The Weather Guys

Why is the grass wet in the morning, when it didn’t rain?

Posted on April 24, 2017 by WeatherGuys Editor

Covered with dew, the wind-driven seeds of milkweed rest in a nest of dried leaves. Dew is made of liquid water that has condensed from some of the water vapor in the air. (Photo credit: La Crosse Tribune archives)

Recent spring mornings have produced wet lawns. To explain this, we start with the fact that air contains water in the gas phase, called water vapor. Dew is made of liquid water that has condensed from some of the water vapor in the air.

Dew occurs when objects cool. A common example of dew formation is when drops of liquid form on the glass of an ice-cold drink. Dew forms when the object, such as the glass, cools down to the dew point temperature.

Water molecules in the air continually bombard surfaces, like blades of grass. Some of the molecules stick, forming a very thin film of water. This film may not last long, as the water evaporates. The evaporation rate depends on the temperature of the water, which is the same temperature as the blades of grass.

So, condensation depends on the state of the atmosphere, such as its temperature and moisture, while evaporation depends on the temperature of the object.

If the object gets cold enough, and there is enough moisture in the air, condensation is much greater than evaporation and the film grows into dew drops.

Each night the weather report includes the temperature and the dew point temperature.

If the two temperatures are close, it is likely that dew will form during the night.

Dew also tends to form on calm nights that come with clear skies. Windy conditions and cloudy skies keep the ground from cooling.

Category: Meteorology, Phenomena

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What are the plant hardy zones and how do they relate to climate?

Posted on April 17, 2017 by WeatherGuys Editor

Signe Knudsen, right, talks with Ia Xiong, in Troy Community Gardens in Madison. Hardiness zones let gardeners know the type of plants capable of surviving their climatic conditions, including the ability to survive a minimum temperature range. (Photo credit: Michelle Stocker, The Capital Times Archives)

If you are involved with gardening, you probably are aware of the hardy zones listed on seed packets.

Southern Wisconsin is largely in zones 5a and 4b, while northern Wisconsin lies in zones 3a to 4a.

The U.S. Department of Agriculture developed the zones and first published them in 1960.

A hardiness zone provides information on the type of plants capable of surviving certain climatic conditions. These conditions include the ability of a plant to survive a minimum temperature range.

Regions with hardiness zone 4b have an average annual minimum temperature range between minus-25 and minus-20 degrees, while zone 5a has a range between minus-20 to minus-15 degrees.

The climate zones are determined from temperature records kept by National Atmospheric and Oceanic Administration (NOAA).

Recognizing that climate varies over long time periods, every 10 years NOAA computes a revised 30-year average temperature and extreme temperatures for the U.S. Between 1961-1990 and 1976-2005, the 30-year average minimum winter temperatures increased at nearly all locations in the continental U.S. This suggests appropriate changes in plant hardiness zones.

The USDA Plant Hardiness Zone Map developed in 1990 was updated in 2012. Previously, south-central Wisconsin was in zone 4b, in the revised map we are zone 5a. Given the continued warming of our minimum wintertime temperatures, when the USDA next revises the hardy zone map, the zones in Wisconsin may again shift northward.

While the hardiness zones are useful, they cannot account for all climate and weather conditions such as snowfall, which can insulate the plants during a cold winter, or severe summer heat. In addition to knowing your plant hardiness zone, it is also very useful to talk with local master gardeners and nurseries as you plan your plantings this season.

Category: Climate, Meteorology, Seasons

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When does tornado season start?

Posted on April 10, 2017 by WeatherGuys Editor

The tornado that struck Stoughton on August 18, 2005, is seen southwest of Highway 51 as it approached the city. (Photo credit: Dale Bernstein, NOAA/NWS)

Wisconsin has had tornadoes in every month of the year except February. We can have tornadoes almost anytime, although the chances of having one in winter are pretty small.

June is the month with the largest number of tornadoes in Wisconsin, with 95 percent of tornadoes occurring between the months of April and September.

We typically define the tornado season for a particular area as the time of year when tornado occurrence there is at its peak. Tornado season peaks in March and April in the Southeast but not until late June or early July in the Upper Midwest. For the entire U.S., May is the month with the greatest number of tornadoes.

For a tornado to occur there must be the right ingredients coming together in a particular place. Severe weather requires warm, moist air near the ground and a change in wind speed and direction, or wind shear, with height above the surface. Since springtime is when the lower atmosphere is warming up, that’s when we start hearing about tornadoes. This continues through summer.

So far, 2017 has seen an active tornado season for the nation. As of the weekend, the U.S. has seen 413 tornadoes that have led to 27 deaths. On average, Wisconsin averages 19 tornadoes a year. There have been 1,537 documented tornadoes in Wisconsin since 1884, although sightings prior to 1950 are very incomplete, as many in rural areas were not reported.

While any tornado should be considered life threatening, most tornadoes that occur in Wisconsin are considered weak, with wind speeds less than 110 mph.

— 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: Meteorology, Seasons, Severe Weather

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Do Major League games get canceled due to snow?

Posted on April 3, 2017 by WeatherGuys Editor

Snow removal begins in the infield of Wrigley Field on March 2, 2015, as the Chicago Cubs’ opening day approaches. (Photo credit: Charles Rex Arbogast, Associated Press)

In this country, Opening Day of the Major League Baseball season is more likely to be considered the start of spring than the vernal equinox that precedes it by two weeks each year.

However, as committed baseball fans surely know, the start of the season does not mandate the start of the warm season. In fact, though no definitive record of the total number of Major League games postponed by snow exists, quite a number of opening weeks have experienced widespread disruption as a result of early season snow. Most recently, opening weeks in 2007 and 2013 were plagued by numerous cancellations across both leagues.

One of the more colorful examples of snow in the early season occurred in 1907 when the Philadelphia Phillies traveled to New York’s Polo Grounds to begin the season against the hometown Giants.

Opening Day was the day after a major snowstorm, and groundskeepers had been forced to shovel large drifts of snow onto the outer edges of the field. After seeing their team fall behind 3-0, disgruntled Giants fans began throwing snowballs onto the field. These first volleys quickly became a full-fledged snowball fight. Before long, home plate umpire Bill Klem was struck in the head by a snowball and immediately called a forfeit in favor of the Phillies.

The threat of snow during the opening week of the season seems minimal this year, though it is almost certain a few cold days and nights will greet the first few weeks of the season in Milwaukee as easterly winds off Lake Michigan can be bone-chilling right through late May.

Category: Seasons, Severe Weather

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Is springtime pothole season?

Posted on March 27, 2017 by WeatherGuys Editor

The spring freeze-and-thaw cycle of pavements accelerates the formation of potholes. (Photo credit: State Journal archives)

Many drivers probably consider early spring to be pothole season.

A pothole is a structural failure in a road surface.

Potholes result from a combination of traffic and water in the underlying soil.

Many roadways are constructed in layers.

The top layer is water resistant and curved to drain water off the road and onto the shoulder.

Underlying the layer we drive on is gravel and soil. Water can seep below the top surface into the underlying materials. Water weakens the soil that lies beneath the pavement, and then stresses due to traffic weaken the pavement and with time, and heavy loads, the pavement breaks up.

We live in an area that experiences freezing and thawing, particularly in spring. This freeze-and-thaw cycle of the pavements accelerates this pothole-forming process.

Water can seep into the upper soil level, but not penetrate the frozen soil layer below. If this water freezes, it expands and takes up more space under the pavement, causing the pavement to expand, bend or crack. This weakens the pavement material.

Then, when ice melts during the day, there is a gap under the pavement where the water was trapped, weakening the support of the pavement. The weight of the vehicles passing over this weak spot in the road can displace and break the pavement, creating a pothole.

Repairing potholes is a challenge, as one should not only fill the hole but also seal it to keep water from getting into any cracks. Cleaning out the loose debris and filling it with a hot and cold asphalt patch is a common approach to fixing a pothole.

Madison offers a website for pothole-reporting by citizens at www.cityofmadison.com/transportation/potholepatrol.

Category: Seasons, Weather Dangers

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