The Weather Guys

Is there a windiest time of year in Madison?

Posted on April 15, 2019 by WeatherGuys Editor

Matt Erdmann, of Madison, takes advantage of a winter wind as he kiteboards at Olbrich Park along the shore of Lake Monona. The climatology suggests that March and April are the windiest months, with average wind speeds of 11.3 and 11.4 mph, respectively, but November through February are not far behind, logging a four-month average of 10.5 mph. (Photo credit: Amber Arnold, State Journal Archives)

Given our recent weather, and with the pollen season dawning in southern Wisconsin, one may wonder if there actually is a windiest time of year in Madison.

Of course, a windy day can come along just about any time of year (the record gust of 83 mph in Madison occurred in June 1975) but the climatology suggests that March and April are the windiest months of the year with average wind speeds of 11.3 and 11.4 mph, respectively. November through February are not far behind, logging a four-month average of 10.5 mph.

The fact that it’s windier during the cold season is not surprising as that time of year is characterized by the highest frequency of mid-latitude cyclone activity. Mid-latitude cyclones are large in scale, covering several states simultaneously, and are often associated with very large differences in sea-level pressure over small distances. It is these pressure differences that drive the wind.

The particular maximum in March and April may reflect the fact that late winter and early spring storms have a higher chance of containing thunderstorms that can locally enhance the winds. It may also be that on a sunny March or April day the daytime heating contributes to a slightly breezier day than might be observed on a similarly sunny day in the depth of winter.

With regard to the pollen issue, it is interesting that the windiest month (April) occurs when plants are most in need of assistance in spreading pollen and reproducing. The power of evolution!

Category: Climate, Seasons, Weather Dangers

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When does winter really end?

Posted on April 8, 2019 by WeatherGuys Editor

A large snow sculpture graces State Street Mall the day after Madison saw 7.2 inches of snow last year on April 18, more than doubling the day’s previous record for snowfall of 3.4 inches in 1912. (Photo credit: Steve Apps, State Journal Archive)

Last year’s 7 inches of snow on April 18 provided vivid evidence that, though early April often brings the first string of nice spring days to southern Wisconsin, we are not truly out of the woods until the end of the month.

Despite the possibility of such an outlier event, recent research at UW-Madison has considered the variability of the end of winter based purely on temperatures above the surface.

Specifically, we have considered the areal extent of air colder than 23 degrees at about 1 mile above the surface over the Great Lakes states.

We use the average areal extent of such air in the region, over the last 70 winters (December, January and February), as a threshold by which to consider the end of winter.

Any day in March or April of any year on which that day’s areal extent of cold air exceeds the 70-year December-January-February average is deemed a “qualifying cold day.” The last day of the last three-day or longer streak of such “qualifying cold days” in a given winter season is considered the end of winter.

Though the end of winter defined this way is highly variable from one year to the next — and bears absolutely no relation to how severe or mild the preceding months of that winter have been — over the last seven decades the average end of winter has become earlier by just over one week in our region.

This regional trend is consistent with the overall global trend of slight warming but, as we all no doubt remember well, does not preserve us from the occasional late-in-the-season reminder that winter leaves only grudgingly.

Category: Meteorology, Seasons

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What is the Coriolis force?

Posted on April 1, 2019 by WeatherGuys Editor

We need to account for the Coriolis force to correctly analyze the large-scale movement of air in the atmosphere and water currents in the ocean. (Photo credit: Jim Buchta, Minneapolis Star Tribune)

Newton’s laws of motion mathematically describe how objects move when forces are exerted on them.

Earth is spinning like a top, even though to us who are standing on Earth, it seems that we are not moving. Newton did not account for Earth’s spin in his equations. The Coriolis force appears as an extra term when Newton’s laws are transformed to account for Earth’s spin.

Italian scientist Giovanni Battista Riccioli described the effect in 1651, explaining that Earth’s rotation causes a cannonball fired to the north to deflect to the east. Gaspard-Gustave Coriolis published a paper in 1835 describing the force mathematically.

The Coriolis force acts in a direction perpendicular to Earth’s rotational axis. Objects in the Northern Hemisphere are deflected to the right, while objects in the Southern Hemisphere are deflected to the left.

The magnitude of the Coriolis force depends on the speed of the object and its latitude. The Coriolis force is zero at the equator and increases toward the poles.

The Coriolis force also is proportional to Earth’s rotation rate. Earth completes one rotation per day, so for everyday motions, like throwing a ball or an apple falling from a tree, the Coriolis force is very small compared to other forces and is negligible. Its effects become noticeable only for motions occurring over large distances and long periods of time.

Because Earth spins, we need to account for the Coriolis force to correctly analyze the large-scale movement of air in the atmosphere and water currents in the ocean. It is too small to explain the rotation of draining water in sinks and toilets.

Category: Meteorology, Phenomena

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What weather is hazardous to flying?

Posted on March 25, 2019 by WeatherGuys Editor

Thunderstorms create multiple hazards for aviation such as updrafts, turbulence, lightning and hail.

While there are weather hazards, flying is pretty safe.

Thunderstorms are a potential hazard, and aircraft avoid thunderstorms to avoid any potential threats or dangers.

Jet aircraft carry weather radars to detect and thus avoid the violent updrafts of storms. These updrafts have strong, turbulent winds and can carry hail.

You probably have experienced hail falling on the ground, but hail can also cause damage to planes flying at 10,000 or even 30,000 feet. On August 7, 2015, an Airbus A320 carrying 119 passengers and five crew members encountered hail, turbulence and lightning within a rapidly developing thunderstorm . It suffered cracked windshields and failed weather radar. No one was injured, although the pilot made an emergency landing at the Denver airport and the nose of the plane suffered hail damage.

Turbulence is also a danger to high-flying jets. Turbulence generated by a thunderstorm can cause the aircraft to violently “shake” even if only flying close to a storm. Clear-air turbulence, or CAT, is the turbulent movement of air in the absence of any visual clues, such as clouds.

Lightning can strike a plane, but generally doesn’t do damage as the bolt enters the aircraft extremities, such as the nose, tail or wing tips.

Icing is particularly dangerous to planes. When a plane is flying in temperatures below freezing while inside a cloud, very cold liquid water drops freeze onto the aircraft as soon as they strike the plane. Ice on the wings can disrupt the lift of the plane causing it to lose altitude. De-icing a plane before take-off prevents build up of ice on the plane.

The downdrafts of a storm can be very dangerous to aircraft on take off or landing. A downdraft can result in rapid wind shift from a tail wind to a head wind causing the aircraft to rapidly lose altitude. Fortunately, airports are equipped with instruments that can detect hazardous conditions and send warnings that keep the planes from encountering these dangerous conditions.

Category: Severe Weather, Weather Dangers

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When is spring?

Posted on March 19, 2019 by WeatherGuys Editor

Satellite view of Earth at the Equinox.

The seasons result from the tilt of the Earth and its yearly circling of the Sun. According to the astronomical definition, spring occurs when the Sun’s rays strike the equator at noon at an angle that is directly overhead. This particular time varies from year to year due to variations in the Earth’s orbit about the Sun. In the Northern Hemisphere the Vernal (or spring) Equinox (equi, ‘equal,’ and nox, ‘night’) occurs sometime between March 19 and 23, but often on March 20 or 21. This year astronomical spring arrives on March 20 at around 4:58 P.M. CDT.

During the equinoxes all locations on Earth experience 12 hours of daylight and 12 hours of darkness. The Sun rises due East and sets due West. Equinoxes are the only two times a year that Sun only rises due east and sets due west for every location on Earth! After the Spring equinox, the Northern Hemisphere tilts toward the Sun, and we start to get longer, sunnier days.

Spring marks the transition from winter to summer. Meteorologists divide the year into quarters to compare seasonal and monthly statistics from one year to the next. Meteorological spring is defined as March through May and so begins on March 1. We might also define spring as the day on which, if there is precipitation, it is more likely to be in the form of rain than snow. For southern Wisconsin, that occurs later in the month of March.

We may also define spring based on the appearance of a particular flower, the blooming of certain trees, or the return of specific migrating birds. These are the phenological signs of spring. Some mark spring by the increase in the number of pot-holes.

Whatever the definition, during spring the length of daylight hours is increasing and the air is warming. That’s welcome news for many people, particularly after our recent cold and damp weather.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison Department of Atmospheric and Oceanic Sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.

Category: Climate, Seasons

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