How does weather impact forest fires?

Posted on May 9, 2016 by WeatherGuys Editor

Wildfires south of Fort McMurray in Alberta Canada on 5/7/16. (Credit: Jonathan Hayward –  Canadian Press)

Fires require fuel to burn, heat to ignite and oxygen to feed the chemical reaction. Weather plays a key role in all of these requirements to start and spread a forest fire.

Weather and climate are important in making fuel available by determining the moisture content of the vegetation. A long period without rain dries out the vegetation, making it easier to burn and thus a better fuel source.

Low humidity makes it easier to start and spread a fire. Trees and shrubs wet from rains make it more difficult to start a fire.

Lightning from thunderstorms often starts a wild fire in remote areas. Hot days will warm and dry potential fuels, making the fire easier to start and increasing the rate at which it can spread.

Winds also play an important role in starting and spreading a fire by fanning the flames of the initial spark from a lightning bolt. The winds provide fresh oxygen that can also stir a fire that has died down. Winds help steer, along with fuel availability and topography, the direction and progress of a fire.

A large fire can generate a wind pattern of its own that can help to spread the fire. Rotating winds can develop along the edge of a hot fire. These vortices are a result of the contrast between the hot air associated with the fire edge and the cooler air over the adjacent, non-burning region.

Fire tornadoes can be generated when the vortices are tilted from the horizontal to a vertical direction. Fire tornadoes can be composed of flames or black smoke and can toss burning debris into the non-burning area, helping to spread the fire.

The substantial influence that the fire has on the winds surrounding it makes the precise direction and speed of the spread of the fire difficult to forecast.

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, Phenomena, Weather Dangers

Comments Off on How does weather impact forest fires?

Is our growing season getting longer?

Posted on May 2, 2016 by WeatherGuys Editor
Khadga Acharya, right, and her mother-in-law, Kala Acharya, harvest an 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)

Khadga Acharya, right, and her mother-in-law, Kala Acharya, harvest an early season crop of herbs from their family’s garden plot in Madison.  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)

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

Comments Off on Is our growing season getting longer?

When can it get really hot in southern Wisconsin?

Posted on April 25, 2016 by WeatherGuys Editor
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

Comments Off on When can it get really hot in southern Wisconsin?

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

Posted on April 18, 2016 by WeatherGuys Editor
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

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

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

Posted on April 11, 2016 by WeatherGuys Editor
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

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