How Does Hail Form?

Posted on April 10, 2011 by Weather Guys Editor

Hail is precipitation in the form of large balls or lumps of ice. There was a storm that produced hail early last week. Hailstones begin as small ice particles that grow primarily by accretion; to grow large, they require abundant water droplets. As the hailstone moves up and down through a storm, it collides with water droplets, growing larger with each collision.  Hailstones can be as large as oranges and grapefruits.

When a hailstone is cut in half, you can see rings of ice. Some rings are milky white; others are clear. This ringed structure suggests that a hailstone can grow by two different processes, wet growth and dry growth.

In wet growth, the hailstone is in a region of the storm where the air temperature is below freezing, but not super cold. When the hailstone collides with a drop of water, the water does not freeze on the ice immediately. Instead, the liquid water spreads over the hailstones and slowly freezes. Because the water freezes slowly, air bubbles can escape, resulting in a layer of clear ice.

Dry growth of hailstones occurs when the air temperature is well below freezing. In these conditions a water droplet freezes immediately as it collides with the hailstone. This quick freezing leads to air bubbles “frozen” in place, leaving cloudy ice. Counting the layers of clear and milky white ice gives an indication of how many times the hailstone traveled to the top of the storm.

Category: Phenomena
Tags ,
Comments Off on How Does Hail Form?

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.

Category: Weather Dangers
Tags , , , ,
Comments Off on What is Freezing Rain?

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.

Category: Phenomena
Tags , ,
Comments Off on What are the Northern Lights?

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.

Category: Weather Dangers
Tags , ,
Comments Off on Why do Bridges Ice before the Road?

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.

Category: Weather Dangers
Tags , , , ,
Comments Off on What is the Coldest Wind-Chill Temperature Ever Recorded in Madison?