What is Freezing Rain?

Posted on March 27, 2011 by Weather Guys Editor

When particles falling from clouds reach the surface as precipitation, they do so primarily as rain, snow, freezing rain, or sleet. Why are there different types of precipitation? It has to do with the ability of water to change phase. Where we live, precipitation usually begins falling out of a cloud as ice particles. What determines the precipitation type is the temperature between the cloud bottom and the ground.

If the temperature remains above freezing from the cloud bottom to the ground, precipitation particles melt into liquid droplets and are called rain. If the temperature underneath a cloud stays below freezing all the way to the ground, the ice crystals never melt and snow falls. Ice storms occur when precipitation particles melt and then fall through a layer of cold air near the ground. The two precipitation types most common during ice storms are freezing rain and sleet. Freezing rain forms when a thin layer of cold air near the surface causes melted precipitation to become super cold. It then freezes on contact with exposed objects on the ground whose temperature is below freezing. Sleet consists of translucent balls of ice that are frozen raindrops. It occurs when the layer of subfreezing air at the surface is deep enough for the raindrop to freeze.

When sleet hits the surface, it bounces and does not coat objects with a sheet of ice, as freezing rain does. Freezing rain covers everything in a sheet of ice, creating shimmering landscapes. However, even a little freezing rain causes treacherous road conditions and tree and power lines to snap.

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What are the Northern Lights?

Posted on March 20, 2011 by Weather Guys Editor

The northern lights, also called aurora borealis, are an evening light show seen as a diffuse glow or as overlapping curtains of greenish-white and sometimes red light. Auroras are triggered when the surface of the sun ejects a cloud of gas, called a coronal mass ejection. It takes about 2 to 3 days for the charged particles in this gas to reach Earth. Earth’s magnetic field deflects these particles towards the North and South Poles.  When these charged particles collide with a molecule or atom they can excite them. When these molecules or atoms shift back down to their normal energy states they light.

Auroras form between 60 and 250 miles above the Earth’s surface when these charged solar particles collide with two abundant constituents of our atmosphere: nitrogen and oxygen. Nitrogen molecules emit pinkish or magenta light, while oxygen atoms emit greenish light. A majority of the required collisions occur near the poles, so the northern lights are usually seen at the higher latitudes of Canada and Alaska. When a large number of particles are emitted by the sun, which usually happens after a solar flare, the lights from the collisions can be seen at our latitude.

Our sun goes through active and quiet periods. The time between 2008 and 2010 was a very quite time, but now the sun is becoming more active so we may have an opportunity to see more of these great natural light shows. Here is a recent photo of the northern lights taken on March 10, 2011 from Middleton by J. Zhou http://tinyurl.com/67beo59.

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Why do Bridges Ice before the Road?

Posted on February 28, 2011 by Weather Guys Editor

Living in a cold climate, we are used to seeing signs that say “bridge freezes before road.”  The fundamental reason is that a bridge hangs above the ground, while the roadway rests on the ground. Water on a road or bridge will freeze once the surface becomes colder enough. So, the road must cool faster than the roadway.

Whether something warms or cools is related to its energy gains and losses. So, as you stand facing an evening bonfire, your front warms because it gains more energy than it loses, while your back cools as it loses more energy to the cooler night air than it gains.

The energy losses from a bridge occur along the top surface and also along its side and bottom. Compared to a roadway, a bridge has more surface area to exchange energy with the atmosphere, and thus will cool down to the air temperature quicker. Many bridges are made of metal and concrete, both of which are good heat conductors. Thus, when the cold air comes in contact with the bridge surfaces, heat is quickly transferred from the bridge to the colder air, cooling the bridge and its surfaces.

A roadway also loses heat from its surface to the cold air above. However, the road surface also gains energy from the ground. So, while the roadway will cool down, it does not cool as fast because of the energy gains it gets from the warmer ground below. Because of those extra energy gains, the roadway cools more slowly and doesn’t form ice as quickly as the bridge.

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What is the Coldest Wind-Chill Temperature Ever Recorded in Madison?

Posted on February 20, 2011 by Weather Guys Editor

The wind-chill temperature describes the increased loss of heat by the movement of the air. The wind-chill is relevant to humans and other animals that need to maintain a constant temperature that is higher than their surroundings.

The cooling power of the wind cannot be measured with a thermometer; it must be computed. The wind-chill temperature translates your body’s heat losses under the current temperature and wind conditions into the heat loses your body would feel if exposed to the existing air temperature and only a 3-knot (or about 3.5 mph) wind. This is not an easy conversion. The original wind-chill formula was devised by Antarctic explorer Paul Siple in 1945; however, research has revealed some flaws in Siple’s work, such as assuming that the wind at face level is equal to the wind at 33 feet above the surface. For this and other reasons, the National Weather Service updated its wind-chill temperature calculation in November 2001.

Dr. Ed Hopkins, Wisconsin’s assistant state climatologist, computed the wind-chill temperatures for all the hourly temperature and wind speed combinations available from Truax/Dane County Regional airport since January 1948. According to the new formulation, the lowest wind-chill temperature was -54.3 F at 4:00 and 5:00 am on January 20, 1985; the same day of President Reagan’s second inaugural, which was held indoors because of the cold Washington weather and because it was a Sunday. That was also the day of Super Bowl XIX, which was held in Palo Alto CA where the game time temperature was 53 F.

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How Much does Snow Evaporate?

Posted on February 13, 2011 by Weather Guys Editor

I notice that when the snow builds up in 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 into the cold, dry air?

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

On warmer days, when temperatures are above freezing, we can see the melting process as snow leaves water behind on surfaces, which then evaporates or gets absorbed by the ground. We do not see the sublimation process because the snow goes directly into water vapor without first melting into liquid water.  However, we do notice that the snow amount is decreasing, so snow sometimes seems to disappear on cold winter days.

The rate of sublimation is a function of the weather conditions. It takes a lot of energy to turn ice into a gas called water vapor: about 7 times the amount of energy needed to boil that water. The energy needed to sublimate the snow off your patio comes primarily from the sun. So, sunny weather is the best weather for sublimating snow.  Windy days are also good, as the wind helps to remove the water molecules once they leave the snow and enter into the atmosphere. A low humidity also helps to increase the rate of snow loss.  So, the amount of snow that sublimates back into the air depends on the typical winter weather for a given location.

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