Is our growing season getting longer?

Khadga Acharya, right, and her mother-in-law, Kala Acharya, harvest and early season crop of herbs from their family's garden plot at the Fountain of Live Community Garden in Madison last June.  Wisconsin's growing season has lengthened by about 12 days, allowing production of longer-season crop types and varieties. (Photo credit:  John Hart - State Journal archives)

Khadga Acharya, right, and her mother-in-law, Kala Acharya, harvest and early season crop of herbs from their family’s garden plot at the Fountain of Live Community Garden in Madison last June. Wisconsin’s growing season has lengthened by about 12 days, allowing production of longer-season crop types and varieties. (Photo credit: John Hart – State Journal archives)

Yes. There are several studies that demonstrate shifts in the timing and length of the growing season.

One way to measure the length of the growing season is to count the number of days between the last frost in spring and the first frost in fall. By this measure, Wisconsin’s growing season lengthened by about 12 days between 1950 and 2006.

Longer growing seasons allow production of longer-season crop types and varieties.

The Growing Degree Days, or GDD, is a heat index that is related to plant development and is used to predict when a crop will reach maturity.

Each day’s GDD is calculated by subtracting a reference temperature, which varies with plant species, from the daily mean temperature, setting values less than zero to zero.

The reference temperature for a given plant is the temperature below which development for that plant either slows or stops.For example, cool season plants, like peas, have a reference temperature of 40 degrees, while warm season plants, like sweet corn and soybeans, have a reference temperature of 50 degrees.

The development of plants depends on the accumulation of heat. Since cool season plants have a lower reference temperature, they accumulate GDDs faster than warm season plants.

When drought or pests do not overly stress plants, the summation of the GDD can be used to measure the accumulation of heat and thus predict when a crop will reach maturity.

GDDs can be computed using climatic temperatures of an area. With that computation, we can estimate good crops to grow in a given region, similar to plant hardy zones. The longer growing season and warmer temperatures results in a changing GDD that is favorable to warm season plants.

Category: Climate, Seasons

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When can it get really hot in southern Wisconsin?

Kate Werner, left, and Maurissa Myers get a visit from a mallard, who stops and poses for a photo, while they enjoy the warm weather at James Madison Park on Lake Mendota last May.  From May 26 through Sept. 21, there is no calendar day on which the record high temperature in Madison is not at least 90. (Photo credit - Amber Arnold, State Journal archives)

Kate Werner, left, and Maurissa Myers get a visit from a mallard, who stops and poses for a photo, while they enjoy the warm weather at James Madison Park on Lake Mendota last May. From May 26 through Sept. 21, there is no calendar day on which the record high temperature in Madison is not at least 90. (Photo credit – Amber Arnold, State Journal archives)

Two of those occurred in April 1952 when the last four days of the month recorded high temperatures of 85, 90, 87 and 90. The real standout warm April day occurred on April 22, 1980, when the temperature soared to 94. On that same day, Milwaukee set its all-time April high record at 91, having reached 90 only one other time, on April 10, 1930.

Summer potential really arrives in Madison (and Milwaukee) in May when record highs above 90 have occurred on 17 of the 31 calendar days (15 in Milwaukee). Interestingly, from May 26 through Sept. 21 there is no calendar day on which the record high temperature in Madison is not at least 90. For Milwaukee, a similar interval stretches from May 24 through Sept. 24.

These unbroken streaks of days with the potential to exceed 90 are probably consistent with most people’s sense of the boundaries of the warm season in southern Wisconsin and so actually serve as decent operational definitions of summer for both cities.

Category: Climate, Meteorology, Seasons

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What connection does UW-Madison have with the National Weather Service?

Dr. Louis Uccellini (Photo credit:  National Oceanic and Atmospheric Administration)

Dr. Louis Uccellini (Photo credit: National Oceanic and Atmospheric Administration)

Last week, the director of the National Weather Service (NWS), Louis W. Uccellini, visited his alma mater as the Department of Atmospheric and Oceanic Sciences inaugural Distinguished Alumni Award winner.

 

Uccellini presented the story of the intellectual and professional journey that led him to the leadership of this extraordinarily important government agency.

He reminded us of some of the rich history that connects the professional weather and climate forecasting services of today with the vision of some of Wisconsin’s first scientists, including professor Increase Lapham, who was among the more influential protagonists arguing for a National Weather Service during the 1860s. That history also includes the first weather satellites, foundational insights into the physics of weather systems in both the middle latitudes and the tropics, as well as pioneering work on understanding the climate system.

As his two-day visit came to a close, we were filled with a great pride that, because of his UW roots, we were able to engage hundreds of students in personal meetings with the director of the NWS. It reminded us that our department and our university are both institutions that aspire to change the world for the better.

UW-Madison is not great by accident but by choice. For more than 150 years the people of our state have chosen to make its success a priority because they realize it is one of the drivers of the state’s success. As Uccellini’s visit reminded us all, we strive to do great things at Wisconsin and we usually succeed.

Category: Meteorology

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What is the effect of snow cover on weather and climate?

Snow surrounds spring crocuses in bloom.  Even as a few flurries filled the sky on Friday, it is almost the end of the snow season for southern Wisconsin. (Photo credit:  Gregory Shaver)

Snow surrounds spring crocuses in bloom. Even as a few flurries filled the sky on Friday, it is almost the end of the snow season for southern Wisconsin. (Photo credit: Gregory Shaver)

Snow is very good at reflecting incoming sunlight as anyone who has ever found themselves near a snow field on a sunny winter day can attest.

In fact, the percentage of incoming sunshine that is reflected, a fraction known as the albedo, is near or above 90 percent for a fresh snow cover. That means that 90 percent of that incoming sunshine, otherwise available to help warm the ground and then, in turn the air above the ground, is no longer able to warm the daytime.

Snow is also exceptionally good at emitting and absorbing longer wave, infrared radiation – the kind emitted by the Earth. Any layer of snow has two sides, a side that makes contact with the ground and another that faces the sky. For snow depths of just a few inches, the side facing the ground absorbs infrared from the surface of the Earth and prohibits that energy from reaching the top surface of the snow.

Meanwhile the top surface emits lots of infrared energy, which cools the air over the snow to very low temperatures. As a result of these two effects, the presence of snow cover lowers both the daytime high and the overnight low temperatures.

Recent research by a colleague of ours has shown that with both the arrival of snow in mid-November and the disappearance of it in late March, the temperature of the southern Prairie Provinces of Canada changes by about 18 degrees within a week. So, as snow loses its grip on the Northern Hemisphere, the warming we all expect from spring can come on quickly.

Category: Meteorology, Seasons

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How do you measure weather far above the ground?

A monitor displays temperature and humidity data being sent back from a weather balloon at the Cooperative Institute of Meteorological Satellite Studies at UW-Madison. (Photo credit:  Andy Manis)

A monitor displays temperature and humidity data being sent back from a weather balloon at the Cooperative Institute of Meteorological Satellite Studies at UW-Madison. (Photo credit: Andy Manis)

Temperature and relative humidity are measured electronically; a small aneroid barometer measures pressure. Wind speed and direction are determined by tracking the position of the balloon.

When we also measure the winds, the observation is called a rawinsonde. The vertical distribution of rawinsonde observations above a location is known as a sounding.

Rawinsonde measurements are made worldwide at several hundred locations twice each day at the same time. They are only launched from land-based weather stations, which leaves much of the world unmeasured.

Because of this limitation of observations in space and time, scientists also use satellite observations to monitor the weather far above the ground. A few satellites can make global observations many times a day. Winds are determined by tracking the movement of clouds and water vapor features in the atmosphere. Temperature and humidity can be inferred under clear-sky conditions by detailed measurements in the infrared.

All these observations become critical measurements for weather forecasting. They provide a measurement of the initial state of the atmosphere at the start of the forecast.

Category: Meteorology

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