Is severe weather season about to begin?

As the threat of winter snows recedes across the country, it is replaced by the threat of severe weather (i.e., thunderstorms with hail, damaging winds and tornadoes.)

GOES-East satellite image of severe storms
in Oklahoma from March 2017. (CIMSS/SSEC)

The severe weather season, though broadly spanning March through August across the United States, is actually quite regional. It begins in March in the Southern states, moves to the southern Plains during April and May, and then heads further north toward the Great Lakes states during the summer. One of the basic underlying reasons for this northward migration of the severe weather threat during the spring and summer is the fact that the jet stream follows a similar seasonal cycle.

The jet stream is a ribbon of high wind speeds located near the top of the troposphere (some six miles above the surface of the Earth). The jet stream position is strongly tied to the southern edge of the dome of cold air that is centered on the North Pole. During the depths of winter, that cold dome expands considerably, extending nearly to the Gulf of Mexico. As the winter ends and spring approaches, the hemisphere begins to warm up and the cold dome shrinks dramatically. Its southern edge moves to central Canada by early summer.

The jet stream is associated with vigorous vertical circulations (upward and downward motions). The upward vertical motions are instrumental in producing thunderstorms. Thus, when the jet stream migrates north as the weather warms in spring and summer, so does the greatest concentration of severe weather outbreaks.

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.
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What is an ice fog?

Photo credit: Amber Arnold

Each year about 700 fatalities occur in the United States as a result of traffic accidents during fog. A combination of high speed and low visibility is often to blame.

A small patch of fog contributed to a fatal multi-car pileup near Verona Wisconsin on February 12.

Ice fog is fine ice crystals suspended in the air.
Fogbow photo credit: Nate Miller

Fog is a cloud in contact with the ground. When the relative humidity approaches 100 percent, water vapor condenses on tiny particles suspended in the air to form a suspension of small water drops. The air in contact with the ground can reach high humidity if it cools or when water from the surface evaporates into it. Either of these processes increases the relative humidity of the air.

Ice fog is a type of fog consisting of fine ice crystals suspended in the air. It occurs only in cold areas of the world, as water droplets suspended in the air can remain liquid down to minus 40 degrees.

More common than an ice fog is a freezing fog. A freezing fog occurs when liquid fog droplets freeze to surfaces, forming white soft or hard rime. If there is a very light wind on a day with freezing fog, the wind can blow the droplets in one particular direction. As a result, small spikes of ice can grow into the wind on objects like trees and fences.The freezing fog can make roadways very slippery and dangerous. The danger is compounded by the poor visibility that accompanies the fog.

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.

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Does weather impact beer making?

Silver Hops farm near Sun Prairie Wisconsin. 
(Photo credit: Alyssa Stang)

Weather impacts all crops, as all plants prefer certain climatic regimes.

Fermented grain is an important ingredient in beer brewing. It defines the alcohol content.

Hops are just as critical, as the flower buds of hop plants add unique flavors, aroma and bitterness to the beer.

Hops grow very fast and require a lot of water. As the Pacific Northwest of the U.S. gets a lot of precipitation, this is a major hop-growing region as irrigation needs are limited. Washington State produces almost three-quarters of the hops used in US beers, with Oregon and Idaho contributing the second most hops. A drought in those regions will significantly reduce yields of hops and impact the beer brewing economy, leading to higher prices and a change in taste of some brands.

In 2015 the hops growing region of the Northwest had an unusually warm winter that led to widespread drought. As a result of the warm winter, the Cascades got mostly rain and a substantially reduced snowpack resulted. By May, an important time for the hops to be watered, the area was starved for water as there was no snow to melt.

Hops are grown on a type of hemp plant and grow best in USDA hardiness zones 5 through 9, which includes Wisconsin. The Wisconsin Hop Exchange Cooperative is one group that is working to establish the state as a producer of high-quality hops.

If you are a beer drinker who likes hoppy IPAs, you might hope for the exchange to prosper and for the Northwest to avoid developing a drier climate.

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.
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How much water is in the atmosphere?

Sitting nearly in the middle of the vast North American continent, Madison has what is known as a continental climate. Continental climates are characterized by large annual extremes in temperature and humidity as well as very distinct seasons.

The continental nature of Madison’s climate is what makes a year’s worth of weather in Madison usually a lot more varied than a year’s worth in Seattle, for instance. There is an astounding 144 degrees difference between the all-time highest (107 on July 13, 1936) and all-time lowest (-37 on Jan. 30, 1951) temperature in Madison.

In addition, the amount of water vapor in the air can range from the barely detectable level in the midst of a deep winter cold spell to as much as 3.5 percent of every breath you take during a severe July heatwave. No matter what the season, the vast majority of the invisible water vapor in the atmosphere is contained in the lowest mile or two from its source at the surface.

At any one instant, the Earth’s atmosphere contains 37.5 million-billion gallons of water vapor – enough to cover the entire surface of the planet with 1 inch of rain if condensed. This amount is recycled, through evaporation powered by the Sun, 40 times each year in what is known as the hydrologic cycle.

In each of these 40 cycles, enough energy is expended to power the U.S. — the largest consumer of energy in the world — for 3,441 years! A truly astounding amount of energy.

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Is winter winding down?

Ice accumulations formed by winds and sub-freezing temperatures hold to tree branches along the Lakeshore Path on the campus of UW-Madison in November. After mid-Feburary, it is rather unlikely we will experience another cold air event. (Photo credit:: John Hart, State Journal Archives)

Despite a prolonged deep freeze that straddled the end of December and the first week of January, during which we had below-zero morning low temperatures on 12 of 13 consecutive days, the month of January is likely to end at just about normal for Madison.

We are not, however, out of the woods just yet. Climatologically, the last week of January/first week of February is the coldest time of the year as a result of several physical factors.

First, the number of daylight hours at high latitudes in the Northern Hemisphere is very small – reaching its annual minimum on Dec. 21. For nearly a month, the increase in sunlight is very meager.

This means that very little input of solar energy is available in high latitude locations and nearly continual nighttime cooling prevails. Consequently, the generation of cold air masses is at its peak during this first four to six weeks of winter.

In line with these factors, Madison’s all-time lowest temperature, minus-37, was recorded on Jan. 30, 1951.

After the first week of February, however, the daily increase in daylight accelerates everywhere – this is even noticeable in Madison as the time of sunset is appreciably later by then.

As a result, it becomes more difficult to manufacture truly frigid air at high latitudes and this, in turn, greatly reduces our chances of an additional prolonged blast of arctic air.

So, even though the medium-range forecast suggests we will have another cold air event in the coming 10 days, take solace in the fact that it is rather unlikely that an additional such event will follow after mid-February. The cosmological and climatological decks are stacked against it.

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