What causes wind gusts?

Posted on January 9, 2012 by Weather Guys Editor

A wind gust is a sudden, brief increase in the speed of the wind followed by a lull. According to National Weather Service observing practice, gusts are reported when the peak wind speed reaches at least 18 mph and the variation in wind speed between the peaks and lulls is at least about 10 mph. The strongest wind gust recorded occurred at Australia’s Barrow Island, with a gust of 253 mph that occurred during tropical cyclone Olivia on April 10, 1996.

Gusts at the ground are caused by either turbulence due to friction, wind shear or by solar heating of the ground. These three mechanisms can force the wind to quickly change speed as well as direction.

In the case of friction, gusts are generated when wind blows around buildings, trees or other obstacles. This type of gustiness is generally largest near tall buildings and alley ways and least over large water bodies.

But air over water can still be gusty. This can be caused by wind shear. A wind shear is a change in the wind over a distance. This can be a change in wind direction, wind speed or both.

On sunny days wind gusts can be generated by rising air currents when the ground is heated. This can generate a thermal of warm air that rises, with air from above sinking to replace the rising thermal. This descending air can cause wind gusts.

The duration of a gust is usually less than 20 seconds. Whatever the mechanism, wind gusts often do things that you’d rather didn’t happen — like destroy an umbrella or blow your new hat blown away.

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What were the top Wisconsin Weather Events for 2011?

Posted on January 2, 2012 by Weather Guys Editor

Each season in 2011 had a memorable weather event, some with negative impacts. There were two good snow storms in February. The Groundhog Day blizzard had wind gusts of up to 60 mph and snowfall depths ranging from 1 to 2 feet. That resulted in snow drifts of 6 to 10 feet. The second storm occurred on February 20-21 with snow accumulations between 8 and 15 inches. Freezing rain and sleet also fell across southern Wisconsin.

There were 38 documented tornadoes, making the year the third busiest on record. A record-breaking tornado event occurred on April 10 with at least 15 tornadoes, including four strong tornadoes. This was the largest total number of tornadoes in one April day. There were 11 confirmed tornadoes on May 22.

Summer weather included straight line winds and a heat wave. The heat wave of July 17-21 covered most of the state with heat indices of 100 degrees to 117 degrees. A tornado-water spout was observed over Lake Mendota on August 8. Waterspouts were also observed east of Milwaukee Harbor on Sept. 24. A severe thunderstorm hit western Waukesha County on June 21 with 100+ mph winds, which leveled or damaged thousands of trees.

Labor Day saw relatively cool temperatures with lows in the 30s. Throughout October, November and December, Wisconsin experienced temperatures that were above our 30-year climatic mean.

During the year, 10 people were directly killed by weather events: one person from a blizzard, one from lightning, one from a tornado, one from non-tornadic thunderstorm winds, one from flooding and five during the July heat wave.

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How do snowflakes grow?

Posted on December 26, 2011 by Weather Guys Editor

A snowflake can be an individual ice crystal or an aggregate of ice crystals. Ice crystals can grow by water vapor deposition on the crystal or by collisions with other cloud particles.

An ice crystal can grow if the air around it has a relative humidity near 100 percent. The ice particle grows by water vapor deposition. Growth by deposition is generally slow. If you find nicely shaped snowflakes, they likely were produced by vapor deposition.

Collisions can produce large snowflakes. When an ice crystal falls through a cloud it may collide with and collect super-cooled liquid water drops. This process of ice crystal growth by sweeping up water drops is called accretion. When ice crystals collide with super-cooled drops, the drops freeze almost instantly, providing a mechanism for the particle to grow quickly.

An ice particle produced by the accretion process that has a size between 0.04 to 0.2 inches and no discernible crystal shape, or habit, is called graupel. On collision and freezing, the super-cooled water often traps air bubbles. Because of this trapped air, the density of a graupel is low and it can easily be crushed, unlike a hailstone.

Aggregation is the process by which ice crystals collide and form a single larger ice particle. The probability that two crystals will stick together depends on the shape of the crystals. If two crystals with spiky appendices collide, it is likely that their branches will become entangled and the two crystals will stick together. Snowflakes composed of aggregates can sometimes reach 3 or 4 inches in size.

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When is the Winter Solstice?

Posted on December 19, 2011 by Weather Guys Editor

The winter solstice (from the Latin sol, or “sun,” and stice, or “come to a stop”) is the day of the year with the fewest hours of daylight. This year, this occurs for the Northern Hemisphere at 11:30 pm Wednesday.

As Earth orbits the sun, its axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because Earth’s axis of spin always points in the same direction — toward the North Star — the orientation of Earth’s axis to the sun is always changing as it orbits the sun.

As this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude. This links the amount of solar energy reaching a particular location to the time of year and causes some months of the year to always be warmer than others — in other words, the seasons.

On the Northern Hemisphere’s winter solstice, the northern spin axis is pointed away from the sun, and latitudes north of the Arctic Circle (66.5 degrees north) have 24 hours of darkness.

The winter solstice is often referred to as the first day of winter. But there are other definitions of winter. For example, the beginning of winter might be defined as the calendar day, on average, when precipitation has an equal chance of falling as rain or snow. For Madison, that calendar day is November 15. Meteorologists often define the three months of winter as December, January and February — the coldest months of the year.

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Why does the moon look red during a lunar eclipse?

Posted on December 12, 2011 by Weather Guys Editor

A total lunar eclipse could be seen in cloud-free regions across most of the United States and Canada on Saturday morning, Dec. 10. In a total lunar eclipse the sun, Earth and moon line up and the Earth casts its shadow on the moon. The moon is always a full moon and it never goes completely dark during a total lunar eclipse. It appears reddish for the same reason that sunsets and sunrises often have a red tint.

The Earth’s shadow has two parts: the umbra and the penumbra. A small amount of the sun’s energy shines directly within the penumbra. Within the umbra, none of the sun’s rays shine directly in this part of the shadow, so it is the darkest part of the shadow. A total lunar eclipse occurs when the moon travels through the Earth’s umbra.

The umbra is not pitch black because of the Earth’s atmosphere. Sunlight is scattered, or redirected, in all directions by the Earth’s atmosphere into the penumbra and umbra. As light passes through the atmosphere, the blue colors are scattered out of the path. Red and orange light pass through the atmosphere and are scattered into the shadow zone. If our planet had no atmosphere, then the moon would be completely dark during a total lunar eclipse.

If you missed seeing this recent total lunar eclipse, the next total lunar eclipse that is viewable from our area occurs in 2014.

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