What rare weather event did we experience last month?

Posted on January 6, 2015 by WeatherGuys Editor

Madison’s overnight low temperature of minus 3 on Dec. 28 was a relatively rare event, all things considered.

As we all know, the temperature dipping below zero in winter is not unusual. But it doesn’t usually happen without snow on the ground, and there are good reasons for that.

Snow on the ground works double duty to keep the air near the ground very cold. First, since the ground is a vast storehouse of heat, the presence of snow insulates the air above from that heat.

The fact that snow is an excellent absorber and emitter of infrared energy — the kind of radiation that nearly everything on Earth emits, including the ground — gives this insulation effect an added radiative dimension. With a couple of inches or more of snow cover, the infrared energy emitted by the ground is absorbed by the overlying snow.

The snow then emits that energy in all directions, some of which are back toward the underlying ground. Thus, much less than half of the energy that would otherwise be emitted from the ground to the overlying atmosphere never gets there.

Secondly, since the top surface of the snow is emitting infrared exceptionally well, the air in contact with that snow surface can get really, really cold. The result is extremely cold surface air.

In our last cold spell, the coldness was imported from locations where this radiative cooling had been in operation — places to our north. This coming week, our low temperatures will be partly “home-brewed” by the snow we just received.

Category: Meteorology, Seasons

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What are geomagnetic storms?

Posted on December 29, 2014 by WeatherGuys Editor

Our sun is an active star that has storms. Sometimes the sun ejects a cloud of gas, called a coronal mass ejection or CME. CMEs are often associated with solar flares, and it takes about two to three days for the charged particles in this gas to reach Earth. Earth’s magnetic field deflects these particles toward the North and South Poles.

Unfortunately, solar electrons and protons from CMEs collide with the Earth’s magnetic field and atmosphere and can stir up electromagnetic storms in the Earth’s magnetosphere. These geomagnetic storms can result in the Northern Lights but also disrupt satellite-based navigation, communications, air travel, power grids and even pipelines. A geomagnetic storm in March 1989 shut down the Hydro-Quebec electric grid in Canada, leaving people without electricity.

In a worst-case scenario, the costs of an extreme geomagnetic storm in the United States could be in the trillions of dollars and would require a recovery period of up to a decade. As a result, the U.S. is developing plans to reliably address impacts of geomagnetic storms on our bulk power system. Networks are being set up across the globe to monitor geomagnetic conditions.

An emerging discipline known as space weather attempts to forecast the effects of solar activity on the upper atmosphere so that impacts to society can be minimized. Instruments on satellites can observe ejections of high-energy plasma from the sun. These can result in NOAA’s Space Weather Prediction Center issuing a warning of a coming magnetic storm, estimating time of arrival and intensity.

Category: Meteorology, Phenomena

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Are ‘atmospheric rivers’ and the ‘Pineapple Express’ related?

Posted on December 15, 2014 by WeatherGuys Editor

Last week, central and southern California got hit with a substantial rain event, welcome news for a severely drought-stricken area that has not seen heavy rains in many months.

A high-profile element of the storm system that affected the region was a so-called “atmospheric river,” or the local variant of this feature sometimes known as the “Pineapple Express.”

These names refer to the fairly narrow (sometimes only a few hundred miles wide) streams of high atmospheric water vapor content that can result from the encroachment of a middle latitude weather system into the northern subtropics — latitudes of about 30 degrees North. The middle latitude weather system has associated with it a robust circulation that drives air northward on its eastern side and toward the equator on its western side.

This same characteristic is observed with the passage of such storms over Wisconsin at any time of year. When such a circulation finds itself as far south as 30 degrees North, the air that it drives northward on its eastern side is pure tropical air, laden with enormous amounts of water vapor. This results in narrow tongues of exceptional water vapor content rushing northward in what resemble “rivers” of water, but in the atmosphere.

On the Pacific coast of North America, the tropical air can sometimes originate near the Hawaiian Islands, hence the name Pineapple Express. When these rivers of high water vapor content are forced up the sides of the Sierra Nevada mountains in California and Nevada, the water vapor is condensed into liquid and solid form, bringing much needed rains to the area.

So, though this recent storm was powerful, it was not unprecedented and, in fact, is a rather common occurrence along the entire Pacific coast in the late autumn and early winter.

Category: Meteorology, Severe Weather

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What is a ‘panhandle hooker,’ and why is it called that?

Posted on December 8, 2014 by WeatherGuys Editor

A panhandle hooker, also called a Texas hooker, is a name for a storm that forms in the panhandles of Oklahoma or Texas.

These storms bring interesting weather to the Midwest and Great Lake regions. The weather system gets its name from where the storm forms and intensifies.

The “hook” describes the curved path that these cyclones take, first bending to the southeast and then curving northeast to Missouri across Iowa and into southern Wisconsin and on to the Great Lakes.

Cold and warm fronts are associated with these mid-latitude cyclones.

These storms occur from late fall through early spring. They are not very common and bring some of the most memorable weather to our region, including heavy snow, strong winds and blizzard conditions.

As they pass over the Great Lakes, the warm waters can intensify the storm, generating dangerous shipping conditions.

The storm that sank the Edmund Fitzgerald ship on Lake Superior on Nov. 10, 1975, was a panhandle hooker.

The weather preceding the arrival of these storms can be very pleasant.

The Armistice Day Storm of Nov. 11, 1940, was another example of a panhandle hooker. Many Midwesterners woke on that morning to surprisingly warm weather and a day off from work in honor of Armistice Day. Thousands of fishermen and duck hunters headed outdoors to take advantage of the near-perfect conditions.

Most people did not realize that the warm air was the harbinger of a powerful panhandle hooker. Before 2 p.m., the cold front chasing the warm air mass arrived in Wisconsin, Minnesota and Michigan. Temperatures dropped rapidly, in some cases more than 50 degrees.

Rain, sleet, and snow began to fall. High winds brought down trees and wires, whipped up snow into blizzard conditions, and made navigating lakes and rivers nearly impossible.

A total of 145 deaths were blamed on that storm.

Category: Meteorology, Seasons, Severe Weather

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What is the windchill temperature?

Posted on December 1, 2014 by WeatherGuys Editor

What is the windchill temperature?

The windchill describes the increased loss of heat by the movement of the air.

The windchill is relevant to humans and other animals that need to maintain a constant temperature that is higher than their surroundings.

The windchill cannot be measured with a thermometer; it must be computed.

The windchill temperature index, expressed in degrees, translates your body’s heat losses under the current temperature and wind conditions into the air temperature with a 3 mph wind that would produce equivalent heat losses.

The original windchill formula, devised by Antarctic explorer Paul Siple in 1945, was based on research involving the time it took water in a plastic container to freeze.

The National Weather Service updated the formula in November 2001.

This modern-day windchill temperature index takes into account a calculated wind speed based at the average height of a human face (not at the winds measured at the national standard height of 33 feet), the exposure of a human face to cold vs. a plastic container, incorporates heat transfer theory, lowers the calm wind threshold from 4 mph to 3 mph, and has a consistent standard for skin tissue resistance.

As an example, a temperature of 0 degrees and a wind of 30 mph has a windchill temperature index of -26.

If left outside, an object will never reach a temperature of -26, it will cool down to 20 at a cooling rate equivalent to an outside temperature of -26 and a wind at 3 mph.

Under these conditions, frostbite would occur within 30 minutes.

Frostbite occurs when your skin and other tissues cool down to the point that ice crystals form in your bodily fluids.

Cold temperatures combined with strong winds can cause frostbite in a matter of minutes, and the windchill temperature index is used to determine cold hazardous conditions.

Category: Meteorology, Seasons

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