Can we predict tornadoes?

Posted on July 27, 2015 by WeatherGuys Editor
Photo of Stoughton Tornado, 2005.

The tornado that struck Stoughton on August 18, 2005, is seen southwest of Highway 51 as it approached the city. Advances in radar technologies have helped to identify storms that are producing a tornadoes, or about to produce a tornado. Photo credit: Dale Bernstein NOAA/NWS

A tornado is a powerful, narrow column of winds that rotate around a center of low pressure. The winds inside a tornado spiral inward and upward, often exceeding speeds of 300 mph.

We cannot always tell if a developing storm will produce a tornado. Advances in radar technologies have helped to identify storms that are producing a tornadoes, or about to produce a tornado.

We do know the necessary conditions needed for tornado formation. And we are getting better at predicting those conditions a few days in advance, enabling forecasters to identify counties where there is a threat of severe weather sometimes as many as three days in advance.

For a thunderstorm to produce a tornado requires warm humid air near the surface with cold dry air above.

These conditions make the atmosphere very unstable, in the sense that once air near the ground is forced upward, it moves upward quickly and forms a storm.

Severe thunderstorm conditions also include a layer of hot dry air between the warm humid air near the ground and the cool dry air aloft.

This hot layer acts as a lid that allows the sun to further heat the warm humid air — making the atmosphere even more unstable.

To form a tornado, the host thunderstorm must also rotate. From below, a rotating bottom of the cloud looks like someone is stirring the storm from above. This happens in a storm when wind at the ground is moving in a different direction and speed than the air above.

The change in wind speed and direction with height is known as wind shear. Both wind shear and atmospheric instability are needed for tornado formation.

Category: Meteorology, Phenomena, Severe Weather, Weather Dangers

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When is the warmest day of the year?

Posted on July 20, 2015 by WeatherGuys Editor

As we move past mid-July, the climatologically warmest day of the year in Madison (i.e. the day with the highest average high temperature) is in our immediate wake.

Every 10 years the 30-year climatology is updated in the following way: In 1972, we used the 1941-1970 average as climatology. By 1982 we were using the 1951-1980 average as climatology. Currently, we are using the 1981-2010 average as climatology.

Though the particular averaging period has changed over the past four decades, the average warmest day of the year has consistently fallen around July 15.

This summer we haven’t had much in the way of heat, recording only two days with a temperature at or above 90 degrees (June 9 at 90 and Friday at 91).

June was just 0.1 degrees above normal — mostly as a consequence of warmer-than-normal overnight low temperatures — and July has averaged 6.2 degrees cooler than normal through the first third of the month.

The coolness thus far does not mean that the rest of the summer will remain moderate, as only 12 of the last 30 summers have recorded more days at or above 90 degrees before July 15 than after it. In fact, in the 13-year period from 1985-97, nine summers recorded the bulk of their 90-degree days before mid-July. From 1998 to 2014, only three such summers have occurred (2007, ’08, and ’12). So, front-loading 90-degree days in the summer has become increasingly rare this century. We’ll see what 2015 brings in the next few weeks.

Category: Meteorology, Seasons

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Why have the skies been so hazy?

Posted on July 13, 2015 by WeatherGuys Editor
The state Capitol is enshrouded in haze July 7 as lingering smoke plumes from wildfires in Canada continue to move through the area. Photo Credit: John Hart, Wisconsin State Journal

The state Capitol is enshrouded in haze July 7 as lingering smoke plumes from wildfires in Canada continue to move through the area.
Photo Credit: John Hart, Wisconsin State Journal

Summer skies often look hazy because of the high humidity, which condenses in the sky and forms small liquid water particles that scatter light, creating that hazy effect.

But there’s a different reason our skies have not been a nice blue color when they’re cloud-free: smoke.

It’s coming from wildfires in the forests of the Northwest Territories in Canada, which were started naturally by lightning strikes. The winds have moved this smoke our way, defining which areas would be affected by the smoke.

In early July, the winds transported the dense smoke south and southeast; the leading edge of the smoke made it as far south as Iowa and northern Illinois. While most of the smoke is high above ground, some smoke reached the ground.

Last week the smoke was so thick and widely spread that some could smell it, and triggered a poor air quality warning for southern Wisconsin. Surface visibility was reduced to 3-5 miles at some locations in North Dakota.

The smoke has made for some very red suns during sunset, particularly last Tuesday and Wednesday.

You may have noticed that if smoke is not too thick, it takes on a bluish tint. That is because the small smoke particles scatter blue colors in sunlight. Smoke above us will scatter the blue colors in all directions, eventually removing nearly all the blue light from sunlight. So, as you look at the sun as it begins to set, the smoke removes all the blue light from the sun’s rays heading toward your eyes, leaving the reds and oranges to transmit through the atmosphere. This makes the sun appear very red.

Category: Meteorology, Phenomena, Weather Dangers

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How do raindrops form?

Posted on July 8, 2015 by WeatherGuys Editor
Raindrops form when microscopic water droplets bump into each other in clouds. The more turbulent the clouds, the bigger the raindrops get.

Raindrops form when microscopic water droplets bump into each other in clouds. The more turbulent the clouds, the bigger the raindrops get.

To explain how this process works, let’s consider water droplets in the upward-moving air of a cloud. Water droplets in clouds with different sizes move at different speeds, as gravity and vertical motions within the cloud act on the particles. The difference in speed increases the chance of collisions, just as the combination of fast trucks and slow cars increases the chance of collisions on a highway. Turbulent motions in the cloud can also cause the droplets to collide.

The process of combining cloud droplets through collision-coalescence is an important mechanism for forming precipitation in clouds composed solely of liquid water droplets.

Aggregation is the process by which ice crystals collide and form a single larger ice particle. In the summer, the collections of crystals may form in the cloud but then melt as they fall to the ground, forming rain. When an ice crystal falls through a cloud, it may collide with and collect supercooled water droplets. This process is called accretion and is a mechanism to quickly form large particles.

How big a droplet or crystal grows depends on how long it stays in the cloud. The longer a particle is in the cloud, the more particles it can collect and the larger it grows. The strength of the vertical motions and the thickness of the cloud determine how long it stays in the cloud. This is why only tall clouds with strong updrafts, such as thunderstorms, produce large precipitation particles.

Category: Meteorology

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How do you stay safe from lightning?

Posted on June 29, 2015 by WeatherGuys Editor

Lightning can be fun to watch but it is also very dangerous.

Approximately 300 people are injured by lightning each year, and about 62 people are killed. On average there is about one death caused by lightning in Wisconsin annually.

While your chances of getting hit by lightning are only about 1 in a million in a given year, it is good to keep some safety tips in mind.

Lightning generates thunder so remember the saying: “When thunder roars, go indoors.” If you are outside and hear thunder, you are at risk and should seek shelter in a large building or enclosed vehicle. You are not safe anywhere outdoors. A house is a completely safe place to be during a lightning storm but stay away from anything that conducts electricity like plumbing, metal doors and TV cables.

If you are caught outside during a storm, you should run to a building or hard-topped vehicle — not a convertible. Most cars are a safe place to be, but not because of the rubber tires. It is the metal roof and metal sides that protect you, not the tires. If lightning were to strike the car, the energy would go through the metal frame into the ground. So don’t lean on the doors.

If caught outside with no access to buildings or cars, there is little you can do to substantially reduce your risk of getting struck. You should avoid open fields and ridge tops, stay away from tall isolated trees, and, if in a group, spread out. If someone in your group were to get struck, they are not electrified. If you touch them, you will not be electrocuted as the body does not store electricity. You should see if they need resuscitation like CPR.

Wisconsin gets hit by lightning about 300,000 times a year, mostly during the spring and summer. That’s about five flashes for each square mile in the state.

Don’t take chances with lightning. Stay safe by having a safety plan.

Category: Phenomena, Severe Weather, Weather Dangers

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