Why was it so windy last week?

Posted on October 24, 2011 by Weather Guys Editor

The wind is air in motion. Moving anything requires a force. Violent destructive winds, as well as gentle summer breezes, result from a complex interplay of different forces. One of these forces results from a pressure gradient, or how fast pressure changes over distance. Strong winds almost always result from large pressure gradients. Recently, southern Wisconsin has been under the influence of a weather pattern that has strong pressure gradients, and thus strong winds.

Near the Earth’s surface, the friction plays a pivotal role in wind, acting to slow the wind. Over the open lakes the wind will be faster than through a stand of trees, where it will be slowed by friction. In the presence of buildings, the air can be funneled between buildings and pick up speed.

The air flows from high atmospheric pressures to low pressures. The Coriolis force pulls the wind to the right so that winds blow counterclockwise around lows and clockwise around highs in the Northern Hemisphere. Friction slows down the wind and weakens the Coriolis force. So, if you stand with your back to the wind, and then turn about 30 percent to your right, low pressure will be on your left-hand side

Thunderstorms also can cause strong winds. Thunderstorms have upward air motions, called updrafts. There are also downdrafts, or sinking air in a storm. Such downdrafts carry air from high elevations in the atmosphere rapidly to the ground. Since wind speed is nearly always much larger at high elevations, the downdrafts carry very high-velocity air to the surface creating the winds of more than 100 mph.

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What is the status of the ozone hole?

Posted on October 17, 2011 by Weather Guys Editor

The winter atmosphere above Antarctica is very cold. The cold temperatures result in a temperature gradient between the South Pole and the Southern Hemisphere middle latitudes.

These temperature gradients lead to a belt of strong westerly stratospheric winds that encircle the South Pole region. These strong winds prevent the transport of warm equatorial air to the polar latitudes.

The extremely cold temperatures confined inside the ring of strong winds help to form unique types of clouds called Polar Stratospheric Clouds, or PSC. In the southern hemisphere winter, chemical reactions on the particles composing PSCs remove the chlorine from atmospheric compounds such as man-made CFCs. When the sun returns to the Antarctic stratosphere in the spring, sunlight splits the chlorine molecules into highly reactive chlorine atoms and ozone is rapidly depleted. Destruction is so rapid over the South Pole region in the Southern Hemisphere springtime that it has been termed a “hole in the ozone layer,” and it is seen every October.

Ozone depletion now is widespread over Antarctica. The Antarctic ozone hole varies in size each year. Current measurements indicate the size of the ozone hole is about five times the size of California. This is anomalously large in comparison to recent years.

Representatives from 23 nations met in 1987 in Montreal to address concerns of ozone depletion by CFCs. The resulting Montreal Protocol, and subsequent international agreements have limited usage and production of CFCs. Although the use of these chemicals has declined, their concentrations in the atmosphere have not responded as quickly. This is because CFCs are very stable molecules and will stay in the atmosphere for nearly 100 years after their release before they decompose. Despite this waiting period, the Montreal Protocol successfully put a stop to the rapid ozone loss that was occurring and has set the stage for recovery of this important chemical.

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How common is beautiful, sunny October?

Posted on October 10, 2011 by Weather Guys Editor

After a fairly dreary end of September, October has dawned with some gorgeous fall weather in our area. In fact, it appears as though we will rack up 11 straight days of not even a trace of precipitation in Madison to begin October 2011. One might wonder how common such a long string of rain-free, sunny days is in a Madison October. A very limited look back into the recent past suggests it is not at all common.

The longest such streak in 2006 occurred from October 5-9 during which the average high temperature was 64.4 degrees – perfectly average for this time of year. A warmer five-day streak occurred in October 2007 (3rd – 7th) when the average high was 81 degrees – more than 16 degrees above normal.

Slightly less impressive was the four-day streak in October 2008 in which the average high was 74.8 degrees, just a bit more than 6 degrees above normal. We were well into the middle of October 2009 before we had even a three-day precipitation-free streak (17th – 19th). During this period the average high temperature was 56 degrees, almost 4 degrees below normal. It is worth noting that this short streak occurred after the first snowflakes had fallen in Madison.

October 2010 was legendary for the major windstorm that occurred on the 26th. It also makes the record books for logging the longest rain-free streak in this short record at 17 straight days (3rd – 19th). During that period the average high was 69.9 degrees, 3 degrees above normal. So, enjoy this incredible string of beautiful fall days; recent history suggests it is pretty unusual.

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Why is Fog Usually Seen in the Morning?

Posted on September 26, 2011 by Weather Guys Editor

A fog is just a cloud at the ground. Fog formation can occur in two ways. First, the air is cooled to the dew point which leads to the formation of fog droplets. When the air temperature is the same as the dew point temperature, condensation occurs on tiny particles floating in the air. The second method of fog formation requires water to evaporate from the surface into the air, raising the dew point until condensation occurs.

Fog often dissipates with daylight. This is sometimes referred to as the fog “burning off” but that analogy is not correct. When the sun rises, the air and ground warm up. This leads to the air temperature being warmer than the dew point temperature, which causes the fog droplets to evaporate.

As the nights are getting longer in autumn, we often have clear and cloudless skies at night. The conditions help the ground, and the air near the ground, to rapidly cool during evening are clear and cloudless skies and long nights. These are the type of conditions we often have in autumn. As the air cools during the longer night the relative humidity increases, which can result in to fog formation. Windy mornings are fog free as strong winds mix the air near the ground with the drier, warmer air above.

As autumn progresses, we will see a fog during the day forming over the unfrozen lakes. This fog, called evaporation fog, forms when colder air moves over warmer water. Evaporation fog over a lake gives the appearance of steam rising out of the water and is sometimes referred to as a steam fog.

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How is Smoke from a Wildfire in Minnesota Affecting Wisconsin Weather?

Posted on September 19, 2011 by Weather Guys Editor

On Tuesday a large part of our state either saw or smelled smoke from a raging wildfire in the Boundary Waters Canoe Area in northern Minnesota. The smoke was particularly easy to detect in the northern parts of Wisconsin, but it spread southward during the day and eventually led to the Milwaukee Brewers deciding to close the Miller Park roof on an otherwise delightful early fall evening just to keep the smoke out.

This was a good decision, as prolonged exposure to smoke plume from wildfires is a health hazard. In fact, officials were recommending that people in the northern parts of the state limit their outdoor activities in the face the invading smoke.

The fire responsible for this pall of smoke was actually initiated by a lightning strike in the Boundary Waters on August 18 and has burned, slowly and over a relatively small portion of that unpopulated area, for nearly a month before a vigorous cyclone passed just to its north over the weekend. The strong winds associated with that storm fanned the flames, resulting in the fire racing 16 miles east between Monday and Tuesday, quadrupling in size. The same storm accounted for our abrupt and welcome change to fall-like conditions early this week.

The weather is a major factor in the growth of wildfires as dryness preconditions the fuels at the ground and winds quickly spread the fire. Though not an everyday occurrence, long-range transport of smoke from remote fires is far from unusual. The proximity of this fire to our state accounts for the pungent nature of this remote smoke event.

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