we approach the end of January. We suspect this will be enough to keep the monthly average temperature below the normal for January.

Though there is really no way to accurately forecast the monthly average for February, experimental forecasts made by the Climate Prediction Center, issued Thursday, predict a 50-50 chance for a normal February in Wisconsin.

Though detailed records are not immediately available, anecdotally it has been at least 20 years since a cold season has flipped from below normal to above normal in consecutive months from November to February. There is some chance that this will be such a year.

Category: Climate, Meteorology, Seasons

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Are the Great Lakes’ water levels normal?

For the first time in about 25 years, the water level of the all the Great Lakes is above normal. Lakes Superior, Michigan and Huron are about 5 inches above the long term average.

This ends a 15-year period where lake levels have been below historic averages.

Lakes Huron and Michigan were at record low levels in January 2013; that is a rapid rise in water level to be above normal two years later. Such a rapid increase has not been measured since observations began in the mid-1800s.

The water levels of the Great Lakes are determined by the amount of water flowing in and out of the lakes.

Precipitation, runoff, and water from streams and groundwater supply water to the lakes, while evaporation and water flowing out of the Great Lakes system are water losses.

When the input exceeds the output, the levels rise.

The water cycle of the lakes is complex, and weather has played a role in this turnaround in lake levels.

Above-average precipitation and above average runoff in the Great Lakes watershed, particularly in the springs of 2013 and 2014, helped to restore lake levels.

The frigid winter of 2013-2014 also helped by reducing evaporation.

Ice on the lake and cold waters reduce evaporation, which also reduces snowfall in the snow-belt regions of the lakes.

Information on and forecasts of Great Lakes water levels is available from several agencies in the United States and Canada.

The forecast for the water levels is to continue to be above average, though levels could change relatively quickly.

Category: Climate, Meteorology, Seasons

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What rare weather event did we experience last month?

Madison’s overnight low temperature of minus 3 on Dec. 28 was a relatively rare event, all things considered.

As we all know, the temperature dipping below zero in winter is not unusual. But it doesn’t usually happen without snow on the ground, and there are good reasons for that.

Snow on the ground works double duty to keep the air near the ground very cold. First, since the ground is a vast storehouse of heat, the presence of snow insulates the air above from that heat.

The fact that snow is an excellent absorber and emitter of infrared energy — the kind of radiation that nearly everything on Earth emits, including the ground — gives this insulation effect an added radiative dimension. With a couple of inches or more of snow cover, the infrared energy emitted by the ground is absorbed by the overlying snow.

The snow then emits that energy in all directions, some of which are back toward the underlying ground. Thus, much less than half of the energy that would otherwise be emitted from the ground to the overlying atmosphere never gets there.

Secondly, since the top surface of the snow is emitting infrared exceptionally well, the air in contact with that snow surface can get really, really cold. The result is extremely cold surface air.

In our last cold spell, the coldness was imported from locations where this radiative cooling had been in operation — places to our north. This coming week, our low temperatures will be partly “home-brewed” by the snow we just received.

Category: Meteorology, Seasons

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What are geomagnetic storms?

Our sun is an active star that has storms. Sometimes the sun ejects a cloud of gas, called a coronal mass ejection or CME. CMEs are often associated with solar flares, and it takes about two to three days for the charged particles in this gas to reach Earth. Earth’s magnetic field deflects these particles toward the North and South Poles.

Unfortunately, solar electrons and protons from CMEs collide with the Earth’s magnetic field and atmosphere and can stir up electromagnetic storms in the Earth’s magnetosphere. These geomagnetic storms can result in the Northern Lights but also disrupt satellite-based navigation, communications, air travel, power grids and even pipelines. A geomagnetic storm in March 1989 shut down the Hydro-Quebec electric grid in Canada, leaving people without electricity.

In a worst-case scenario, the costs of an extreme geomagnetic storm in the United States could be in the trillions of dollars and would require a recovery period of up to a decade. As a result, the U.S. is developing plans to reliably address impacts of geomagnetic storms on our bulk power system. Networks are being set up across the globe to monitor geomagnetic conditions.

An emerging discipline known as space weather attempts to forecast the effects of solar activity on the upper atmosphere so that impacts to society can be minimized. Instruments on satellites can observe ejections of high-energy plasma from the sun. These can result in NOAA’s Space Weather Prediction Center issuing a warning of a coming magnetic storm, estimating time of arrival and intensity.

Category: Meteorology, Phenomena

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