How do you measure snow accumulation in high winds?

Posted on January 23, 2012 by Weather Guys Editor

Accurate and precise measurement of snow accumulation is a difficult task. The measurement tools are simple: a ruler or yardstick that measures in inches and tenths of an inch.

The trick in measuring snow consistently is simply finding a good place to measure and a firm surface for your ruler to set on. It is recommended to use a snow board, which is a square piece of wood 16 inches on a side and painted white. Other surface options are wooden decks, picnic tables and cars.

Your measurement location should be 20 to 30 feet away from the house with an unobstructed view of the sky. Sidewalks are not recommended as good sites as they tend to accelerate melting of the snow. Neither is grass a good site as snow tends to sit up on top of the blades of grass, while the ruler goes down to the ground. With this setup, measuring new snow accumulation is straightforward, if there’s no wind. When the wind blows and the snow drifts, measuring snow is a challenge.

To deal with drifting snow, make measurements at a various places in your yard and then average them to get what is considered a representative measurement.

Snow is also measured with the standard rain gauge — an 8-inch diameter cylinder that collects precipitation. In winter the gauge can be equipped with a known amount of antifreeze so that the snow melts when it enters. This measurement yields the liquid water equivalent of the snowfall.

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Do the tropics influence the weather in Madison?

Posted on January 17, 2012 by Weather Guys Editor

It may seem implausible at first glance, but current research in the Department of Atmospheric and Oceanic Sciences at UW-Madison is exploring connections between tropical cyclones (hurricanes) near the Philippines and extreme weather events in southern Wisconsin.

The connection appears to derive from unusual jet stream structures forced by the outflow from the hurricane at high levels in the atmosphere.

Hurricanes are well-organized collections of tropical thunderstorms that take warm, moist air from the lowest levels of the tropical atmosphere and very quickly thrust it nine miles or more above the surface. This rapid upward motion substantially distorts the shape of the tropopause which, in turn, intensifies the normal jet stream located on the northern periphery of the vast hurricane outflow.

The intensified jet, often a very long-lived structure, can become a major weather event produced in the central United States several days later. Currently, we think this type of connection is most likely to occur in spring and fall.

Our incredible windstorm in October 2010 was a clear example of this tropical connection to Wisconsin weather. We are just beginning to explore these ideas in detail and will keep you informed of our results.

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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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